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Title: On the Origin of Species

Author: Charles Darwin

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ON THE


ORIGIN OF SPECIES.







"But with regard to the material world, we can at least go so far as this--
we can perceive that events are brought about not by insulated
interpositions of Divine power, exerted in each particular case, but by the
establishment of general laws."

W. Whewell: Bridgewater Treatise.



"To conclude, therefore, let no man out of a weak conceit of sobriety, or
an ill-applied moderation, think or maintain, that a man can search too far
or be too well studied in the book of God's word, or in the book of God's
works; divinity or philosophy; but rather let men endeavour an endless
progress or proficience in both."

Bacon: Advancement of Learning.



Down, Bromley, Kent,

October 1st, 1859.




ON


THE ORIGIN OF SPECIES BY MEANS OF NATURAL SELECTION,


OR THE


PRESERVATION OF FAVOURED RACES IN THE STRUGGLE FOR LIFE.


BY CHARLES DARWIN, M.A.,
FELLOW OF THE ROYAL, GEOLOGICAL, LINNAEAN, ETC., SOCIETIES;


AUTHOR OF 'JOURNAL OF RESEARCHES DURING H.M.S. BEAGLE'S VOYAGE
ROUND THE WORLD.'



LONDON:

JOHN MURRAY, ALBEMARLE STREET.

1859.



CONTENTS.


INTRODUCTION.


CHAPTER 1. VARIATION UNDER DOMESTICATION.

Causes of Variability.
Effects of Habit.
Correlation of Growth.
Inheritance.
Character of Domestic Varieties.
Difficulty of distinguishing between Varieties and Species.
Origin of Domestic Varieties from one or more Species.
Domestic Pigeons, their Differences and Origin.
Principle of Selection anciently followed, its Effects.
Methodical and Unconscious Selection.
Unknown Origin of our Domestic Productions.
Circumstances favourable to Man's power of Selection.


CHAPTER 2. VARIATION UNDER NATURE.

Variability.
Individual Differences.
Doubtful species.
Wide ranging, much diffused, and common species vary most.
Species of the larger genera in any country vary more than the species
of the smaller genera.
Many of the species of the larger genera resemble varieties in being
very closely, but unequally, related to each other, and in having
restricted ranges.


CHAPTER 3. STRUGGLE FOR EXISTENCE.

Bears on natural selection.
The term used in a wide sense.
Geometrical powers of increase.
Rapid increase of naturalised animals and plants.
Nature of the checks to increase.
Competition universal.
Effects of climate.
Protection from the number of individuals.
Complex relations of all animals and plants throughout nature.
Struggle for life most severe between individuals and varieties of the
same species; often severe between species of the same genus.
The relation of organism to organism the most important of all
relations.


CHAPTER 4. NATURAL SELECTION.

Natural Selection: its power compared with man's selection, its power
on characters of trifling importance, its power at all ages and on
both sexes.
Sexual Selection.
On the generality of intercrosses between individuals of the same
species.
Circumstances favourable and unfavourable to Natural Selection,
namely, intercrossing, isolation, number of individuals.
Slow action.
Extinction caused by Natural Selection.
Divergence of Character, related to the diversity of inhabitants of
any small area, and to naturalisation.
Action of Natural Selection, through Divergence of Character and
Extinction, on the descendants from a common parent.
Explains the Grouping of all organic beings.


CHAPTER 5. LAWS OF VARIATION.

Effects of external conditions.
Use and disuse, combined with natural selection; organs of flight and
of vision.
Acclimatisation.
Correlation of growth.
Compensation and economy of growth.
False correlations.
Multiple, rudimentary, and lowly organised structures variable.
Parts developed in an unusual manner are highly variable: specific
characters more variable than generic: secondary sexual characters
variable.
Species of the same genus vary in an analogous manner.
Reversions to long-lost characters.
Summary.


CHAPTER 6. DIFFICULTIES ON THEORY.

Difficulties on the theory of descent with modification.
Transitions.
Absence or rarity of transitional varieties.
Transitions in habits of life.
Diversified habits in the same species.
Species with habits widely different from those of their allies.
Organs of extreme perfection.
Means of transition.
Cases of difficulty.
Natura non facit saltum.
Organs of small importance.
Organs not in all cases absolutely perfect.
The law of Unity of Type and of the Conditions of Existence embraced
by the theory of Natural Selection.


CHAPTER 7. INSTINCT.

Instincts comparable with habits, but different in their origin.
Instincts graduated.
Aphides and ants.
Instincts variable.
Domestic instincts, their origin.
Natural instincts of the cuckoo, ostrich, and parasitic bees.
Slave-making ants.
Hive-bee, its cell-making instinct.
Difficulties on the theory of the Natural Selection of instincts.
Neuter or sterile insects.
Summary.


CHAPTER 8. HYBRIDISM.

Distinction between the sterility of first crosses and of hybrids.
Sterility various in degree, not universal, affected by close
interbreeding, removed by domestication.
Laws governing the sterility of hybrids.
Sterility not a special endowment, but incidental on other
differences.
Causes of the sterility of first crosses and of hybrids.
Parallelism between the effects of changed conditions of life and
crossing.
Fertility of varieties when crossed and of their mongrel offspring not
universal.
Hybrids and mongrels compared independently of their fertility.
Summary.


CHAPTER 9. ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.

On the absence of intermediate varieties at the present day.
On the nature of extinct intermediate varieties; on their number.
On the vast lapse of time, as inferred from the rate of deposition and
of denudation.
On the poorness of our palaeontological collections.
On the intermittence of geological formations.
On the absence of intermediate varieties in any one formation.
On the sudden appearance of groups of species.
On their sudden appearance in the lowest known fossiliferous strata.


CHAPTER 10. ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS.

On the slow and successive appearance of new species.
On their different rates of change.
Species once lost do not reappear.
Groups of species follow the same general rules in their appearance
and disappearance as do single species.
On Extinction.
On simultaneous changes in the forms of life throughout the world.
On the affinities of extinct species to each other and to living
species.
On the state of development of ancient forms.
On the succession of the same types within the same areas.
Summary of preceding and present chapters.


CHAPTER 11. GEOGRAPHICAL DISTRIBUTION.

Present distribution cannot be accounted for by differences in
physical conditions.
Importance of barriers.
Affinity of the productions of the same continent.
Centres of creation.
Means of dispersal, by changes of climate and of the level of the
land, and by occasional means.
Dispersal during the Glacial period co-extensive with the world.


CHAPTER 12. GEOGRAPHICAL DISTRIBUTION--continued.

Distribution of fresh-water productions.
On the inhabitants of oceanic islands.
Absence of Batrachians and of terrestrial Mammals.
On the relation of the inhabitants of islands to those of the nearest
mainland.
On colonisation from the nearest source with subsequent modification.
Summary of the last and present chapters.


CHAPTER 13. MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY:
EMBRYOLOGY: RUDIMENTARY
ORGANS.

CLASSIFICATION, groups subordinate to groups.
Natural system.
Rules and difficulties in classification, explained on the theory of
descent with modification.
Classification of varieties.
Descent always used in classification.
Analogical or adaptive characters.
Affinities, general, complex and radiating.
Extinction separates and defines groups.
MORPHOLOGY, between members of the same class, between parts of the
same individual.
EMBRYOLOGY, laws of, explained by variations not supervening at an
early age, and being inherited at a corresponding age.
RUDIMENTARY ORGANS; their origin explained.
Summary.


CHAPTER 14. RECAPITULATION AND CONCLUSION.

Recapitulation of the difficulties on the theory of Natural Selection.
Recapitulation of the general and special circumstances in its favour.
Causes of the general belief in the immutability of species.
How far the theory of natural selection may be extended.
Effects of its adoption on the study of Natural history.
Concluding remarks.



ON THE ORIGIN OF SPECIES.



INTRODUCTION.

When on board H.M.S. 'Beagle,' as naturalist, I was much struck with
certain facts in the distribution of the inhabitants of South America,
and in the geological relations of the present to the past inhabitants
of that continent. These facts seemed to me to throw some light on the
origin of species--that mystery of mysteries, as it has been called by
one of our greatest philosophers. On my return home, it occurred to
me, in 1837, that something might perhaps be made out on this question
by patiently accumulating and reflecting on all sorts of facts which
could possibly have any bearing on it. After five years' work I
allowed myself to speculate on the subject, and drew up some short
notes; these I enlarged in 1844 into a sketch of the conclusions,
which then seemed to me probable: from that period to the present day
I have steadily pursued the same object. I hope that I may be excused
for entering on these personal details, as I give them to show that I
have not been hasty in coming to a decision.

My work is now nearly finished; but as it will take me two or three
more years to complete it, and as my health is far from strong, I have
been urged to publish this Abstract. I have more especially been
induced to do this, as Mr. Wallace, who is now studying the natural
history of the Malay archipelago, has arrived at almost exactly the
same general conclusions that I have on the origin of species. Last
year he sent to me a memoir on this subject, with a request that I
would forward it to Sir Charles Lyell, who sent it to the Linnean
Society, and it is published in the third volume of the Journal of
that Society. Sir C. Lyell and Dr. Hooker, who both knew of my
work--the latter having read my sketch of 1844--honoured me by
thinking it advisable to publish, with Mr. Wallace's excellent memoir,
some brief extracts from my manuscripts.

This Abstract, which I now publish, must necessarily be imperfect. I
cannot here give references and authorities for my several statements;
and I must trust to the reader reposing some confidence in my
accuracy. No doubt errors will have crept in, though I hope I have
always been cautious in trusting to good authorities alone. I can here
give only the general conclusions at which I have arrived, with a few
facts in illustration, but which, I hope, in most cases will suffice.
No one can feel more sensible than I do of the necessity of hereafter
publishing in detail all the facts, with references, on which my
conclusions have been grounded; and I hope in a future work to do
this. For I am well aware that scarcely a single point is discussed in
this volume on which facts cannot be adduced, often apparently leading
to conclusions directly opposite to those at which I have arrived. A
fair result can be obtained only by fully stating and balancing the
facts and arguments on both sides of each question; and this cannot
possibly be here done.

I much regret that want of space prevents my having the satisfaction
of acknowledging the generous assistance which I have received from
very many naturalists, some of them personally unknown to me. I
cannot, however, let this opportunity pass without expressing my deep
obligations to Dr. Hooker, who for the last fifteen years has aided me
in every possible way by his large stores of knowledge and his
excellent judgment.

In considering the Origin of Species, it is quite conceivable that a
naturalist, reflecting on the mutual affinities of organic beings, on
their embryological relations, their geographical distribution,
geological succession, and other such facts, might come to the
conclusion that each species had not been independently created, but
had descended, like varieties, from other species. Nevertheless, such
a conclusion, even if well founded, would be unsatisfactory, until it
could be shown how the innumerable species inhabiting this world have
been modified, so as to acquire that perfection of structure and
coadaptation which most justly excites our admiration. Naturalists
continually refer to external conditions, such as climate, food, etc.,
as the only possible cause of variation. In one very limited sense, as
we shall hereafter see, this may be true; but it is preposterous to
attribute to mere external conditions, the structure, for instance, of
the woodpecker, with its feet, tail, beak, and tongue, so admirably
adapted to catch insects under the bark of trees. In the case of the
misseltoe, which draws its nourishment from certain trees, which has
seeds that must be transported by certain birds, and which has flowers
with separate sexes absolutely requiring the agency of certain insects
to bring pollen from one flower to the other, it is equally
preposterous to account for the structure of this parasite, with its
relations to several distinct organic beings, by the effects of
external conditions, or of habit, or of the volition of the plant
itself.

The author of the 'Vestiges of Creation' would, I presume, say that,
after a certain unknown number of generations, some bird had given
birth to a woodpecker, and some plant to the misseltoe, and that these
had been produced perfect as we now see them; but this assumption
seems to me to be no explanation, for it leaves the case of the
coadaptations of organic beings to each other and to their physical
conditions of life, untouched and unexplained.

It is, therefore, of the highest importance to gain a clear insight
into the means of modification and coadaptation. At the commencement
of my observations it seemed to me probable that a careful study of
domesticated animals and of cultivated plants would offer the best
chance of making out this obscure problem. Nor have I been
disappointed; in this and in all other perplexing cases I have
invariably found that our knowledge, imperfect though it be, of
variation under domestication, afforded the best and safest clue. I
may venture to express my conviction of the high value of such
studies, although they have been very commonly neglected by
naturalists.

From these considerations, I shall devote the first chapter of this
Abstract to Variation under Domestication. We shall thus see that a
large amount of hereditary modification is at least possible, and,
what is equally or more important, we shall see how great is the power
of man in accumulating by his Selection successive slight variations.
I will then pass on to the variability of species in a state of
nature; but I shall, unfortunately, be compelled to treat this subject
far too briefly, as it can be treated properly only by giving long
catalogues of facts. We shall, however, be enabled to discuss what
circumstances are most favourable to variation. In the next chapter
the Struggle for Existence amongst all organic beings throughout the
world, which inevitably follows from their high geometrical powers of
increase, will be treated of. This is the doctrine of Malthus, applied
to the whole animal and vegetable kingdoms. As many more individuals
of each species are born than can possibly survive; and as,
consequently, there is a frequently recurring struggle for existence,
it follows that any being, if it vary however slightly in any manner
profitable to itself, under the complex and sometimes varying
conditions of life, will have a better chance of surviving, and thus
be NATURALLY SELECTED. From the strong principle of inheritance, any
selected variety will tend to propagate its new and modified form.

This fundamental subject of Natural Selection will be treated at some
length in the fourth chapter; and we shall then see how Natural
Selection almost inevitably causes much Extinction of the less
improved forms of life and induces what I have called Divergence of
Character. In the next chapter I shall discuss the complex and little
known laws of variation and of correlation of growth. In the four
succeeding chapters, the most apparent and gravest difficulties on the
theory will be given: namely, first, the difficulties of transitions,
or in understanding how a simple being or a simple organ can be
changed and perfected into a highly developed being or elaborately
constructed organ; secondly the subject of Instinct, or the mental
powers of animals, thirdly, Hybridism, or the infertility of species
and the fertility of varieties when intercrossed; and fourthly, the
imperfection of the Geological Record. In the next chapter I shall
consider the geological succession of organic beings throughout time;
in the eleventh and twelfth, their geographical distribution
throughout space; in the thirteenth, their classification or mutual
affinities, both when mature and in an embryonic condition. In the
last chapter I shall give a brief recapitulation of the whole work,
and a few concluding remarks.

No one ought to feel surprise at much remaining as yet unexplained in
regard to the origin of species and varieties, if he makes due
allowance for our profound ignorance in regard to the mutual relations
of all the beings which live around us. Who can explain why one
species ranges widely and is very numerous, and why another allied
species has a narrow range and is rare? Yet these relations are of the
highest importance, for they determine the present welfare, and, as I
believe, the future success and modification of every inhabitant of
this world. Still less do we know of the mutual relations of the
innumerable inhabitants of the world during the many past geological
epochs in its history. Although much remains obscure, and will long
remain obscure, I can entertain no doubt, after the most deliberate
study and dispassionate judgment of which I am capable, that the view
which most naturalists entertain, and which I formerly
entertained--namely, that each species has been independently
created--is erroneous. I am fully convinced that species are not
immutable; but that those belonging to what are called the same genera
are lineal descendants of some other and generally extinct species, in
the same manner as the acknowledged varieties of any one species are
the descendants of that species. Furthermore, I am convinced that
Natural Selection has been the main but not exclusive means of
modification.


CHAPTER 1. VARIATION UNDER DOMESTICATION.

Causes of Variability.
Effects of Habit.
Correlation of Growth.
Inheritance.
Character of Domestic Varieties.
Difficulty of distinguishing between Varieties and Species.
Origin of Domestic Varieties from one or more Species.
Domestic Pigeons, their Differences and Origin.
Principle of Selection anciently followed, its Effects.
Methodical and Unconscious Selection.
Unknown Origin of our Domestic Productions.
Circumstances favourable to Man's power of Selection.

When we look to the individuals of the same variety or sub-variety of
our older cultivated plants and animals, one of the first points which
strikes us, is, that they generally differ much more from each other,
than do the individuals of any one species or variety in a state of
nature. When we reflect on the vast diversity of the plants and
animals which have been cultivated, and which have varied during all
ages under the most different climates and treatment, I think we are
driven to conclude that this greater variability is simply due to our
domestic productions having been raised under conditions of life not
so uniform as, and somewhat different from, those to which the
parent-species have been exposed under nature. There is, also, I
think, some probability in the view propounded by Andrew Knight, that
this variability may be partly connected with excess of food. It seems
pretty clear that organic beings must be exposed during several
generations to the new conditions of life to cause any appreciable
amount of variation; and that when the organisation has once begun to
vary, it generally continues to vary for many generations. No case is
on record of a variable being ceasing to be variable under
cultivation. Our oldest cultivated plants, such as wheat, still often
yield new varieties: our oldest domesticated animals are still capable
of rapid improvement or modification.

It has been disputed at what period of life the causes of variability,
whatever they may be, generally act; whether during the early or late
period of development of the embryo, or at the instant of conception.
Geoffroy St. Hilaire's experiments show that unnatural treatment of
the embryo causes monstrosities; and monstrosities cannot be separated
by any clear line of distinction from mere variations. But I am
strongly inclined to suspect that the most frequent cause of
variability may be attributed to the male and female reproductive
elements having been affected prior to the act of conception. Several
reasons make me believe in this; but the chief one is the remarkable
effect which confinement or cultivation has on the functions of the
reproductive system; this system appearing to be far more susceptible
than any other part of the organisation, to the action of any change
in the conditions of life. Nothing is more easy than to tame an
animal, and few things more difficult than to get it to breed freely
under confinement, even in the many cases when the male and female
unite. How many animals there are which will not breed, though living
long under not very close confinement in their native country! This is
generally attributed to vitiated instincts; but how many cultivated
plants display the utmost vigour, and yet rarely or never seed! In
some few such cases it has been found out that very trifling changes,
such as a little more or less water at some particular period of
growth, will determine whether or not the plant sets a seed. I cannot
here enter on the copious details which I have collected on this
curious subject; but to show how singular the laws are which determine
the reproduction of animals under confinement, I may just mention that
carnivorous animals, even from the tropics, breed in this country
pretty freely under confinement, with the exception of the
plantigrades or bear family; whereas, carnivorous birds, with the
rarest exceptions, hardly ever lay fertile eggs. Many exotic plants
have pollen utterly worthless, in the same exact condition as in the
most sterile hybrids. When, on the one hand, we see domesticated
animals and plants, though often weak and sickly, yet breeding quite
freely under confinement; and when, on the other hand, we see
individuals, though taken young from a state of nature, perfectly
tamed, long-lived, and healthy (of which I could give numerous
instances), yet having their reproductive system so seriously affected
by unperceived causes as to fail in acting, we need not be surprised
at this system, when it does act under confinement, acting not quite
regularly, and producing offspring not perfectly like their parents or
variable.

Sterility has been said to be the bane of horticulture; but on this
view we owe variability to the same cause which produces sterility;
and variability is the source of all the choicest productions of the
garden. I may add, that as some organisms will breed most freely under
the most unnatural conditions (for instance, the rabbit and ferret
kept in hutches), showing that their reproductive system has not been
thus affected; so will some animals and plants withstand domestication
or cultivation, and vary very slightly--perhaps hardly more than in a
state of nature.

A long list could easily be given of "sporting plants;" by this term
gardeners mean a single bud or offset, which suddenly assumes a new
and sometimes very different character from that of the rest of the
plant. Such buds can be propagated by grafting, etc., and sometimes by
seed. These "sports" are extremely rare under nature, but far from
rare under cultivation; and in this case we see that the treatment of
the parent has affected a bud or offset, and not the ovules or pollen.
But it is the opinion of most physiologists that there is no essential
difference between a bud and an ovule in their earliest stages of
formation; so that, in fact, "sports" support my view, that
variability may be largely attributed to the ovules or pollen, or to
both, having been affected by the treatment of the parent prior to the
act of conception. These cases anyhow show that variation is not
necessarily connected, as some authors have supposed, with the act of
generation.

Seedlings from the same fruit, and the young of the same litter,
sometimes differ considerably from each other, though both the young
and the parents, as Muller has remarked, have apparently been exposed
to exactly the same conditions of life; and this shows how unimportant
the direct effects of the conditions of life are in comparison with
the laws of reproduction, and of growth, and of inheritance; for had
the action of the conditions been direct, if any of the young had
varied, all would probably have varied in the same manner. To judge
how much, in the case of any variation, we should attribute to the
direct action of heat, moisture, light, food, etc., is most difficult:
my impression is, that with animals such agencies have produced very
little direct effect, though apparently more in the case of plants.
Under this point of view, Mr. Buckman's recent experiments on plants
seem extremely valuable. When all or nearly all the individuals
exposed to certain conditions are affected in the same way, the change
at first appears to be directly due to such conditions; but in some
cases it can be shown that quite opposite conditions produce similar
changes of structure. Nevertheless some slight amount of change may, I
think, be attributed to the direct action of the conditions of
life--as, in some cases, increased size from amount of food, colour
from particular kinds of food and from light, and perhaps the
thickness of fur from climate.

Habit also has a decided influence, as in the period of flowering with
plants when transported from one climate to another. In animals it has
a more marked effect; for instance, I find in the domestic duck that
the bones of the wing weigh less and the bones of the leg more, in
proportion to the whole skeleton, than do the same bones in the
wild-duck; and I presume that this change may be safely attributed to
the domestic duck flying much less, and walking more, than its wild
parent. The great and inherited development of the udders in cows and
goats in countries where they are habitually milked, in comparison
with the state of these organs in other countries, is another instance
of the effect of use. Not a single domestic animal can be named which
has not in some country drooping ears; and the view suggested by some
authors, that the drooping is due to the disuse of the muscles of the
ear, from the animals not being much alarmed by danger, seems
probable.

There are many laws regulating variation, some few of which can be
dimly seen, and will be hereafter briefly mentioned. I will here only
allude to what may be called correlation of growth. Any change in the
embryo or larva will almost certainly entail changes in the mature
animal. In monstrosities, the correlations between quite distinct
parts are very curious; and many instances are given in Isidore
Geoffroy St. Hilaire's great work on this subject. Breeders believe
that long limbs are almost always accompanied by an elongated head.
Some instances of correlation are quite whimsical; thus cats with blue
eyes are invariably deaf; colour and constitutional peculiarities go
together, of which many remarkable cases could be given amongst
animals and plants. From the facts collected by Heusinger, it appears
that white sheep and pigs are differently affected from coloured
individuals by certain vegetable poisons. Hairless dogs have imperfect
teeth; long-haired and coarse-haired animals are apt to have, as is
asserted, long or many horns; pigeons with feathered feet have skin
between their outer toes; pigeons with short beaks have small feet,
and those with long beaks large feet. Hence, if man goes on selecting,
and thus augmenting, any peculiarity, he will almost certainly
unconsciously modify other parts of the structure, owing to the
mysterious laws of the correlation of growth.

The result of the various, quite unknown, or dimly seen laws of
variation is infinitely complex and diversified. It is well worth
while carefully to study the several treatises published on some of
our old cultivated plants, as on the hyacinth, potato, even the
dahlia, etc.; and it is really surprising to note the endless points
in structure and constitution in which the varieties and sub-varieties
differ slightly from each other. The whole organisation seems to have
become plastic, and tends to depart in some small degree from that of
the parental type.

Any variation which is not inherited is unimportant for us. But the
number and diversity of inheritable deviations of structure, both
those of slight and those of considerable physiological importance, is
endless. Dr. Prosper Lucas's treatise, in two large volumes, is the
fullest and the best on this subject. No breeder doubts how strong is
the tendency to inheritance: like produces like is his fundamental
belief: doubts have been thrown on this principle by theoretical
writers alone. When a deviation appears not unfrequently, and we see
it in the father and child, we cannot tell whether it may not be due
to the same original cause acting on both; but when amongst
individuals, apparently exposed to the same conditions, any very rare
deviation, due to some extraordinary combination of circumstances,
appears in the parent--say, once amongst several million
individuals--and it reappears in the child, the mere doctrine of
chances almost compels us to attribute its reappearance to
inheritance. Every one must have heard of cases of albinism, prickly
skin, hairy bodies, etc., appearing in several members of the same
family. If strange and rare deviations of structure are truly
inherited, less strange and commoner deviations may be freely admitted
to be inheritable. Perhaps the correct way of viewing the whole
subject, would be, to look at the inheritance of every character
whatever as the rule, and non-inheritance as the anomaly.

The laws governing inheritance are quite unknown; no one can say why
the same peculiarity in different individuals of the same species, and
in individuals of different species, is sometimes inherited and
sometimes not so; why the child often reverts in certain characters to
its grandfather or grandmother or other much more remote ancestor; why
a peculiarity is often transmitted from one sex to both sexes or to
one sex alone, more commonly but not exclusively to the like sex. It
is a fact of some little importance to us, that peculiarities
appearing in the males of our domestic breeds are often transmitted
either exclusively, or in a much greater degree, to males alone. A
much more important rule, which I think may be trusted, is that, at
whatever period of life a peculiarity first appears, it tends to
appear in the offspring at a corresponding age, though sometimes
earlier. In many cases this could not be otherwise: thus the inherited
peculiarities in the horns of cattle could appear only in the
offspring when nearly mature; peculiarities in the silkworm are known
to appear at the corresponding caterpillar or cocoon stage. But
hereditary diseases and some other facts make me believe that the rule
has a wider extension, and that when there is no apparent reason why a
peculiarity should appear at any particular age, yet that it does tend
to appear in the offspring at the same period at which it first
appeared in the parent. I believe this rule to be of the highest
importance in explaining the laws of embryology. These remarks are of
course confined to the first APPEARANCE of the peculiarity, and not to
its primary cause, which may have acted on the ovules or male element;
in nearly the same manner as in the crossed offspring from a
short-horned cow by a long-horned bull, the greater length of horn,
though appearing late in life, is clearly due to the male element.

Having alluded to the subject of reversion, I may here refer to a
statement often made by naturalists--namely, that our domestic
varieties, when run wild, gradually but certainly revert in character
to their aboriginal stocks. Hence it has been argued that no
deductions can be drawn from domestic races to species in a state of
nature. I have in vain endeavoured to discover on what decisive facts
the above statement has so often and so boldly been made. There would
be great difficulty in proving its truth: we may safely conclude that
very many of the most strongly-marked domestic varieties could not
possibly live in a wild state. In many cases we do not know what the
aboriginal stock was, and so could not tell whether or not nearly
perfect reversion had ensued. It would be quite necessary, in order to
prevent the effects of intercrossing, that only a single variety
should be turned loose in its new home. Nevertheless, as our varieties
certainly do occasionally revert in some of their characters to
ancestral forms, it seems to me not improbable, that if we could
succeed in naturalising, or were to cultivate, during many
generations, the several races, for instance, of the cabbage, in very
poor soil (in which case, however, some effect would have to be
attributed to the direct action of the poor soil), that they would to
a large extent, or even wholly, revert to the wild aboriginal stock.
Whether or not the experiment would succeed, is not of great
importance for our line of argument; for by the experiment itself the
conditions of life are changed. If it could be shown that our domestic
varieties manifested a strong tendency to reversion,--that is, to lose
their acquired characters, whilst kept under unchanged conditions, and
whilst kept in a considerable body, so that free intercrossing might
check, by blending together, any slight deviations of structure, in
such case, I grant that we could deduce nothing from domestic
varieties in regard to species. But there is not a shadow of evidence
in favour of this view: to assert that we could not breed our cart and
race-horses, long and short-horned cattle, and poultry of various
breeds, and esculent vegetables, for an almost infinite number of
generations, would be opposed to all experience. I may add, that when
under nature the conditions of life do change, variations and
reversions of character probably do occur; but natural selection, as
will hereafter be explained, will determine how far the new characters
thus arising shall be preserved.

When we look to the hereditary varieties or races of our domestic
animals and plants, and compare them with species closely allied
together, we generally perceive in each domestic race, as already
remarked, less uniformity of character than in true species. Domestic
races of the same species, also, often have a somewhat monstrous
character; by which I mean, that, although differing from each other,
and from the other species of the same genus, in several trifling
respects, they often differ in an extreme degree in some one part,
both when compared one with another, and more especially when compared
with all the species in nature to which they are nearest allied. With
these exceptions (and with that of the perfect fertility of varieties
when crossed,--a subject hereafter to be discussed), domestic races of
the same species differ from each other in the same manner as, only in
most cases in a lesser degree than, do closely-allied species of the
same genus in a state of nature. I think this must be admitted, when
we find that there are hardly any domestic races, either amongst
animals or plants, which have not been ranked by some competent judges
as mere varieties, and by other competent judges as the descendants of
aboriginally distinct species. If any marked distinction existed
between domestic races and species, this source of doubt could not so
perpetually recur. It has often been stated that domestic races do not
differ from each other in characters of generic value. I think it
could be shown that this statement is hardly correct; but naturalists
differ most widely in determining what characters are of generic
value; all such valuations being at present empirical. Moreover, on
the view of the origin of genera which I shall presently give, we have
no right to expect often to meet with generic differences in our
domesticated productions.

When we attempt to estimate the amount of structural difference
between the domestic races of the same species, we are soon involved
in doubt, from not knowing whether they have descended from one or
several parent-species. This point, if it could be cleared up, would
be interesting; if, for instance, it could be shown that the
greyhound, bloodhound, terrier, spaniel, and bull-dog, which we all
know propagate their kind so truly, were the offspring of any single
species, then such facts would have great weight in making us doubt
about the immutability of the many very closely allied and natural
species--for instance, of the many foxes--inhabiting different
quarters of the world. I do not believe, as we shall presently see,
that all our dogs have descended from any one wild species; but, in
the case of some other domestic races, there is presumptive, or even
strong, evidence in favour of this view.

It has often been assumed that man has chosen for domestication
animals and plants having an extraordinary inherent tendency to vary,
and likewise to withstand diverse climates. I do not dispute that
these capacities have added largely to the value of most of our
domesticated productions; but how could a savage possibly know, when
he first tamed an animal, whether it would vary in succeeding
generations, and whether it would endure other climates? Has the
little variability of the ass or guinea-fowl, or the small power of
endurance of warmth by the rein-deer, or of cold by the common camel,
prevented their domestication? I cannot doubt that if other animals
and plants, equal in number to our domesticated productions, and
belonging to equally diverse classes and countries, were taken from a
state of nature, and could be made to breed for an equal number of
generations under domestication, they would vary on an average as
largely as the parent species of our existing domesticated productions
have varied.

In the case of most of our anciently domesticated animals and plants,
I do not think it is possible to come to any definite conclusion,
whether they have descended from one or several species. The argument
mainly relied on by those who believe in the multiple origin of our
domestic animals is, that we find in the most ancient records, more
especially on the monuments of Egypt, much diversity in the breeds;
and that some of the breeds closely resemble, perhaps are identical
with, those still existing. Even if this latter fact were found more
strictly and generally true than seems to me to be the case, what does
it show, but that some of our breeds originated there, four or five
thousand years ago? But Mr. Horner's researches have rendered it in
some degree probable that man sufficiently civilized to have
manufactured pottery existed in the valley of the Nile thirteen or
fourteen thousand years ago; and who will pretend to say how long
before these ancient periods, savages, like those of Tierra del Fuego
or Australia, who possess a semi-domestic dog, may not have existed in
Egypt?

The whole subject must, I think, remain vague; nevertheless, I may,
without here entering on any details, state that, from geographical
and other considerations, I think it highly probable that our domestic
dogs have descended from several wild species. In regard to sheep and
goats I can form no opinion. I should think, from facts communicated
to me by Mr. Blyth, on the habits, voice, and constitution, etc., of
the humped Indian cattle, that these had descended from a different
aboriginal stock from our European cattle; and several competent
judges believe that these latter have had more than one wild parent.
With respect to horses, from reasons which I cannot give here, I am
doubtfully inclined to believe, in opposition to several authors, that
all the races have descended from one wild stock. Mr. Blyth, whose
opinion, from his large and varied stores of knowledge, I should value
more than that of almost any one, thinks that all the breeds of
poultry have proceeded from the common wild Indian fowl (Gallus
bankiva). In regard to ducks and rabbits, the breeds of which differ
considerably from each other in structure, I do not doubt that they
all have descended from the common wild duck and rabbit.

The doctrine of the origin of our several domestic races from several
aboriginal stocks, has been carried to an absurd extreme by some
authors. They believe that every race which breeds true, let the
distinctive characters be ever so slight, has had its wild prototype.
At this rate there must have existed at least a score of species of
wild cattle, as many sheep, and several goats in Europe alone, and
several even within Great Britain. One author believes that there
formerly existed in Great Britain eleven wild species of sheep
peculiar to it! When we bear in mind that Britain has now hardly one
peculiar mammal, and France but few distinct from those of Germany and
conversely, and so with Hungary, Spain, etc., but that each of these
kingdoms possesses several peculiar breeds of cattle, sheep, etc., we
must admit that many domestic breeds have originated in Europe; for
whence could they have been derived, as these several countries do not
possess a number of peculiar species as distinct parent-stocks? So it
is in India. Even in the case of the domestic dogs of the whole world,
which I fully admit have probably descended from several wild species,
I cannot doubt that there has been an immense amount of inherited
variation. Who can believe that animals closely resembling the Italian
greyhound, the bloodhound, the bull-dog, or Blenheim spaniel, etc.--so
unlike all wild Canidae--ever existed freely in a state of nature? It
has often been loosely said that all our races of dogs have been
produced by the crossing of a few aboriginal species; but by crossing
we can get only forms in some degree intermediate between their
parents; and if we account for our several domestic races by this
process, we must admit the former existence of the most extreme forms,
as the Italian greyhound, bloodhound, bull-dog, etc., in the wild
state. Moreover, the possibility of making distinct races by crossing
has been greatly exaggerated. There can be no doubt that a race may be
modified by occasional crosses, if aided by the careful selection of
those individual mongrels, which present any desired character; but
that a race could be obtained nearly intermediate between two
extremely different races or species, I can hardly believe. Sir J.
Sebright expressly experimentised for this object, and failed. The
offspring from the first cross between two pure breeds is tolerably
and sometimes (as I have found with pigeons) extremely uniform, and
everything seems simple enough; but when these mongrels are crossed
one with another for several generations, hardly two of them will be
alike, and then the extreme difficulty, or rather utter hopelessness,
of the task becomes apparent. Certainly, a breed intermediate between
TWO VERY DISTINCT breeds could not be got without extreme care and
long-continued selection; nor can I find a single case on record of a
permanent race having been thus formed.

ON THE BREEDS OF THE DOMESTIC PIGEON.

Believing that it is always best to study some special group, I have,
after deliberation, taken up domestic pigeons. I have kept every breed
which I could purchase or obtain, and have been most kindly favoured
with skins from several quarters of the world, more especially by the
Honourable W. Elliot from India, and by the Honourable C. Murray from
Persia. Many treatises in different languages have been published on
pigeons, and some of them are very important, as being of considerable
antiquity. I have associated with several eminent fanciers, and have
been permitted to join two of the London Pigeon Clubs. The diversity
of the breeds is something astonishing. Compare the English carrier
and the short-faced tumbler, and see the wonderful difference in their
beaks, entailing corresponding differences in their skulls. The
carrier, more especially the male bird, is also remarkable from the
wonderful development of the carunculated skin about the head, and
this is accompanied by greatly elongated eyelids, very large external
orifices to the nostrils, and a wide gape of mouth. The short-faced
tumbler has a beak in outline almost like that of a finch; and the
common tumbler has the singular and strictly inherited habit of flying
at a great height in a compact flock, and tumbling in the air head
over heels. The runt is a bird of great size, with long, massive beak
and large feet; some of the sub-breeds of runts have very long necks,
others very long wings and tails, others singularly short tails. The
barb is allied to the carrier, but, instead of a very long beak, has a
very short and very broad one. The pouter has a much elongated body,
wings, and legs; and its enormously developed crop, which it glories
in inflating, may well excite astonishment and even laughter. The
turbit has a very short and conical beak, with a line of reversed
feathers down the breast; and it has the habit of continually
expanding slightly the upper part of the oesophagus. The Jacobin has
the feathers so much reversed along the back of the neck that they
form a hood, and it has, proportionally to its size, much elongated
wing and tail feathers. The trumpeter and laugher, as their names
express, utter a very different coo from the other breeds. The fantail
has thirty or even forty tail-feathers, instead of twelve or fourteen,
the normal number in all members of the great pigeon family; and these
feathers are kept expanded, and are carried so erect that in good
birds the head and tail touch; the oil-gland is quite aborted. Several
other less distinct breeds might have been specified.

In the skeletons of the several breeds, the development of the bones
of the face in length and breadth and curvature differs enormously.
The shape, as well as the breadth and length of the ramus of the lower
jaw, varies in a highly remarkable manner. The number of the caudal
and sacral vertebrae vary; as does the number of the ribs, together
with their relative breadth and the presence of processes. The size
and shape of the apertures in the sternum are highly variable; so is
the degree of divergence and relative size of the two arms of the
furcula. The proportional width of the gape of mouth, the proportional
length of the eyelids, of the orifice of the nostrils, of the tongue
(not always in strict correlation with the length of beak), the size
of the crop and of the upper part of the oesophagus; the development
and abortion of the oil-gland; the number of the primary wing and
caudal feathers; the relative length of wing and tail to each other
and to the body; the relative length of leg and of the feet; the
number of scutellae on the toes, the development of skin between the
toes, are all points of structure which are variable. The period at
which the perfect plumage is acquired varies, as does the state of the
down with which the nestling birds are clothed when hatched. The shape
and size of the eggs vary. The manner of flight differs remarkably; as
does in some breeds the voice and disposition. Lastly, in certain
breeds, the males and females have come to differ to a slight degree
from each other.

Altogether at least a score of pigeons might be chosen, which if shown
to an ornithologist, and he were told that they were wild birds, would
certainly, I think, be ranked by him as well-defined species.
Moreover, I do not believe that any ornithologist would place the
English carrier, the short-faced tumbler, the runt, the barb, pouter,
and fantail in the same genus; more especially as in each of these
breeds several truly-inherited sub-breeds, or species as he might have
called them, could be shown him.

Great as the differences are between the breeds of pigeons, I am fully
convinced that the common opinion of naturalists is correct, namely,
that all have descended from the rock-pigeon (Columba livia),
including under this term several geographical races or sub-species,
which differ from each other in the most trifling respects. As several
of the reasons which have led me to this belief are in some degree
applicable in other cases, I will here briefly give them. If the
several breeds are not varieties, and have not proceeded from the
rock-pigeon, they must have descended from at least seven or eight
aboriginal stocks; for it is impossible to make the present domestic
breeds by the crossing of any lesser number: how, for instance, could
a pouter be produced by crossing two breeds unless one of the
parent-stocks possessed the characteristic enormous crop? The supposed
aboriginal stocks must all have been rock-pigeons, that is, not
breeding or willingly perching on trees. But besides C. livia, with
its geographical sub-species, only two or three other species of
rock-pigeons are known; and these have not any of the characters of
the domestic breeds. Hence the supposed aboriginal stocks must either
still exist in the countries where they were originally domesticated,
and yet be unknown to ornithologists; and this, considering their
size, habits, and remarkable characters, seems very improbable; or
they must have become extinct in the wild state. But birds breeding on
precipices, and good fliers, are unlikely to be exterminated; and the
common rock-pigeon, which has the same habits with the domestic
breeds, has not been exterminated even on several of the smaller
British islets, or on the shores of the Mediterranean. Hence the
supposed extermination of so many species having similar habits with
the rock-pigeon seems to me a very rash assumption. Moreover, the
several above-named domesticated breeds have been transported to all
parts of the world, and, therefore, some of them must have been
carried back again into their native country; but not one has ever
become wild or feral, though the dovecot-pigeon, which is the
rock-pigeon in a very slightly altered state, has become feral in
several places. Again, all recent experience shows that it is most
difficult to get any wild animal to breed freely under domestication;
yet on the hypothesis of the multiple origin of our pigeons, it must
be assumed that at least seven or eight species were so thoroughly
domesticated in ancient times by half-civilized man, as to be quite
prolific under confinement.

An argument, as it seems to me, of great weight, and applicable in
several other cases, is, that the above-specified breeds, though
agreeing generally in constitution, habits, voice, colouring, and in
most parts of their structure, with the wild rock-pigeon, yet are
certainly highly abnormal in other parts of their structure: we may
look in vain throughout the whole great family of Columbidae for a
beak like that of the English carrier, or that of the short-faced
tumbler, or barb; for reversed feathers like those of the jacobin; for
a crop like that of the pouter; for tail-feathers like those of the
fantail. Hence it must be assumed not only that half-civilized man
succeeded in thoroughly domesticating several species, but that he
intentionally or by chance picked out extraordinarily abnormal
species; and further, that these very species have since all become
extinct or unknown. So many strange contingencies seem to me
improbable in the highest degree.

Some facts in regard to the colouring of pigeons well deserve
consideration. The rock-pigeon is of a slaty-blue, and has a white
rump (the Indian sub-species, C. intermedia of Strickland, having it
bluish); the tail has a terminal dark bar, with the bases of the outer
feathers externally edged with white; the wings have two black bars;
some semi-domestic breeds and some apparently truly wild breeds have,
besides the two black bars, the wings chequered with black. These
several marks do not occur together in any other species of the whole
family. Now, in every one of the domestic breeds, taking thoroughly
well-bred birds, all the above marks, even to the white edging of the
outer tail-feathers, sometimes concur perfectly developed. Moreover,
when two birds belonging to two distinct breeds are crossed, neither
of which is blue or has any of the above-specified marks, the mongrel
offspring are very apt suddenly to acquire these characters; for
instance, I crossed some uniformly white fantails with some uniformly
black barbs, and they produced mottled brown and black birds; these I
again crossed together, and one grandchild of the pure white fantail
and pure black barb was of as beautiful a blue colour, with the white
rump, double black wing-bar, and barred and white-edged tail-feathers,
as any wild rock-pigeon! We can understand these facts, on the
well-known principle of reversion to ancestral characters, if all the
domestic breeds have descended from the rock-pigeon. But if we deny
this, we must make one of the two following highly improbable
suppositions. Either, firstly, that all the several imagined
aboriginal stocks were coloured and marked like the rock-pigeon,
although no other existing species is thus coloured and marked, so
that in each separate breed there might be a tendency to revert to the
very same colours and markings. Or, secondly, that each breed, even
the purest, has within a dozen or, at most, within a score of
generations, been crossed by the rock-pigeon: I say within a dozen or
twenty generations, for we know of no fact countenancing the belief
that the child ever reverts to some one ancestor, removed by a greater
number of generations. In a breed which has been crossed only once
with some distinct breed, the tendency to reversion to any character
derived from such cross will naturally become less and less, as in
each succeeding generation there will be less of the foreign blood;
but when there has been no cross with a distinct breed, and there is a
tendency in both parents to revert to a character, which has been lost
during some former generation, this tendency, for all that we can see
to the contrary, may be transmitted undiminished for an indefinite
number of generations. These two distinct cases are often confounded
in treatises on inheritance.

Lastly, the hybrids or mongrels from between all the domestic breeds
of pigeons are perfectly fertile. I can state this from my own
observations, purposely made on the most distinct breeds. Now, it is
difficult, perhaps impossible, to bring forward one case of the hybrid
offspring of two animals CLEARLY DISTINCT being themselves perfectly
fertile. Some authors believe that long-continued domestication
eliminates this strong tendency to sterility: from the history of the
dog I think there is some probability in this hypothesis, if applied
to species closely related together, though it is unsupported by a
single experiment. But to extend the hypothesis so far as to suppose
that species, aboriginally as distinct as carriers, tumblers, pouters,
and fantails now are, should yield offspring perfectly fertile, inter
se, seems to me rash in the extreme.

From these several reasons, namely, the improbability of man having
formerly got seven or eight supposed species of pigeons to breed
freely under domestication; these supposed species being quite unknown
in a wild state, and their becoming nowhere feral; these species
having very abnormal characters in certain respects, as compared with
all other Columbidae, though so like in most other respects to the
rock-pigeon; the blue colour and various marks occasionally appearing
in all the breeds, both when kept pure and when crossed; the mongrel
offspring being perfectly fertile;--from these several reasons, taken
together, I can feel no doubt that all our domestic breeds have
descended from the Columba livia with its geographical sub-species.

In favour of this view, I may add, firstly, that C. livia, or the
rock-pigeon, has been found capable of domestication in Europe and in
India; and that it agrees in habits and in a great number of points of
structure with all the domestic breeds. Secondly, although an English
carrier or short-faced tumbler differs immensely in certain characters
from the rock-pigeon, yet by comparing the several sub-breeds of these
breeds, more especially those brought from distant countries, we can
make an almost perfect series between the extremes of structure.
Thirdly, those characters which are mainly distinctive of each breed,
for instance the wattle and length of beak of the carrier, the
shortness of that of the tumbler, and the number of tail-feathers in
the fantail, are in each breed eminently variable; and the explanation
of this fact will be obvious when we come to treat of selection.
Fourthly, pigeons have been watched, and tended with the utmost care,
and loved by many people. They have been domesticated for thousands of
years in several quarters of the world; the earliest known record of
pigeons is in the fifth Aegyptian dynasty, about 3000 B.C., as was
pointed out to me by Professor Lepsius; but Mr. Birch informs me that
pigeons are given in a bill of fare in the previous dynasty. In the
time of the Romans, as we hear from Pliny, immense prices were given
for pigeons; "nay, they are come to this pass, that they can reckon up
their pedigree and race." Pigeons were much valued by Akber Khan in
India, about the year 1600; never less than 20,000 pigeons were taken
with the court. "The monarchs of Iran and Turan sent him some very
rare birds;" and, continues the courtly historian, "His Majesty by
crossing the breeds, which method was never practised before, has
improved them astonishingly." About this same period the Dutch were as
eager about pigeons as were the old Romans. The paramount importance
of these considerations in explaining the immense amount of variation
which pigeons have undergone, will be obvious when we treat of
Selection. We shall then, also, see how it is that the breeds so often
have a somewhat monstrous character. It is also a most favourable
circumstance for the production of distinct breeds, that male and
female pigeons can be easily mated for life; and thus different breeds
can be kept together in the same aviary.

I have discussed the probable origin of domestic pigeons at some, yet
quite insufficient, length; because when I first kept pigeons and
watched the several kinds, knowing well how true they bred, I felt
fully as much difficulty in believing that they could ever have
descended from a common parent, as any naturalist could in coming to a
similar conclusion in regard to the many species of finches, or other
large groups of birds, in nature. One circumstance has struck me much;
namely, that all the breeders of the various domestic animals and the
cultivators of plants, with whom I have ever conversed, or whose
treatises I have read, are firmly convinced that the several breeds to
which each has attended, are descended from so many aboriginally
distinct species. Ask, as I have asked, a celebrated raiser of
Hereford cattle, whether his cattle might not have descended from long
horns, and he will laugh you to scorn. I have never met a pigeon, or
poultry, or duck, or rabbit fancier, who was not fully convinced that
each main breed was descended from a distinct species. Van Mons, in
his treatise on pears and apples, shows how utterly he disbelieves
that the several sorts, for instance a Ribston-pippin or Codlin-apple,
could ever have proceeded from the seeds of the same tree. Innumerable
other examples could be given. The explanation, I think, is simple:
from long-continued study they are strongly impressed with the
differences between the several races; and though they well know that
each race varies slightly, for they win their prizes by selecting such
slight differences, yet they ignore all general arguments, and refuse
to sum up in their minds slight differences accumulated during many
successive generations. May not those naturalists who, knowing far
less of the laws of inheritance than does the breeder, and knowing no
more than he does of the intermediate links in the long lines of
descent, yet admit that many of our domestic races have descended from
the same parents--may they not learn a lesson of caution, when they
deride the idea of species in a state of nature being lineal
descendants of other species?

SELECTION.

Let us now briefly consider the steps by which domestic races have
been produced, either from one or from several allied species. Some
little effect may, perhaps, be attributed to the direct action of the
external conditions of life, and some little to habit; but he would be
a bold man who would account by such agencies for the differences of a
dray and race horse, a greyhound and bloodhound, a carrier and tumbler
pigeon. One of the most remarkable features in our domesticated races
is that we see in them adaptation, not indeed to the animal's or
plant's own good, but to man's use or fancy. Some variations useful to
him have probably arisen suddenly, or by one step; many botanists, for
instance, believe that the fuller's teazle, with its hooks, which
cannot be rivalled by any mechanical contrivance, is only a variety of
the wild Dipsacus; and this amount of change may have suddenly arisen
in a seedling. So it has probably been with the turnspit dog; and this
is known to have been the case with the ancon sheep. But when we
compare the dray-horse and race-horse, the dromedary and camel, the
various breeds of sheep fitted either for cultivated land or mountain
pasture, with the wool of one breed good for one purpose, and that of
another breed for another purpose; when we compare the many breeds of
dogs, each good for man in very different ways; when we compare the
game-cock, so pertinacious in battle, with other breeds so little
quarrelsome, with "everlasting layers" which never desire to sit, and
with the bantam so small and elegant; when we compare the host of
agricultural, culinary, orchard, and flower-garden races of plants,
most useful to man at different seasons and for different purposes, or
so beautiful in his eyes, we must, I think, look further than to mere
variability. We cannot suppose that all the breeds were suddenly
produced as perfect and as useful as we now see them; indeed, in
several cases, we know that this has not been their history. The key
is man's power of accumulative selection: nature gives successive
variations; man adds them up in certain directions useful to him. In
this sense he may be said to make for himself useful breeds.

The great power of this principle of selection is not hypothetical. It
is certain that several of our eminent breeders have, even within a
single lifetime, modified to a large extent some breeds of cattle and
sheep. In order fully to realise what they have done, it is almost
necessary to read several of the many treatises devoted to this
subject, and to inspect the animals. Breeders habitually speak of an
animal's organisation as something quite plastic, which they can model
almost as they please. If I had space I could quote numerous passages
to this effect from highly competent authorities. Youatt, who was
probably better acquainted with the works of agriculturalists than
almost any other individual, and who was himself a very good judge of
an animal, speaks of the principle of selection as "that which enables
the agriculturist, not only to modify the character of his flock, but
to change it altogether. It is the magician's wand, by means of which
he may summon into life whatever form and mould he pleases." Lord
Somerville, speaking of what breeders have done for sheep, says:--"It
would seem as if they had chalked out upon a wall a form perfect in
itself, and then had given it existence." That most skilful breeder,
Sir John Sebright, used to say, with respect to pigeons, that "he
would produce any given feather in three years, but it would take him
six years to obtain head and beak." In Saxony the importance of the
principle of selection in regard to merino sheep is so fully
recognised, that men follow it as a trade: the sheep are placed on a
table and are studied, like a picture by a connoisseur; this is done
three times at intervals of months, and the sheep are each time marked
and classed, so that the very best may ultimately be selected for
breeding.

What English breeders have actually effected is proved by the enormous
prices given for animals with a good pedigree; and these have now been
exported to almost every quarter of the world. The improvement is by
no means generally due to crossing different breeds; all the best
breeders are strongly opposed to this practice, except sometimes
amongst closely allied sub-breeds. And when a cross has been made, the
closest selection is far more indispensable even than in ordinary
cases. If selection consisted merely in separating some very distinct
variety, and breeding from it, the principle would be so obvious as
hardly to be worth notice; but its importance consists in the great
effect produced by the accumulation in one direction, during
successive generations, of differences absolutely inappreciable by an
uneducated eye--differences which I for one have vainly attempted to
appreciate. Not one man in a thousand has accuracy of eye and judgment
sufficient to become an eminent breeder. If gifted with these
qualities, and he studies his subject for years, and devotes his
lifetime to it with indomitable perseverance, he will succeed, and may
make great improvements; if he wants any of these qualities, he will
assuredly fail. Few would readily believe in the natural capacity and
years of practice requisite to become even a skilful pigeon-fancier.

The same principles are followed by horticulturists; but the
variations are here often more abrupt. No one supposes that our
choicest productions have been produced by a single variation from the
aboriginal stock. We have proofs that this is not so in some cases, in
which exact records have been kept; thus, to give a very trifling
instance, the steadily-increasing size of the common gooseberry may be
quoted. We see an astonishing improvement in many florists' flowers,
when the flowers of the present day are compared with drawings made
only twenty or thirty years ago. When a race of plants is once pretty
well established, the seed-raisers do not pick out the best plants,
but merely go over their seed-beds, and pull up the "rogues," as they
call the plants that deviate from the proper standard. With animals
this kind of selection is, in fact, also followed; for hardly any one
is so careless as to allow his worst animals to breed.

In regard to plants, there is another means of observing the
accumulated effects of selection--namely, by comparing the diversity
of flowers in the different varieties of the same species in the
flower-garden; the diversity of leaves, pods, or tubers, or whatever
part is valued, in the kitchen-garden, in comparison with the flowers
of the same varieties; and the diversity of fruit of the same species
in the orchard, in comparison with the leaves and flowers of the same
set of varieties. See how different the leaves of the cabbage are, and
how extremely alike the flowers; how unlike the flowers of the
heartsease are, and how alike the leaves; how much the fruit of the
different kinds of gooseberries differ in size, colour, shape, and
hairiness, and yet the flowers present very slight differences. It is
not that the varieties which differ largely in some one point do not
differ at all in other points; this is hardly ever, perhaps never, the
case. The laws of correlation of growth, the importance of which
should never be overlooked, will ensure some differences; but, as a
general rule, I cannot doubt that the continued selection of slight
variations, either in the leaves, the flowers, or the fruit, will
produce races differing from each other chiefly in these characters.

It may be objected that the principle of selection has been reduced to
methodical practice for scarcely more than three-quarters of a
century; it has certainly been more attended to of late years, and
many treatises have been published on the subject; and the result, I
may add, has been, in a corresponding degree, rapid and important. But
it is very far from true that the principle is a modern discovery. I
could give several references to the full acknowledgment of the
importance of the principle in works of high antiquity. In rude and
barbarous periods of English history choice animals were often
imported, and laws were passed to prevent their exportation: the
destruction of horses under a certain size was ordered, and this may
be compared to the "roguing" of plants by nurserymen. The principle of
selection I find distinctly given in an ancient Chinese encyclopaedia.
Explicit rules are laid down by some of the Roman classical writers.
From passages in Genesis, it is clear that the colour of domestic
animals was at that early period attended to. Savages now sometimes
cross their dogs with wild canine animals, to improve the breed, and
they formerly did so, as is attested by passages in Pliny. The savages
in South Africa match their draught cattle by colour, as do some of
the Esquimaux their teams of dogs. Livingstone shows how much good
domestic breeds are valued by the negroes of the interior of Africa
who have not associated with Europeans. Some of these facts do not
show actual selection, but they show that the breeding of domestic
animals was carefully attended to in ancient times, and is now
attended to by the lowest savages. It would, indeed, have been a
strange fact, had attention not been paid to breeding, for the
inheritance of good and bad qualities is so obvious.

At the present time, eminent breeders try by methodical selection,
with a distinct object in view, to make a new strain or sub-breed,
superior to anything existing in the country. But, for our purpose, a
kind of Selection, which may be called Unconscious, and which results
from every one trying to possess and breed from the best individual
animals, is more important. Thus, a man who intends keeping pointers
naturally tries to get as good dogs as he can, and afterwards breeds
from his own best dogs, but he has no wish or expectation of
permanently altering the breed. Nevertheless I cannot doubt that this
process, continued during centuries, would improve and modify any
breed, in the same way as Bakewell, Collins, etc., by this very same
process, only carried on more methodically, did greatly modify, even
during their own lifetimes, the forms and qualities of their cattle.
Slow and insensible changes of this kind could never be recognised
unless actual measurements or careful drawings of the breeds in
question had been made long ago, which might serve for comparison. In
some cases, however, unchanged or but little changed individuals of
the same breed may be found in less civilised districts, where the
breed has been less improved. There is reason to believe that King
Charles's spaniel has been unconsciously modified to a large extent
since the time of that monarch. Some highly competent authorities are
convinced that the setter is directly derived from the spaniel, and
has probably been slowly altered from it. It is known that the English
pointer has been greatly changed within the last century, and in this
case the change has, it is believed, been chiefly effected by crosses
with the fox-hound; but what concerns us is, that the change has been
effected unconsciously and gradually, and yet so effectually, that,
though the old Spanish pointer certainly came from Spain, Mr. Borrow
has not seen, as I am informed by him, any native dog in Spain like
our pointer.

By a similar process of selection, and by careful training, the whole
body of English racehorses have come to surpass in fleetness and size
the parent Arab stock, so that the latter, by the regulations for the
Goodwood Races, are favoured in the weights they carry. Lord Spencer
and others have shown how the cattle of England have increased in
weight and in early maturity, compared with the stock formerly kept in
this country. By comparing the accounts given in old pigeon treatises
of carriers and tumblers with these breeds as now existing in Britain,
India, and Persia, we can, I think, clearly trace the stages through
which they have insensibly passed, and come to differ so greatly from
the rock-pigeon.

Youatt gives an excellent illustration of the effects of a course of
selection, which may be considered as unconsciously followed, in so
far that the breeders could never have expected or even have wished to
have produced the result which ensued--namely, the production of two
distinct strains. The two flocks of Leicester sheep kept by Mr.
Buckley and Mr. Burgess, as Mr. Youatt remarks, "have been purely bred
from the original stock of Mr. Bakewell for upwards of fifty years.
There is not a suspicion existing in the mind of any one at all
acquainted with the subject that the owner of either of them has
deviated in any one instance from the pure blood of Mr. Bakewell's
flock, and yet the difference between the sheep possessed by these two
gentlemen is so great that they have the appearance of being quite
different varieties."

If there exist savages so barbarous as never to think of the inherited
character of the offspring of their domestic animals, yet any one
animal particularly useful to them, for any special purpose, would be
carefully preserved during famines and other accidents, to which
savages are so liable, and such choice animals would thus generally
leave more offspring than the inferior ones; so that in this case
there would be a kind of unconscious selection going on. We see the
value set on animals even by the barbarians of Tierra del Fuego, by
their killing and devouring their old women, in times of dearth, as of
less value than their dogs.

In plants the same gradual process of improvement, through the
occasional preservation of the best individuals, whether or not
sufficiently distinct to be ranked at their first appearance as
distinct varieties, and whether or not two or more species or races
have become blended together by crossing, may plainly be recognised in
the increased size and beauty which we now see in the varieties of the
heartsease, rose, pelargonium, dahlia, and other plants, when compared
with the older varieties or with their parent-stocks. No one would
ever expect to get a first-rate heartsease or dahlia from the seed of
a wild plant. No one would expect to raise a first-rate melting pear
from the seed of a wild pear, though he might succeed from a poor
seedling growing wild, if it had come from a garden-stock. The pear,
though cultivated in classical times, appears, from Pliny's
description, to have been a fruit of very inferior quality. I have
seen great surprise expressed in horticultural works at the wonderful
skill of gardeners, in having produced such splendid results from such
poor materials; but the art, I cannot doubt, has been simple, and, as
far as the final result is concerned, has been followed almost
unconsciously. It has consisted in always cultivating the best known
variety, sowing its seeds, and, when a slightly better variety has
chanced to appear, selecting it, and so onwards. But the gardeners of
the classical period, who cultivated the best pear they could procure,
never thought what splendid fruit we should eat; though we owe our
excellent fruit, in some small degree, to their having naturally
chosen and preserved the best varieties they could anywhere find.

A large amount of change in our cultivated plants, thus slowly and
unconsciously accumulated, explains, as I believe, the well-known
fact, that in a vast number of cases we cannot recognise, and
therefore do not know, the wild parent-stocks of the plants which have
been longest cultivated in our flower and kitchen gardens. If it has
taken centuries or thousands of years to improve or modify most of our
plants up to their present standard of usefulness to man, we can
understand how it is that neither Australia, the Cape of Good Hope,
nor any other region inhabited by quite uncivilised man, has afforded
us a single plant worth culture. It is not that these countries, so
rich in species, do not by a strange chance possess the aboriginal
stocks of any useful plants, but that the native plants have not been
improved by continued selection up to a standard of perfection
comparable with that given to the plants in countries anciently
civilised.

In regard to the domestic animals kept by uncivilised man, it should
not be overlooked that they almost always have to struggle for their
own food, at least during certain seasons. And in two countries very
differently circumstanced, individuals of the same species, having
slightly different constitutions or structure, would often succeed
better in the one country than in the other, and thus by a process of
"natural selection," as will hereafter be more fully explained, two
sub-breeds might be formed. This, perhaps, partly explains what has
been remarked by some authors, namely, that the varieties kept by
savages have more of the character of species than the varieties kept
in civilised countries.

On the view here given of the all-important part which selection by
man has played, it becomes at once obvious, how it is that our
domestic races show adaptation in their structure or in their habits
to man's wants or fancies. We can, I think, further understand the
frequently abnormal character of our domestic races, and likewise
their differences being so great in external characters and relatively
so slight in internal parts or organs. Man can hardly select, or only
with much difficulty, any deviation of structure excepting such as is
externally visible; and indeed he rarely cares for what is internal.
He can never act by selection, excepting on variations which are first
given to him in some slight degree by nature. No man would ever try to
make a fantail, till he saw a pigeon with a tail developed in some
slight degree in an unusual manner, or a pouter till he saw a pigeon
with a crop of somewhat unusual size; and the more abnormal or unusual
any character was when it first appeared, the more likely it would be
to catch his attention. But to use such an expression as trying to
make a fantail, is, I have no doubt, in most cases, utterly incorrect.
The man who first selected a pigeon with a slightly larger tail, never
dreamed what the descendants of that pigeon would become through
long-continued, partly unconscious and partly methodical selection.
Perhaps the parent bird of all fantails had only fourteen
tail-feathers somewhat expanded, like the present Java fantail, or
like individuals of other and distinct breeds, in which as many as
seventeen tail-feathers have been counted. Perhaps the first
pouter-pigeon did not inflate its crop much more than the turbit now
does the upper part of its oesophagus,--a habit which is disregarded
by all fanciers, as it is not one of the points of the breed.

Nor let it be thought that some great deviation of structure would be
necessary to catch the fancier's eye: he perceives extremely small
differences, and it is in human nature to value any novelty, however
slight, in one's own possession. Nor must the value which would
formerly be set on any slight differences in the individuals of the
same species, be judged of by the value which would now be set on
them, after several breeds have once fairly been established. Many
slight differences might, and indeed do now, arise amongst pigeons,
which are rejected as faults or deviations from the standard of
perfection of each breed. The common goose has not given rise to any
marked varieties; hence the Thoulouse and the common breed, which
differ only in colour, that most fleeting of characters, have lately
been exhibited as distinct at our poultry-shows.

I think these views further explain what has sometimes been
noticed--namely that we know nothing about the origin or history of
any of our domestic breeds. But, in fact, a breed, like a dialect of a
language, can hardly be said to have had a definite origin. A man
preserves and breeds from an individual with some slight deviation of
structure, or takes more care than usual in matching his best animals
and thus improves them, and the improved individuals slowly spread in
the immediate neighbourhood. But as yet they will hardly have a
distinct name, and from being only slightly valued, their history will
be disregarded. When further improved by the same slow and gradual
process, they will spread more widely, and will get recognised as
something distinct and valuable, and will then probably first receive
a provincial name. In semi-civilised countries, with little free
communication, the spreading and knowledge of any new sub-breed will
be a slow process. As soon as the points of value of the new sub-breed
are once fully acknowledged, the principle, as I have called it, of
unconscious selection will always tend,--perhaps more at one period
than at another, as the breed rises or falls in fashion,--perhaps more
in one district than in another, according to the state of
civilisation of the inhabitants--slowly to add to the characteristic
features of the breed, whatever they may be. But the chance will be
infinitely small of any record having been preserved of such slow,
varying, and insensible changes.

I must now say a few words on the circumstances, favourable, or the
reverse, to man's power of selection. A high degree of variability is
obviously favourable, as freely giving the materials for selection to
work on; not that mere individual differences are not amply
sufficient, with extreme care, to allow of the accumulation of a large
amount of modification in almost any desired direction. But as
variations manifestly useful or pleasing to man appear only
occasionally, the chance of their appearance will be much increased by
a large number of individuals being kept; and hence this comes to be
of the highest importance to success. On this principle Marshall has
remarked, with respect to the sheep of parts of Yorkshire, that "as
they generally belong to poor people, and are mostly IN SMALL LOTS,
they never can be improved." On the other hand, nurserymen, from
raising large stocks of the same plants, are generally far more
successful than amateurs in getting new and valuable varieties. The
keeping of a large number of individuals of a species in any country
requires that the species should be placed under favourable conditions
of life, so as to breed freely in that country. When the individuals
of any species are scanty, all the individuals, whatever their quality
may be, will generally be allowed to breed, and this will effectually
prevent selection. But probably the most important point of all, is,
that the animal or plant should be so highly useful to man, or so much
valued by him, that the closest attention should be paid to even the
slightest deviation in the qualities or structure of each individual.
Unless such attention be paid nothing can be effected. I have seen it
gravely remarked, that it was most fortunate that the strawberry began
to vary just when gardeners began to attend closely to this plant. No
doubt the strawberry had always varied since it was cultivated, but
the slight varieties had been neglected. As soon, however, as
gardeners picked out individual plants with slightly larger, earlier,
or better fruit, and raised seedlings from them, and again picked out
the best seedlings and bred from them, then, there appeared (aided by
some crossing with distinct species) those many admirable varieties of
the strawberry which have been raised during the last thirty or forty
years.

In the case of animals with separate sexes, facility in preventing
crosses is an important element of success in the formation of new
races,--at least, in a country which is already stocked with other
races. In this respect enclosure of the land plays a part. Wandering
savages or the inhabitants of open plains rarely possess more than one
breed of the same species. Pigeons can be mated for life, and this is
a great convenience to the fancier, for thus many races may be kept
true, though mingled in the same aviary; and this circumstance must
have largely favoured the improvement and formation of new breeds.
Pigeons, I may add, can be propagated in great numbers and at a very
quick rate, and inferior birds may be freely rejected, as when killed
they serve for food. On the other hand, cats, from their nocturnal
rambling habits, cannot be matched, and, although so much valued by
women and children, we hardly ever see a distinct breed kept up; such
breeds as we do sometimes see are almost always imported from some
other country, often from islands. Although I do not doubt that some
domestic animals vary less than others, yet the rarity or absence of
distinct breeds of the cat, the donkey, peacock, goose, etc., may be
attributed in main part to selection not having been brought into
play: in cats, from the difficulty in pairing them; in donkeys, from
only a few being kept by poor people, and little attention paid to
their breeding; in peacocks, from not being very easily reared and a
large stock not kept; in geese, from being valuable only for two
purposes, food and feathers, and more especially from no pleasure
having been felt in the display of distinct breeds.

To sum up on the origin of our Domestic Races of animals and plants. I
believe that the conditions of life, from their action on the
reproductive system, are so far of the highest importance as causing
variability. I do not believe that variability is an inherent and
necessary contingency, under all circumstances, with all organic
beings, as some authors have thought. The effects of variability are
modified by various degrees of inheritance and of reversion.
Variability is governed by many unknown laws, more especially by that
of correlation of growth. Something may be attributed to the direct
action of the conditions of life. Something must be attributed to use
and disuse. The final result is thus rendered infinitely complex. In
some cases, I do not doubt that the intercrossing of species,
aboriginally distinct, has played an important part in the origin of
our domestic productions. When in any country several domestic breeds
have once been established, their occasional intercrossing, with the
aid of selection, has, no doubt, largely aided in the formation of new
sub-breeds; but the importance of the crossing of varieties has, I
believe, been greatly exaggerated, both in regard to animals and to
those plants which are propagated by seed. In plants which are
temporarily propagated by cuttings, buds, etc., the importance of the
crossing both of distinct species and of varieties is immense; for the
cultivator here quite disregards the extreme variability both of
hybrids and mongrels, and the frequent sterility of hybrids; but the
cases of plants not propagated by seed are of little importance to us,
for their endurance is only temporary. Over all these causes of Change
I am convinced that the accumulative action of Selection, whether
applied methodically and more quickly, or unconsciously and more
slowly, but more efficiently, is by far the predominant Power.


CHAPTER 2. VARIATION UNDER NATURE.

Variability.
Individual differences.
Doubtful species.
Wide ranging, much diffused, and common species vary most.
Species of the larger genera in any country vary more than the species
of the smaller genera.
Many of the species of the larger genera resemble varieties in being
very closely, but unequally, related to each other, and in having
restricted ranges.

Before applying the principles arrived at in the last chapter to
organic beings in a state of nature, we must briefly discuss whether
these latter are subject to any variation. To treat this subject at
all properly, a long catalogue of dry facts should be given; but these
I shall reserve for my future work. Nor shall I here discuss the
various definitions which have been given of the term species. No one
definition has as yet satisfied all naturalists; yet every naturalist
knows vaguely what he means when he speaks of a species. Generally the
term includes the unknown element of a distinct act of creation. The
term "variety" is almost equally difficult to define; but here
community of descent is almost universally implied, though it can
rarely be proved. We have also what are called monstrosities; but they
graduate into varieties. By a monstrosity I presume is meant some
considerable deviation of structure in one part, either injurious to
or not useful to the species, and not generally propagated. Some
authors use the term "variation" in a technical sense, as implying a
modification directly due to the physical conditions of life; and
"variations" in this sense are supposed not to be inherited: but who
can say that the dwarfed condition of shells in the brackish waters of
the Baltic, or dwarfed plants on Alpine summits, or the thicker fur of
an animal from far northwards, would not in some cases be inherited
for at least some few generations? and in this case I presume that the
form would be called a variety.

Again, we have many slight differences which may be called individual
differences, such as are known frequently to appear in the offspring
from the same parents, or which may be presumed to have thus arisen,
from being frequently observed in the individuals of the same species
inhabiting the same confined locality. No one supposes that all the
individuals of the same species are cast in the very same mould. These
individual differences are highly important for us, as they afford
materials for natural selection to accumulate, in the same manner as
man can accumulate in any given direction individual differences in
his domesticated productions. These individual differences generally
affect what naturalists consider unimportant parts; but I could show
by a long catalogue of facts, that parts which must be called
important, whether viewed under a physiological or classificatory
point of view, sometimes vary in the individuals of the same species.
I am convinced that the most experienced naturalist would be surprised
at the number of the cases of variability, even in important parts of
structure, which he could collect on good authority, as I have
collected, during a course of years. It should be remembered that
systematists are far from pleased at finding variability in important
characters, and that there are not many men who will laboriously
examine internal and important organs, and compare them in many
specimens of the same species. I should never have expected that the
branching of the main nerves close to the great central ganglion of an
insect would have been variable in the same species; I should have
expected that changes of this nature could have been effected only by
slow degrees: yet quite recently Mr. Lubbock has shown a degree of
variability in these main nerves in Coccus, which may almost be
compared to the irregular branching of the stem of a tree. This
philosophical naturalist, I may add, has also quite recently shown
that the muscles in the larvae of certain insects are very far from
uniform. Authors sometimes argue in a circle when they state that
important organs never vary; for these same authors practically rank
that character as important (as some few naturalists have honestly
confessed) which does not vary; and, under this point of view, no
instance of an important part varying will ever be found: but under
any other point of view many instances assuredly can be given.

There is one point connected with individual differences, which seems
to me extremely perplexing: I refer to those genera which have
sometimes been called "protean" or "polymorphic," in which the species
present an inordinate amount of variation; and hardly two naturalists
can agree which forms to rank as species and which as varieties. We
may instance Rubus, Rosa, and Hieracium amongst plants, several genera
of insects, and several genera of Brachiopod shells. In most
polymorphic genera some of the species have fixed and definite
characters. Genera which are polymorphic in one country seem to be,
with some few exceptions, polymorphic in other countries, and
likewise, judging from Brachiopod shells, at former periods of time.
These facts seem to be very perplexing, for they seem to show that
this kind of variability is independent of the conditions of life. I
am inclined to suspect that we see in these polymorphic genera
variations in points of structure which are of no service or
disservice to the species, and which consequently have not been seized
on and rendered definite by natural selection, as hereafter will be
explained.

Those forms which possess in some considerable degree the character of
species, but which are so closely similar to some other forms, or are
so closely linked to them by intermediate gradations, that naturalists
do not like to rank them as distinct species, are in several respects
the most important for us. We have every reason to believe that many
of these doubtful and closely-allied forms have permanently retained
their characters in their own country for a long time; for as long, as
far as we know, as have good and true species. Practically, when a
naturalist can unite two forms together by others having intermediate
characters, he treats the one as a variety of the other, ranking the
most common, but sometimes the one first described, as the species,
and the other as the variety. But cases of great difficulty, which I
will not here enumerate, sometimes occur in deciding whether or not to
rank one form as a variety of another, even when they are closely
connected by intermediate links; nor will the commonly-assumed hybrid
nature of the intermediate links always remove the difficulty. In very
many cases, however, one form is ranked as a variety of another, not
because the intermediate links have actually been found, but because
analogy leads the observer to suppose either that they do now
somewhere exist, or may formerly have existed; and here a wide door
for the entry of doubt and conjecture is opened.

Hence, in determining whether a form should be ranked as a species or
a variety, the opinion of naturalists having sound judgment and wide
experience seems the only guide to follow. We must, however, in many
cases, decide by a majority of naturalists, for few well-marked and
well-known varieties can be named which have not been ranked as
species by at least some competent judges.

That varieties of this doubtful nature are far from uncommon cannot be
disputed. Compare the several floras of Great Britain, of France or of
the United States, drawn up by different botanists, and see what a
surprising number of forms have been ranked by one botanist as good
species, and by another as mere varieties. Mr. H. C. Watson, to whom I
lie under deep obligation for assistance of all kinds, has marked for
me 182 British plants, which are generally considered as varieties,
but which have all been ranked by botanists as species; and in making
this list he has omitted many trifling varieties, but which
nevertheless have been ranked by some botanists as species, and he has
entirely omitted several highly polymorphic genera. Under genera,
including the most polymorphic forms, Mr. Babington gives 251 species,
whereas Mr. Bentham gives only 112,--a difference of 139 doubtful
forms! Amongst animals which unite for each birth, and which are
highly locomotive, doubtful forms, ranked by one zoologist as a
species and by another as a variety, can rarely be found within the
same country, but are common in separated areas. How many of those
birds and insects in North America and Europe, which differ very
slightly from each other, have been ranked by one eminent naturalist
as undoubted species, and by another as varieties, or, as they are
often called, as geographical races! Many years ago, when comparing,
and seeing others compare, the birds from the separate islands of the
Galapagos Archipelago, both one with another, and with those from the
American mainland, I was much struck how entirely vague and arbitrary
is the distinction between species and varieties. On the islets of the
little Madeira group there are many insects which are characterized as
varieties in Mr. Wollaston's admirable work, but which it cannot be
doubted would be ranked as distinct species by many entomologists.
Even Ireland has a few animals, now generally regarded as varieties,
but which have been ranked as species by some zoologists. Several most
experienced ornithologists consider our British red grouse as only a
strongly-marked race of a Norwegian species, whereas the greater
number rank it as an undoubted species peculiar to Great Britain. A
wide distance between the homes of two doubtful forms leads many
naturalists to rank both as distinct species; but what distance, it
has been well asked, will suffice? if that between America and Europe
is ample, will that between the Continent and the Azores, or Madeira,
or the Canaries, or Ireland, be sufficient? It must be admitted that
many forms, considered by highly-competent judges as varieties, have
so perfectly the character of species that they are ranked by other
highly-competent judges as good and true species. But to discuss
whether they are rightly called species or varieties, before any
definition of these terms has been generally accepted, is vainly to
beat the air.

Many of the cases of strongly-marked varieties or doubtful species
well deserve consideration; for several interesting lines of argument,
from geographical distribution, analogical variation, hybridism, etc.,
have been brought to bear on the attempt to determine their rank. I
will here give only a single instance,--the well-known one of the
primrose and cowslip, or Primula veris and elatior. These plants
differ considerably in appearance; they have a different flavour and
emit a different odour; they flower at slightly different periods;
they grow in somewhat different stations; they ascend mountains to
different heights; they have different geographical ranges; and
lastly, according to very numerous experiments made during several
years by that most careful observer Gartner, they can be crossed only
with much difficulty. We could hardly wish for better evidence of the
two forms being specifically distinct. On the other hand, they are
united by many intermediate links, and it is very doubtful whether
these links are hybrids; and there is, as it seems to me, an
overwhelming amount of experimental evidence, showing that they
descend from common parents, and consequently must be ranked as
varieties.

Close investigation, in most cases, will bring naturalists to an
agreement how to rank doubtful forms. Yet it must be confessed, that
it is in the best-known countries that we find the greatest number of
forms of doubtful value. I have been struck with the fact, that if any
animal or plant in a state of nature be highly useful to man, or from
any cause closely attract his attention, varieties of it will almost
universally be found recorded. These varieties, moreover, will be
often ranked by some authors as species. Look at the common oak, how
closely it has been studied; yet a German author makes more than a
dozen species out of forms, which are very generally considered as
varieties; and in this country the highest botanical authorities and
practical men can be quoted to show that the sessile and pedunculated
oaks are either good and distinct species or mere varieties.

When a young naturalist commences the study of a group of organisms
quite unknown to him, he is at first much perplexed to determine what
differences to consider as specific, and what as varieties; for he
knows nothing of the amount and kind of variation to which the group
is subject; and this shows, at least, how very generally there is some
variation. But if he confine his attention to one class within one
country, he will soon make up his mind how to rank most of the
doubtful forms. His general tendency will be to make many species, for
he will become impressed, just like the pigeon or poultry-fancier
before alluded to, with the amount of difference in the forms which he
is continually studying; and he has little general knowledge of
analogical variation in other groups and in other countries, by which
to correct his first impressions. As he extends the range of his
observations, he will meet with more cases of difficulty; for he will
encounter a greater number of closely-allied forms. But if his
observations be widely extended, he will in the end generally be
enabled to make up his own mind which to call varieties and which
species; but he will succeed in this at the expense of admitting much
variation,--and the truth of this admission will often be disputed by
other naturalists. When, moreover, he comes to study allied forms
brought from countries not now continuous, in which case he can hardly
hope to find the intermediate links between his doubtful forms, he
will have to trust almost entirely to analogy, and his difficulties
will rise to a climax.

Certainly no clear line of demarcation has as yet been drawn between
species and sub-species--that is, the forms which in the opinion of
some naturalists come very near to, but do not quite arrive at the
rank of species; or, again, between sub-species and well-marked
varieties, or between lesser varieties and individual differences.
These differences blend into each other in an insensible series; and a
series impresses the mind with the idea of an actual passage.

Hence I look at individual differences, though of small interest to
the systematist, as of high importance for us, as being the first step
towards such slight varieties as are barely thought worth recording in
works on natural history. And I look at varieties which are in any
degree more distinct and permanent, as steps leading to more strongly
marked and more permanent varieties; and at these latter, as leading
to sub-species, and to species. The passage from one stage of
difference to another and higher stage may be, in some cases, due
merely to the long-continued action of different physical conditions
in two different regions; but I have not much faith in this view; and
I attribute the passage of a variety, from a state in which it differs
very slightly from its parent to one in which it differs more, to the
action of natural selection in accumulating (as will hereafter be more
fully explained) differences of structure in certain definite
directions. Hence I believe a well-marked variety may be justly called
an incipient species; but whether this belief be justifiable must be
judged of by the general weight of the several facts and views given
throughout this work.

It need not be supposed that all varieties or incipient species
necessarily attain the rank of species. They may whilst in this
incipient state become extinct, or they may endure as varieties for
very long periods, as has been shown to be the case by Mr. Wollaston
with the varieties of certain fossil land-shells in Madeira. If a
variety were to flourish so as to exceed in numbers the parent
species, it would then rank as the species, and the species as the
variety; or it might come to supplant and exterminate the parent
species; or both might co-exist, and both rank as independent species.
But we shall hereafter have to return to this subject.

From these remarks it will be seen that I look at the term species, as
one arbitrarily given for the sake of convenience to a set of
individuals closely resembling each other, and that it does not
essentially differ from the term variety, which is given to less
distinct and more fluctuating forms. The term variety, again, in
comparison with mere individual differences, is also applied
arbitrarily, and for mere convenience sake.

Guided by theoretical considerations, I thought that some interesting
results might be obtained in regard to the nature and relations of the
species which vary most, by tabulating all the varieties in several
well-worked floras. At first this seemed a simple task; but Mr. H. C.
Watson, to whom I am much indebted for valuable advice and assistance
on this subject, soon convinced me that there were many difficulties,
as did subsequently Dr. Hooker, even in stronger terms. I shall
reserve for my future work the discussion of these difficulties, and
the tables themselves of the proportional numbers of the varying
species. Dr. Hooker permits me to add, that after having carefully
read my manuscript, and examined the tables, he thinks that the
following statements are fairly well established. The whole subject,
however, treated as it necessarily here is with much brevity, is
rather perplexing, and allusions cannot be avoided to the "struggle
for existence," "divergence of character," and other questions,
hereafter to be discussed.

Alph. De Candolle and others have shown that plants which have very
wide ranges generally present varieties; and this might have been
expected, as they become exposed to diverse physical conditions, and
as they come into competition (which, as we shall hereafter see, is a
far more important circumstance) with different sets of organic
beings. But my tables further show that, in any limited country, the
species which are most common, that is abound most in individuals, and
the species which are most widely diffused within their own country
(and this is a different consideration from wide range, and to a
certain extent from commonness), often give rise to varieties
sufficiently well-marked to have been recorded in botanical works.
Hence it is the most flourishing, or, as they may be called, the
dominant species,--those which range widely over the world, are the
most diffused in their own country, and are the most numerous in
individuals,--which oftenest produce well-marked varieties, or, as I
consider them, incipient species. And this, perhaps, might have been
anticipated; for, as varieties, in order to become in any degree
permanent, necessarily have to struggle with the other inhabitants of
the country, the species which are already dominant will be the most
likely to yield offspring which, though in some slight degree
modified, will still inherit those advantages that enabled their
parents to become dominant over their compatriots.

If the plants inhabiting a country and described in any Flora be
divided into two equal masses, all those in the larger genera being
placed on one side, and all those in the smaller genera on the other
side, a somewhat larger number of the very common and much diffused or
dominant species will be found on the side of the larger genera. This,
again, might have been anticipated; for the mere fact of many species
of the same genus inhabiting any country, shows that there is
something in the organic or inorganic conditions of that country
favourable to the genus; and, consequently, we might have expected to
have found in the larger genera, or those including many species, a
large proportional number of dominant species. But so many causes tend
to obscure this result, that I am surprised that my tables show even a
small majority on the side of the larger genera. I will here allude to
only two causes of obscurity. Fresh-water and salt-loving plants have
generally very wide ranges and are much diffused, but this seems to be
connected with the nature of the stations inhabited by them, and has
little or no relation to the size of the genera to which the species
belong. Again, plants low in the scale of organisation are generally
much more widely diffused than plants higher in the scale; and here
again there is no close relation to the size of the genera. The cause
of lowly-organised plants ranging widely will be discussed in our
chapter on geographical distribution.

From looking at species as only strongly-marked and well-defined
varieties, I was led to anticipate that the species of the larger
genera in each country would oftener present varieties, than the
species of the smaller genera; for wherever many closely related
species (i.e. species of the same genus) have been formed, many
varieties or incipient species ought, as a general rule, to be now
forming. Where many large trees grow, we expect to find saplings.
Where many species of a genus have been formed through variation,
circumstances have been favourable for variation; and hence we might
expect that the circumstances would generally be still favourable to
variation. On the other hand, if we look at each species as a special
act of creation, there is no apparent reason why more varieties should
occur in a group having many species, than in one having few.

To test the truth of this anticipation I have arranged the plants of
twelve countries, and the coleopterous insects of two districts, into
two nearly equal masses, the species of the larger genera on one side,
and those of the smaller genera on the other side, and it has
invariably proved to be the case that a larger proportion of the
species on the side of the larger genera present varieties, than on
the side of the smaller genera. Moreover, the species of the large
genera which present any varieties, invariably present a larger
average number of varieties than do the species of the small genera.
Both these results follow when another division is made, and when all
the smallest genera, with from only one to four species, are
absolutely excluded from the tables. These facts are of plain
signification on the view that species are only strongly marked and
permanent varieties; for wherever many species of the same genus have
been formed, or where, if we may use the expression, the manufactory
of species has been active, we ought generally to find the manufactory
still in action, more especially as we have every reason to believe
the process of manufacturing new species to be a slow one. And this
certainly is the case, if varieties be looked at as incipient species;
for my tables clearly show as a general rule that, wherever many
species of a genus have been formed, the species of that genus present
a number of varieties, that is of incipient species, beyond the
average. It is not that all large genera are now varying much, and are
thus increasing in the number of their species, or that no small
genera are now varying and increasing; for if this had been so, it
would have been fatal to my theory; inasmuch as geology plainly tells
us that small genera have in the lapse of time often increased greatly
in size; and that large genera have often come to their maxima,
declined, and disappeared. All that we want to show is, that where
many species of a genus have been formed, on an average many are still
forming; and this holds good.

There are other relations between the species of large genera and
their recorded varieties which deserve notice. We have seen that there
is no infallible criterion by which to distinguish species and
well-marked varieties; and in those cases in which intermediate links
have not been found between doubtful forms, naturalists are compelled
to come to a determination by the amount of difference between them,
judging by analogy whether or not the amount suffices to raise one or
both to the rank of species. Hence the amount of difference is one
very important criterion in settling whether two forms should be
ranked as species or varieties. Now Fries has remarked in regard to
plants, and Westwood in regard to insects, that in large genera the
amount of difference between the species is often exceedingly small. I
have endeavoured to test this numerically by averages, and, as far as
my imperfect results go, they always confirm the view. I have also
consulted some sagacious and most experienced observers, and, after
deliberation, they concur in this view. In this respect, therefore,
the species of the larger genera resemble varieties, more than do the
species of the smaller genera. Or the case may be put in another way,
and it may be said, that in the larger genera, in which a number of
varieties or incipient species greater than the average are now
manufacturing, many of the species already manufactured still to a
certain extent resemble varieties, for they differ from each other by
a less than usual amount of difference.

Moreover, the species of the large genera are related to each other,
in the same manner as the varieties of any one species are related to
each other. No naturalist pretends that all the species of a genus are
equally distinct from each other; they may generally be divided into
sub-genera, or sections, or lesser groups. As Fries has well remarked,
little groups of species are generally clustered like satellites
around certain other species. And what are varieties but groups of
forms, unequally related to each other, and clustered round certain
forms--that is, round their parent-species? Undoubtedly there is one
most important point of difference between varieties and species;
namely, that the amount of difference between varieties, when compared
with each other or with their parent-species, is much less than that
between the species of the same genus. But when we come to discuss the
principle, as I call it, of Divergence of Character, we shall see how
this may be explained, and how the lesser differences between
varieties will tend to increase into the greater differences between
species.

There is one other point which seems to me worth notice. Varieties
generally have much restricted ranges: this statement is indeed
scarcely more than a truism, for if a variety were found to have a
wider range than that of its supposed parent-species, their
denominations ought to be reversed. But there is also reason to
believe, that those species which are very closely allied to other
species, and in so far resemble varieties, often have much restricted
ranges. For instance, Mr. H. C. Watson has marked for me in the
well-sifted London Catalogue of plants (4th edition) 63 plants which
are therein ranked as species, but which he considers as so closely
allied to other species as to be of doubtful value: these 63 reputed
species range on an average over 6.9 of the provinces into which Mr.
Watson has divided Great Britain. Now, in this same catalogue, 53
acknowledged varieties are recorded, and these range over 7.7
provinces; whereas, the species to which these varieties belong range
over 14.3 provinces. So that the acknowledged varieties have very
nearly the same restricted average range, as have those very closely
allied forms, marked for me by Mr. Watson as doubtful species, but
which are almost universally ranked by British botanists as good and
true species.

Finally, then, varieties have the same general characters as species,
for they cannot be distinguished from species,--except, firstly, by
the discovery of intermediate linking forms, and the occurrence of
such links cannot affect the actual characters of the forms which they
connect; and except, secondly, by a certain amount of difference, for
two forms, if differing very little, are generally ranked as
varieties, notwithstanding that intermediate linking forms have not
been discovered; but the amount of difference considered necessary to
give to two forms the rank of species is quite indefinite. In genera
having more than the average number of species in any country, the
species of these genera have more than the average number of
varieties. In large genera the species are apt to be closely, but
unequally, allied together, forming little clusters round certain
species. Species very closely allied to other species apparently have
restricted ranges. In all these several respects the species of large
genera present a strong analogy with varieties. And we can clearly
understand these analogies, if species have once existed as varieties,
and have thus originated: whereas, these analogies are utterly
inexplicable if each species has been independently created.

We have, also, seen that it is the most flourishing and dominant
species of the larger genera which on an average vary most; and
varieties, as we shall hereafter see, tend to become converted into
new and distinct species. The larger genera thus tend to become
larger; and throughout nature the forms of life which are now dominant
tend to become still more dominant by leaving many modified and
dominant descendants. But by steps hereafter to be explained, the
larger genera also tend to break up into smaller genera. And thus, the
forms of life throughout the universe become divided into groups
subordinate to groups.


CHAPTER 3. STRUGGLE FOR EXISTENCE.

Bears on natural selection.
The term used in a wide sense.
Geometrical powers of increase.
Rapid increase of naturalised animals and plants.
Nature of the checks to increase.
Competition universal.
Effects of climate.
Protection from the number of individuals.
Complex relations of all animals and plants throughout nature.
Struggle for life most severe between individuals and varieties of the
same species; often severe between species of the same genus.
The relation of organism to organism the most important of all
relations.

Before entering on the subject of this chapter, I must make a few
preliminary remarks, to show how the struggle for existence bears on
Natural Selection. It has been seen in the last chapter that amongst
organic beings in a state of nature there is some individual
variability; indeed I am not aware that this has ever been disputed.
It is immaterial for us whether a multitude of doubtful forms be
called species or sub-species or varieties; what rank, for instance,
the two or three hundred doubtful forms of British plants are entitled
to hold, if the existence of any well-marked varieties be admitted.
But the mere existence of individual variability and of some few
well-marked varieties, though necessary as the foundation for the
work, helps us but little in understanding how species arise in
nature. How have all those exquisite adaptations of one part of the
organisation to another part, and to the conditions of life, and of
one distinct organic being to another being, been perfected? We see
these beautiful co-adaptations most plainly in the woodpecker and
missletoe; and only a little less plainly in the humblest parasite
which clings to the hairs of a quadruped or feathers of a bird; in the
structure of the beetle which dives through the water; in the plumed
seed which is wafted by the gentlest breeze; in short, we see
beautiful adaptations everywhere and in every part of the organic
world.

Again, it may be asked, how is it that varieties, which I have called
incipient species, become ultimately converted into good and distinct
species, which in most cases obviously differ from each other far more
than do the varieties of the same species? How do those groups of
species, which constitute what are called distinct genera, and which
differ from each other more than do the species of the same genus,
arise? All these results, as we shall more fully see in the next
chapter, follow inevitably from the struggle for life. Owing to this
struggle for life, any variation, however slight and from whatever
cause proceeding, if it be in any degree profitable to an individual
of any species, in its infinitely complex relations to other organic
beings and to external nature, will tend to the preservation of that
individual, and will generally be inherited by its offspring. The
offspring, also, will thus have a better chance of surviving, for, of
the many individuals of any species which are periodically born, but a
small number can survive. I have called this principle, by which each
slight variation, if useful, is preserved, by the term of Natural
Selection, in order to mark its relation to man's power of selection.
We have seen that man by selection can certainly produce great
results, and can adapt organic beings to his own uses, through the
accumulation of slight but useful variations, given to him by the hand
of Nature. But Natural Selection, as we shall hereafter see, is a
power incessantly ready for action, and is as immeasurably superior to
man's feeble efforts, as the works of Nature are to those of Art.

We will now discuss in a little more detail the struggle for
existence. In my future work this subject shall be treated, as it well
deserves, at much greater length. The elder De Candolle and Lyell have
largely and philosophically shown that all organic beings are exposed
to severe competition. In regard to plants, no one has treated this
subject with more spirit and ability than W. Herbert, Dean of
Manchester, evidently the result of his great horticultural knowledge.
Nothing is easier than to admit in words the truth of the universal
struggle for life, or more difficult--at least I have found it
so--than constantly to bear this conclusion in mind. Yet unless it be
thoroughly engrained in the mind, I am convinced that the whole
economy of nature, with every fact on distribution, rarity, abundance,
extinction, and variation, will be dimly seen or quite misunderstood.
We behold the face of nature bright with gladness, we often see
superabundance of food; we do not see, or we forget, that the birds
which are idly singing round us mostly live on insects or seeds, and
are thus constantly destroying life; or we forget how largely these
songsters, or their eggs, or their nestlings, are destroyed by birds
and beasts of prey; we do not always bear in mind, that though food
may be now superabundant, it is not so at all seasons of each
recurring year.

I should premise that I use the term Struggle for Existence in a large
and metaphorical sense, including dependence of one being on another,
and including (which is more important) not only the life of the
individual, but success in leaving progeny. Two canine animals in a
time of dearth, may be truly said to struggle with each other which
shall get food and live. But a plant on the edge of a desert is said
to struggle for life against the drought, though more properly it
should be said to be dependent on the moisture. A plant which annually
produces a thousand seeds, of which on an average only one comes to
maturity, may be more truly said to struggle with the plants of the
same and other kinds which already clothe the ground. The missletoe is
dependent on the apple and a few other trees, but can only in a
far-fetched sense be said to struggle with these trees, for if too
many of these parasites grow on the same tree, it will languish and
die. But several seedling missletoes, growing close together on the
same branch, may more truly be said to struggle with each other. As
the missletoe is disseminated by birds, its existence depends on
birds; and it may metaphorically be said to struggle with other
fruit-bearing plants, in order to tempt birds to devour and thus
disseminate its seeds rather than those of other plants. In these
several senses, which pass into each other, I use for convenience sake
the general term of struggle for existence.

A struggle for existence inevitably follows from the high rate at
which all organic beings tend to increase. Every being, which during
its natural lifetime produces several eggs or seeds, must suffer
destruction during some period of its life, and during some season or
occasional year, otherwise, on the principle of geometrical increase,
its numbers would quickly become so inordinately great that no country
could support the product. Hence, as more individuals are produced
than can possibly survive, there must in every case be a struggle for
existence, either one individual with another of the same species, or
with the individuals of distinct species, or with the physical
conditions of life. It is the doctrine of Malthus applied with
manifold force to the whole animal and vegetable kingdoms; for in this
case there can be no artificial increase of food, and no prudential
restraint from marriage. Although some species may be now increasing,
more or less rapidly, in numbers, all cannot do so, for the world
would not hold them.

There is no exception to the rule that every organic being naturally
increases at so high a rate, that if not destroyed, the earth would
soon be covered by the progeny of a single pair. Even slow-breeding
man has doubled in twenty-five years, and at this rate, in a few
thousand years, there would literally not be standing room for his
progeny. Linnaeus has calculated that if an annual plant produced only
two seeds--and there is no plant so unproductive as this--and their
seedlings next year produced two, and so on, then in twenty years
there would be a million plants. The elephant is reckoned to be the
slowest breeder of all known animals, and I have taken some pains to
estimate its probable minimum rate of natural increase: it will be
under the mark to assume that it breeds when thirty years old, and
goes on breeding till ninety years old, bringing forth three pair of
young in this interval; if this be so, at the end of the fifth century
there would be alive fifteen million elephants, descended from the
first pair.

But we have better evidence on this subject than mere theoretical
calculations, namely, the numerous recorded cases of the astonishingly
rapid increase of various animals in a state of nature, when
circumstances have been favourable to them during two or three
following seasons. Still more striking is the evidence from our
domestic animals of many kinds which have run wild in several parts of
the world: if the statements of the rate of increase of slow-breeding
cattle and horses in South America, and latterly in Australia, had not
been well authenticated, they would have been quite incredible. So it
is with plants: cases could be given of introduced plants which have
become common throughout whole islands in a period of less than ten
years. Several of the plants now most numerous over the wide plains of
La Plata, clothing square leagues of surface almost to the exclusion
of all other plants, have been introduced from Europe; and there are
plants which now range in India, as I hear from Dr. Falconer, from
Cape Comorin to the Himalaya, which have been imported from America
since its discovery. In such cases, and endless instances could be
given, no one supposes that the fertility of these animals or plants
has been suddenly and temporarily increased in any sensible degree.
The obvious explanation is that the conditions of life have been very
favourable, and that there has consequently been less destruction of
the old and young, and that nearly all the young have been enabled to
breed. In such cases the geometrical ratio of increase, the result of
which never fails to be surprising, simply explains the
extraordinarily rapid increase and wide diffusion of naturalised
productions in their new homes.

In a state of nature almost every plant produces seed, and amongst
animals there are very few which do not annually pair. Hence we may
confidently assert, that all plants and animals are tending to
increase at a geometrical ratio, that all would most rapidly stock
every station in which they could any how exist, and that the
geometrical tendency to increase must be checked by destruction at
some period of life. Our familiarity with the larger domestic animals
tends, I think, to mislead us: we see no great destruction falling on
them, and we forget that thousands are annually slaughtered for food,
and that in a state of nature an equal number would have somehow to be
disposed of.

The only difference between organisms which annually produce eggs or
seeds by the thousand, and those which produce extremely few, is, that
the slow-breeders would require a few more years to people, under
favourable conditions, a whole district, let it be ever so large. The
condor lays a couple of eggs and the ostrich a score, and yet in the
same country the condor may be the more numerous of the two: the
Fulmar petrel lays but one egg, yet it is believed to be the most
numerous bird in the world. One fly deposits hundreds of eggs, and
another, like the hippobosca, a single one; but this difference does
not determine how many individuals of the two species can be supported
in a district. A large number of eggs is of some importance to those
species, which depend on a rapidly fluctuating amount of food, for it
allows them rapidly to increase in number. But the real importance of
a large number of eggs or seeds is to make up for much destruction at
some period of life; and this period in the great majority of cases is
an early one. If an animal can in any way protect its own eggs or
young, a small number may be produced, and yet the average stock be
fully kept up; but if many eggs or young are destroyed, many must be
produced, or the species will become extinct. It would suffice to keep
up the full number of a tree, which lived on an average for a thousand
years, if a single seed were produced once in a thousand years,
supposing that this seed were never destroyed, and could be ensured to
germinate in a fitting place. So that in all cases, the average number
of any animal or plant depends only indirectly on the number of its
eggs or seeds.

In looking at Nature, it is most necessary to keep the foregoing
considerations always in mind--never to forget that every single
organic being around us may be said to be striving to the utmost to
increase in numbers; that each lives by a struggle at some period of
its life; that heavy destruction inevitably falls either on the young
or old, during each generation or at recurrent intervals. Lighten any
check, mitigate the destruction ever so little, and the number of the
species will almost instantaneously increase to any amount. The face
of Nature may be compared to a yielding surface, with ten thousand
sharp wedges packed close together and driven inwards by incessant
blows, sometimes one wedge being struck, and then another with greater
force.

What checks the natural tendency of each species to increase in number
is most obscure. Look at the most vigorous species; by as much as it
swarms in numbers, by so much will its tendency to increase be still
further increased. We know not exactly what the checks are in even one
single instance. Nor will this surprise any one who reflects how
ignorant we are on this head, even in regard to mankind, so
incomparably better known than any other animal. This subject has been
ably treated by several authors, and I shall, in my future work,
discuss some of the checks at considerable length, more especially in
regard to the feral animals of South America. Here I will make only a
few remarks, just to recall to the reader's mind some of the chief
points. Eggs or very young animals seem generally to suffer most, but
this is not invariably the case. With plants there is a vast
destruction of seeds, but, from some observations which I have made, I
believe that it is the seedlings which suffer most from germinating in
ground already thickly stocked with other plants. Seedlings, also, are
destroyed in vast numbers by various enemies; for instance, on a piece
of ground three feet long and two wide, dug and cleared, and where
there could be no choking from other plants, I marked all the
seedlings of our native weeds as they came up, and out of the 357 no
less than 295 were destroyed, chiefly by slugs and insects. If turf
which has long been mown, and the case would be the same with turf
closely browsed by quadrupeds, be let to grow, the more vigorous
plants gradually kill the less vigorous, though fully grown, plants:
thus out of twenty species growing on a little plot of turf (three
feet by four) nine species perished from the other species being
allowed to grow up freely.

The amount of food for each species of course gives the extreme limit
to which each can increase; but very frequently it is not the
obtaining food, but the serving as prey to other animals, which
determines the average numbers of a species. Thus, there seems to be
little doubt that the stock of partridges, grouse, and hares on any
large estate depends chiefly on the destruction of vermin. If not one
head of game were shot during the next twenty years in England, and,
at the same time, if no vermin were destroyed, there would, in all
probability, be less game than at present, although hundreds of
thousands of game animals are now annually killed. On the other hand,
in some cases, as with the elephant and rhinoceros, none are destroyed
by beasts of prey: even the tiger in India most rarely dares to attack
a young elephant protected by its dam.

Climate plays an important part in determining the average numbers of
a species, and periodical seasons of extreme cold or drought, I
believe to be the most effective of all checks. I estimated that the
winter of 1854-55 destroyed four-fifths of the birds in my own
grounds; and this is a tremendous destruction, when we remember that
ten per cent. is an extraordinarily severe mortality from epidemics
with man. The action of climate seems at first sight to be quite
independent of the struggle for existence; but in so far as climate
chiefly acts in reducing food, it brings on the most severe struggle
between the individuals, whether of the same or of distinct species,
which subsist on the same kind of food. Even when climate, for
instance extreme cold, acts directly, it will be the least vigorous,
or those which have got least food through the advancing winter, which
will suffer most. When we travel from south to north, or from a damp
region to a dry, we invariably see some species gradually getting
rarer and rarer, and finally disappearing; and the change of climate
being conspicuous, we are tempted to attribute the whole effect to its
direct action. But this is a very false view: we forget that each
species, even where it most abounds, is constantly suffering enormous
destruction at some period of its life, from enemies or from
competitors for the same place and food; and if these enemies or
competitors be in the least degree favoured by any slight change of
climate, they will increase in numbers, and, as each area is already
fully stocked with inhabitants, the other species will decrease. When
we travel southward and see a species decreasing in numbers, we may
feel sure that the cause lies quite as much in other species being
favoured, as in this one being hurt. So it is when we travel
northward, but in a somewhat lesser degree, for the number of species
of all kinds, and therefore of competitors, decreases northwards;
hence in going northward, or in ascending a mountain, we far oftener
meet with stunted forms, due to the DIRECTLY injurious action of
climate, than we do in proceeding southwards or in descending a
mountain. When we reach the Arctic regions, or snow-capped summits, or
absolute deserts, the struggle for life is almost exclusively with the
elements.

That climate acts in main part indirectly by favouring other species,
we may clearly see in the prodigious number of plants in our gardens
which can perfectly well endure our climate, but which never become
naturalised, for they cannot compete with our native plants, nor
resist destruction by our native animals.

When a species, owing to highly favourable circumstances, increases
inordinately in numbers in a small tract, epidemics--at least, this
seems generally to occur with our game animals--often ensue: and here
we have a limiting check independent of the struggle for life. But
even some of these so-called epidemics appear to be due to parasitic
worms, which have from some cause, possibly in part through facility
of diffusion amongst the crowded animals, been disproportionably
favoured: and here comes in a sort of struggle between the parasite
and its prey.

On the other hand, in many cases, a large stock of individuals of the
same species, relatively to the numbers of its enemies, is absolutely
necessary for its preservation. Thus we can easily raise plenty of
corn and rape-seed, etc., in our fields, because the seeds are in
great excess compared with the number of birds which feed on them; nor
can the birds, though having a superabundance of food at this one
season, increase in number proportionally to the supply of seed, as
their numbers are checked during winter: but any one who has tried,
knows how troublesome it is to get seed from a few wheat or other such
plants in a garden; I have in this case lost every single seed. This
view of the necessity of a large stock of the same species for its
preservation, explains, I believe, some singular facts in nature, such
as that of very rare plants being sometimes extremely abundant in the
few spots where they do occur; and that of some social plants being
social, that is, abounding in individuals, even on the extreme
confines of their range. For in such cases, we may believe, that a
plant could exist only where the conditions of its life were so
favourable that many could exist together, and thus save each other
from utter destruction. I should add that the good effects of frequent
intercrossing, and the ill effects of close interbreeding, probably
come into play in some of these cases; but on this intricate subject I
will not here enlarge.

Many cases are on record showing how complex and unexpected are the
checks and relations between organic beings, which have to struggle
together in the same country. I will give only a single instance,
which, though a simple one, has interested me. In Staffordshire, on
the estate of a relation where I had ample means of investigation,
there was a large and extremely barren heath, which had never been
touched by the hand of man; but several hundred acres of exactly the
same nature had been enclosed twenty-five years previously and planted
with Scotch fir. The change in the native vegetation of the planted
part of the heath was most remarkable, more than is generally seen in
passing from one quite different soil to another: not only the
proportional numbers of the heath-plants were wholly changed, but
twelve species of plants (not counting grasses and carices) flourished
in the plantations, which could not be found on the heath. The effect
on the insects must have been still greater, for six insectivorous
birds were very common in the plantations, which were not to be seen
on the heath; and the heath was frequented by two or three distinct
insectivorous birds. Here we see how potent has been the effect of the
introduction of a single tree, nothing whatever else having been done,
with the exception that the land had been enclosed, so that cattle
could not enter. But how important an element enclosure is, I plainly
saw near Farnham, in Surrey. Here there are extensive heaths, with a
few clumps of old Scotch firs on the distant hill-tops: within the
last ten years large spaces have been enclosed, and self-sown firs are
now springing up in multitudes, so close together that all cannot
live.

When I ascertained that these young trees had not been sown or
planted, I was so much surprised at their numbers that I went to
several points of view, whence I could examine hundreds of acres of
the unenclosed heath, and literally I could not see a single Scotch
fir, except the old planted clumps. But on looking closely between the
stems of the heath, I found a multitude of seedlings and little trees,
which had been perpetually browsed down by the cattle. In one square
yard, at a point some hundred yards distant from one of the old
clumps, I counted thirty-two little trees; and one of them, judging
from the rings of growth, had during twenty-six years tried to raise
its head above the stems of the heath, and had failed. No wonder that,
as soon as the land was enclosed, it became thickly clothed with
vigorously growing young firs. Yet the heath was so extremely barren
and so extensive that no one would ever have imagined that cattle
would have so closely and effectually searched it for food.

Here we see that cattle absolutely determine the existence of the
Scotch fir; but in several parts of the world insects determine the
existence of cattle. Perhaps Paraguay offers the most curious instance
of this; for here neither cattle nor horses nor dogs have ever run
wild, though they swarm southward and northward in a feral state; and
Azara and Rengger have shown that this is caused by the greater number
in Paraguay of a certain fly, which lays its eggs in the navels of
these animals when first born. The increase of these flies, numerous
as they are, must be habitually checked by some means, probably by
birds. Hence, if certain insectivorous birds (whose numbers are
probably regulated by hawks or beasts of prey) were to increase in
Paraguay, the flies would decrease--then cattle and horses would
become feral, and this would certainly greatly alter (as indeed I have
observed in parts of South America) the vegetation: this again would
largely affect the insects; and this, as we just have seen in
Staffordshire, the insectivorous birds, and so onwards in
ever-increasing circles of complexity. We began this series by
insectivorous birds, and we have ended with them. Not that in nature
the relations can ever be as simple as this. Battle within battle must
ever be recurring with varying success; and yet in the long-run the
forces are so nicely balanced, that the face of nature remains uniform
for long periods of time, though assuredly the merest trifle would
often give the victory to one organic being over another. Nevertheless
so profound is our ignorance, and so high our presumption, that we
marvel when we hear of the extinction of an organic being; and as we
do not see the cause, we invoke cataclysms to desolate the world, or
invent laws on the duration of the forms of life!

I am tempted to give one more instance showing how plants and animals,
most remote in the scale of nature, are bound together by a web of
complex relations. I shall hereafter have occasion to show that the
exotic Lobelia fulgens, in this part of England, is never visited by
insects, and consequently, from its peculiar structure, never can set
a seed. Many of our orchidaceous plants absolutely require the visits
of moths to remove their pollen-masses and thus to fertilise them. I
have, also, reason to believe that humble-bees are indispensable to
the fertilisation of the heartsease (Viola tricolor), for other bees
do not visit this flower. From experiments which I have tried, I have
found that the visits of bees, if not indispensable, are at least
highly beneficial to the fertilisation of our clovers; but humble-bees
alone visit the common red clover (Trifolium pratense), as other bees
cannot reach the nectar. Hence I have very little doubt, that if the
whole genus of humble-bees became extinct or very rare in England, the
heartsease and red clover would become very rare, or wholly disappear.
The number of humble-bees in any district depends in a great degree on
the number of field-mice, which destroy their combs and nests; and Mr.
H. Newman, who has long attended to the habits of humble-bees,
believes that "more than two thirds of them are thus destroyed all
over England." Now the number of mice is largely dependent, as every
one knows, on the number of cats; and Mr. Newman says, "Near villages
and small towns I have found the nests of humble-bees more numerous
than elsewhere, which I attribute to the number of cats that destroy
the mice." Hence it is quite credible that the presence of a feline
animal in large numbers in a district might determine, through the
intervention first of mice and then of bees, the frequency of certain
flowers in that district!

In the case of every species, many different checks, acting at
different periods of life, and during different seasons or years,
probably come into play; some one check or some few being generally
the most potent, but all concurring in determining the average number
or even the existence of the species. In some cases it can be shown
that widely-different checks act on the same species in different
districts. When we look at the plants and bushes clothing an entangled
bank, we are tempted to attribute their proportional numbers and kinds
to what we call chance. But how false a view is this! Every one has
heard that when an American forest is cut down, a very different
vegetation springs up; but it has been observed that the trees now
growing on the ancient Indian mounds, in the Southern United States,
display the same beautiful diversity and proportion of kinds as in the
surrounding virgin forests. What a struggle between the several kinds
of trees must here have gone on during long centuries, each annually
scattering its seeds by the thousand; what war between insect and
insect--between insects, snails, and other animals with birds and
beasts of prey--all striving to increase, and all feeding on each
other or on the trees or their seeds and seedlings, or on the other
plants which first clothed the ground and thus checked the growth of
the trees! Throw up a handful of feathers, and all must fall to the
ground according to definite laws; but how simple is this problem
compared to the action and reaction of the innumerable plants and
animals which have determined, in the course of centuries, the
proportional numbers and kinds of trees now growing on the old Indian
ruins!

The dependency of one organic being on another, as of a parasite on
its prey, lies generally between beings remote in the scale of nature.
This is often the case with those which may strictly be said to
struggle with each other for existence, as in the case of locusts and
grass-feeding quadrupeds. But the struggle almost invariably will be
most severe between the individuals of the same species, for they
frequent the same districts, require the same food, and are exposed to
the same dangers. In the case of varieties of the same species, the
struggle will generally be almost equally severe, and we sometimes see
the contest soon decided: for instance, if several varieties of wheat
be sown together, and the mixed seed be resown, some of the varieties
which best suit the soil or climate, or are naturally the most
fertile, will beat the others and so yield more seed, and will
consequently in a few years quite supplant the other varieties. To
keep up a mixed stock of even such extremely close varieties as the
variously coloured sweet-peas, they must be each year harvested
separately, and the seed then mixed in due proportion, otherwise the
weaker kinds will steadily decrease in numbers and disappear. So again
with the varieties of sheep: it has been asserted that certain
mountain-varieties will starve out other mountain-varieties, so that
they cannot be kept together. The same result has followed from
keeping together different varieties of the medicinal leech. It may
even be doubted whether the varieties of any one of our domestic
plants or animals have so exactly the same strength, habits, and
constitution, that the original proportions of a mixed stock could be
kept up for half a dozen generations, if they were allowed to struggle
together, like beings in a state of nature, and if the seed or young
were not annually sorted.

As species of the same genus have usually, though by no means
invariably, some similarity in habits and constitution, and always in
structure, the struggle will generally be more severe between species
of the same genus, when they come into competition with each other,
than between species of distinct genera. We see this in the recent
extension over parts of the United States of one species of swallow
having caused the decrease of another species. The recent increase of
the missel-thrush in parts of Scotland has caused the decrease of the
song-thrush. How frequently we hear of one species of rat taking the
place of another species under the most different climates! In Russia
the small Asiatic cockroach has everywhere driven before it its great
congener. One species of charlock will supplant another, and so in
other cases. We can dimly see why the competition should be most
severe between allied forms, which fill nearly the same place in the
economy of nature; but probably in no one case could we precisely say
why one species has been victorious over another in the great battle
of life.

A corollary of the highest importance may be deduced from the
foregoing remarks, namely, that the structure of every organic being
is related, in the most essential yet often hidden manner, to that of
all other organic beings, with which it comes into competition for
food or residence, or from which it has to escape, or on which it
preys. This is obvious in the structure of the teeth and talons of the
tiger; and in that of the legs and claws of the parasite which clings
to the hair on the tiger's body. But in the beautifully plumed seed of
the dandelion, and in the flattened and fringed legs of the
water-beetle, the relation seems at first confined to the elements of
air and water. Yet the advantage of plumed seeds no doubt stands in
the closest relation to the land being already thickly clothed by
other plants; so that the seeds may be widely distributed and fall on
unoccupied ground. In the water-beetle, the structure of its legs, so
well adapted for diving, allows it to compete with other aquatic
insects, to hunt for its own prey, and to escape serving as prey to
other animals.

The store of nutriment laid up within the seeds of many plants seems
at first sight to have no sort of relation to other plants. But from
the strong growth of young plants produced from such seeds (as peas
and beans), when sown in the midst of long grass, I suspect that the
chief use of the nutriment in the seed is to favour the growth of the
young seedling, whilst struggling with other plants growing vigorously
all around.

Look at a plant in the midst of its range, why does it not double or
quadruple its numbers? We know that it can perfectly well withstand a
little more heat or cold, dampness or dryness, for elsewhere it ranges
into slightly hotter or colder, damper or drier districts. In this
case we can clearly see that if we wished in imagination to give the
plant the power of increasing in number, we should have to give it
some advantage over its competitors, or over the animals which preyed
on it. On the confines of its geographical range, a change of
constitution with respect to climate would clearly be an advantage to
our plant; but we have reason to believe that only a few plants or
animals range so far, that they are destroyed by the rigour of the
climate alone. Not until we reach the extreme confines of life, in the
arctic regions or on the borders of an utter desert, will competition
cease. The land may be extremely cold or dry, yet there will be
competition between some few species, or between the individuals of
the same species, for the warmest or dampest spots.

Hence, also, we can see that when a plant or animal is placed in a new
country amongst new competitors, though the climate may be exactly the
same as in its former home, yet the conditions of its life will
generally be changed in an essential manner. If we wished to increase
its average numbers in its new home, we should have to modify it in a
different way to what we should have done in its native country; for
we should have to give it some advantage over a different set of
competitors or enemies.

It is good thus to try in our imagination to give any form some
advantage over another. Probably in no single instance should we know
what to do, so as to succeed. It will convince us of our ignorance on
the mutual relations of all organic beings; a conviction as necessary,
as it seems to be difficult to acquire. All that we can do, is to keep
steadily in mind that each organic being is striving to increase at a
geometrical ratio; that each at some period of its life, during some
season of the year, during each generation or at intervals, has to
struggle for life, and to suffer great destruction. When we reflect on
this struggle, we may console ourselves with the full belief, that the
war of nature is not incessant, that no fear is felt, that death is
generally prompt, and that the vigorous, the healthy, and the happy
survive and multiply.


CHAPTER 4.

NATURAL SELECTION.

Natural Selection: its power compared with man's selection, its power
on characters of trifling importance, its power at all ages and on
both sexes.
Sexual Selection.
On the generality of intercrosses between individuals of the same
species.
Circumstances favourable and unfavourable to Natural Selection,
namely, intercrossing, isolation, number of individuals.
Slow action.
Extinction caused by Natural Selection.
Divergence of Character, related to the diversity of inhabitants of
any small area, and to naturalisation.
Action of Natural Selection, through Divergence of Character and
Extinction, on the descendants from a common parent.
Explains the Grouping of all organic beings.

How will the struggle for existence, discussed too briefly in the last
chapter, act in regard to variation? Can the principle of selection,
which we have seen is so potent in the hands of man, apply in nature?
I think we shall see that it can act most effectually. Let it be borne
in mind in what an endless number of strange peculiarities our
domestic productions, and, in a lesser degree, those under nature,
vary; and how strong the hereditary tendency is. Under domestication,
it may be truly said that the whole organisation becomes in some
degree plastic. Let it be borne in mind how infinitely complex and
close-fitting are the mutual relations of all organic beings to each
other and to their physical conditions of life. Can it, then, be
thought improbable, seeing that variations useful to man have
undoubtedly occurred, that other variations useful in some way to each
being in the great and complex battle of life, should sometimes occur
in the course of thousands of generations? If such do occur, can we
doubt (remembering that many more individuals are born than can
possibly survive) that individuals having any advantage, however
slight, over others, would have the best chance of surviving and of
procreating their kind? On the other hand, we may feel sure that any
variation in the least degree injurious would be rigidly destroyed.
This preservation of favourable variations and the rejection of
injurious variations, I call Natural Selection. Variations neither
useful nor injurious would not be affected by natural selection, and
would be left a fluctuating element, as perhaps we see in the species
called polymorphic.

We shall best understand the probable course of natural selection by
taking the case of a country undergoing some physical change, for
instance, of climate. The proportional numbers of its inhabitants
would almost immediately undergo a change, and some species might
become extinct. We may conclude, from what we have seen of the
intimate and complex manner in which the inhabitants of each country
are bound together, that any change in the numerical proportions of
some of the inhabitants, independently of the change of climate
itself, would most seriously affect many of the others. If the country
were open on its borders, new forms would certainly immigrate, and
this also would seriously disturb the relations of some of the former
inhabitants. Let it be remembered how powerful the influence of a
single introduced tree or mammal has been shown to be. But in the case
of an island, or of a country partly surrounded by barriers, into
which new and better adapted forms could not freely enter, we should
then have places in the economy of nature which would assuredly be
better filled up, if some of the original inhabitants were in some
manner modified; for, had the area been open to immigration, these
same places would have been seized on by intruders. In such case,
every slight modification, which in the course of ages chanced to
arise, and which in any way favoured the individuals of any of the
species, by better adapting them to their altered conditions, would
tend to be preserved; and natural selection would thus have free scope
for the work of improvement.

We have reason to believe, as stated in the first chapter, that a
change in the conditions of life, by specially acting on the
reproductive system, causes or increases variability; and in the
foregoing case the conditions of life are supposed to have undergone a
change, and this would manifestly be favourable to natural selection,
by giving a better chance of profitable variations occurring; and
unless profitable variations do occur, natural selection can do
nothing. Not that, as I believe, any extreme amount of variability is
necessary; as man can certainly produce great results by adding up in
any given direction mere individual differences, so could Nature, but
far more easily, from having incomparably longer time at her disposal.
Nor do I believe that any great physical change, as of climate, or any
unusual degree of isolation to check immigration, is actually
necessary to produce new and unoccupied places for natural selection
to fill up by modifying and improving some of the varying inhabitants.
For as all the inhabitants of each country are struggling together
with nicely balanced forces, extremely slight modifications in the
structure or habits of one inhabitant would often give it an advantage
over others; and still further modifications of the same kind would
often still further increase the advantage. No country can be named in
which all the native inhabitants are now so perfectly adapted to each
other and to the physical conditions under which they live, that none
of them could anyhow be improved; for in all countries, the natives
have been so far conquered by naturalised productions, that they have
allowed foreigners to take firm possession of the land. And as
foreigners have thus everywhere beaten some of the natives, we may
safely conclude that the natives might have been modified with
advantage, so as to have better resisted such intruders.

As man can produce and certainly has produced a great result by his
methodical and unconscious means of selection, what may not nature
effect? Man can act only on external and visible characters: nature
cares nothing for appearances, except in so far as they may be useful
to any being. She can act on every internal organ, on every shade of
constitutional difference, on the whole machinery of life. Man selects
only for his own good; Nature only for that of the being which she
tends. Every selected character is fully exercised by her; and the
being is placed under well-suited conditions of life. Man keeps the
natives of many climates in the same country; he seldom exercises each
selected character in some peculiar and fitting manner; he feeds a
long and a short beaked pigeon on the same food; he does not exercise
a long-backed or long-legged quadruped in any peculiar manner; he
exposes sheep with long and short wool to the same climate. He does
not allow the most vigorous males to struggle for the females. He does
not rigidly destroy all inferior animals, but protects during each
varying season, as far as lies in his power, all his productions. He
often begins his selection by some half-monstrous form; or at least by
some modification prominent enough to catch his eye, or to be plainly
useful to him. Under nature, the slightest difference of structure or
constitution may well turn the nicely-balanced scale in the struggle
for life, and so be preserved. How fleeting are the wishes and efforts
of man! how short his time! and consequently how poor will his
products be, compared with those accumulated by nature during whole
geological periods. Can we wonder, then, that nature's productions
should be far "truer" in character than man's productions; that they
should be infinitely better adapted to the most complex conditions of
life, and should plainly bear the stamp of far higher workmanship?

It may be said that natural selection is daily and hourly
scrutinising, throughout the world, every variation, even the
slightest; rejecting that which is bad, preserving and adding up all
that is good; silently and insensibly working, whenever and wherever
opportunity offers, at the improvement of each organic being in
relation to its organic and inorganic conditions of life. We see
nothing of these slow changes in progress, until the hand of time has
marked the long lapse of ages, and then so imperfect is our view into
long past geological ages, that we only see that the forms of life are
now different from what they formerly were.

Although natural selection can act only through and for the good of
each being, yet characters and structures, which we are apt to
consider as of very trifling importance, may thus be acted on. When we
see leaf-eating insects green, and bark-feeders mottled-grey; the
alpine ptarmigan white in winter, the red-grouse the colour of
heather, and the black-grouse that of peaty earth, we must believe
that these tints are of service to these birds and insects in
preserving them from danger. Grouse, if not destroyed at some period
of their lives, would increase in countless numbers; they are known to
suffer largely from birds of prey; and hawks are guided by eyesight to
their prey,--so much so, that on parts of the Continent persons are
warned not to keep white pigeons, as being the most liable to
destruction. Hence I can see no reason to doubt that natural selection
might be most effective in giving the proper colour to each kind of
grouse, and in keeping that colour, when once acquired, true and
constant. Nor ought we to think that the occasional destruction of an
animal of any particular colour would produce little effect: we should
remember how essential it is in a flock of white sheep to destroy
every lamb with the faintest trace of black. In plants the down on the
fruit and the colour of the flesh are considered by botanists as
characters of the most trifling importance: yet we hear from an
excellent horticulturist, Downing, that in the United States
smooth-skinned fruits suffer far more from a beetle, a curculio, than
those with down; that purple plums suffer far more from a certain
disease than yellow plums; whereas another disease attacks
yellow-fleshed peaches far more than those with other coloured flesh.
If, with all the aids of art, these slight differences make a great
difference in cultivating the several varieties, assuredly, in a state
of nature, where the trees would have to struggle with other trees and
with a host of enemies, such differences would effectually settle
which variety, whether a smooth or downy, a yellow or purple fleshed
fruit, should succeed.

In looking at many small points of difference between species, which,
as far as our ignorance permits us to judge, seem to be quite
unimportant, we must not forget that climate, food, etc., probably
produce some slight and direct effect. It is, however, far more
necessary to bear in mind that there are many unknown laws of
correlation of growth, which, when one part of the organisation is
modified through variation, and the modifications are accumulated by
natural selection for the good of the being, will cause other
modifications, often of the most unexpected nature.

As we see that those variations which under domestication appear at
any particular period of life, tend to reappear in the offspring at
the same period;--for instance, in the seeds of the many varieties of
our culinary and agricultural plants; in the caterpillar and cocoon
stages of the varieties of the silkworm; in the eggs of poultry, and
in the colour of the down of their chickens; in the horns of our sheep
and cattle when nearly adult;--so in a state of nature, natural
selection will be enabled to act on and modify organic beings at any
age, by the accumulation of profitable variations at that age, and by
their inheritance at a corresponding age. If it profit a plant to have
its seeds more and more widely disseminated by the wind, I can see no
greater difficulty in this being effected through natural selection,
than in the cotton-planter increasing and improving by selection the
down in the pods on his cotton-trees. Natural selection may modify and
adapt the larva of an insect to a score of contingencies, wholly
different from those which concern the mature insect. These
modifications will no doubt affect, through the laws of correlation,
the structure of the adult; and probably in the case of those insects
which live only for a few hours, and which never feed, a large part of
their structure is merely the correlated result of successive changes
in the structure of their larvae. So, conversely, modifications in the
adult will probably often affect the structure of the larva; but in
all cases natural selection will ensure that modifications consequent
on other modifications at a different period of life, shall not be in
the least degree injurious: for if they became so, they would cause
the extinction of the species.

Natural selection will modify the structure of the young in relation
to the parent, and of the parent in relation to the young. In social
animals it will adapt the structure of each individual for the benefit
of the community; if each in consequence profits by the selected
change. What natural selection cannot do, is to modify the structure
of one species, without giving it any advantage, for the good of
another species; and though statements to this effect may be found in
works of natural history, I cannot find one case which will bear
investigation. A structure used only once in an animal's whole life,
if of high importance to it, might be modified to any extent by
natural selection; for instance, the great jaws possessed by certain
insects, and used exclusively for opening the cocoon--or the hard tip
to the beak of nestling birds, used for breaking the egg. It has been
asserted, that of the best short-beaked tumbler-pigeons more perish in
the egg than are able to get out of it; so that fanciers assist in the
act of hatching. Now, if nature had to make the beak of a full-grown
pigeon very short for the bird's own advantage, the process of
modification would be very slow, and there would be simultaneously the
most rigorous selection of the young birds within the egg, which had
the most powerful and hardest beaks, for all with weak beaks would
inevitably perish: or, more delicate and more easily broken shells
might be selected, the thickness of the shell being known to vary like
every other structure.

SEXUAL SELECTION.

Inasmuch as peculiarities often appear under domestication in one sex
and become hereditarily attached to that sex, the same fact probably
occurs under nature, and if so, natural selection will be able to
modify one sex in its functional relations to the other sex, or in
relation to wholly different habits of life in the two sexes, as is
sometimes the case with insects. And this leads me to say a few words
on what I call Sexual Selection. This depends, not on a struggle for
existence, but on a struggle between the males for possession of the
females; the result is not death to the unsuccessful competitor, but
few or no offspring. Sexual selection is, therefore, less rigorous
than natural selection. Generally, the most vigorous males, those
which are best fitted for their places in nature, will leave most
progeny. But in many cases, victory will depend not on general vigour,
but on having special weapons, confined to the male sex. A hornless
stag or spurless cock would have a poor chance of leaving offspring.
Sexual selection by always allowing the victor to breed might surely
give indomitable courage, length to the spur, and strength to the wing
to strike in the spurred leg, as well as the brutal cock-fighter, who
knows well that he can improve his breed by careful selection of the
best cocks. How low in the scale of nature this law of battle
descends, I know not; male alligators have been described as fighting,
bellowing, and whirling round, like Indians in a war-dance, for the
possession of the females; male salmons have been seen fighting all
day long; male stag-beetles often bear wounds from the huge mandibles
of other males. The war is, perhaps, severest between the males of
polygamous animals, and these seem oftenest provided with special
weapons. The males of carnivorous animals are already well armed;
though to them and to others, special means of defence may be given
through means of sexual selection, as the mane to the lion, the
shoulder-pad to the boar, and the hooked jaw to the male salmon; for
the shield may be as important for victory, as the sword or spear.

Amongst birds, the contest is often of a more peaceful character. All
those who have attended to the subject, believe that there is the
severest rivalry between the males of many species to attract by
singing the females. The rock-thrush of Guiana, birds of Paradise, and
some others, congregate; and successive males display their gorgeous
plumage and perform strange antics before the females, which standing
by as spectators, at last choose the most attractive partner. Those
who have closely attended to birds in confinement well know that they
often take individual preferences and dislikes: thus Sir R. Heron has
described how one pied peacock was eminently attractive to all his hen
birds. It may appear childish to attribute any effect to such
apparently weak means: I cannot here enter on the details necessary to
support this view; but if man can in a short time give elegant
carriage and beauty to his bantams, according to his standard of
beauty, I can see no good reason to doubt that female birds, by
selecting, during thousands of generations, the most melodious or
beautiful males, according to their standard of beauty, might produce
a marked effect. I strongly suspect that some well-known laws with
respect to the plumage of male and female birds, in comparison with
the plumage of the young, can be explained on the view of plumage
having been chiefly modified by sexual selection, acting when the
birds have come to the breeding age or during the breeding season; the
modifications thus produced being inherited at corresponding ages or
seasons, either by the males alone, or by the males and females; but I
have not space here to enter on this subject.

Thus it is, as I believe, that when the males and females of any
animal have the same general habits of life, but differ in structure,
colour, or ornament, such differences have been mainly caused by
sexual selection; that is, individual males have had, in successive
generations, some slight advantage over other males, in their weapons,
means of defence, or charms; and have transmitted these advantages to
their male offspring. Yet, I would not wish to attribute all such
sexual differences to this agency: for we see peculiarities arising
and becoming attached to the male sex in our domestic animals (as the
wattle in male carriers, horn-like protuberances in the cocks of
certain fowls, etc.), which we cannot believe to be either useful to
the males in battle, or attractive to the females. We see analogous
cases under nature, for instance, the tuft of hair on the breast of
the turkey-cock, which can hardly be either useful or ornamental to
this bird;--indeed, had the tuft appeared under domestication, it
would have been called a monstrosity.

ILLUSTRATIONS OF THE ACTION OF NATURAL SELECTION.

In order to make it clear how, as I believe, natural selection acts, I
must beg permission to give one or two imaginary illustrations. Let us
take the case of a wolf, which preys on various animals, securing some
by craft, some by strength, and some by fleetness; and let us suppose
that the fleetest prey, a deer for instance, had from any change in
the country increased in numbers, or that other prey had decreased in
numbers, during that season of the year when the wolf is hardest
pressed for food. I can under such circumstances see no reason to
doubt that the swiftest and slimmest wolves would have the best chance
of surviving, and so be preserved or selected,--provided always that
they retained strength to master their prey at this or at some other
period of the year, when they might be compelled to prey on other
animals. I can see no more reason to doubt this, than that man can
improve the fleetness of his greyhounds by careful and methodical
selection, or by that unconscious selection which results from each
man trying to keep the best dogs without any thought of modifying the
breed.

Even without any change in the proportional numbers of the animals on
which our wolf preyed, a cub might be born with an innate tendency to
pursue certain kinds of prey. Nor can this be thought very improbable;
for we often observe great differences in the natural tendencies of
our domestic animals; one cat, for instance, taking to catch rats,
another mice; one cat, according to Mr. St. John, bringing home winged
game, another hares or rabbits, and another hunting on marshy ground
and almost nightly catching woodcocks or snipes. The tendency to catch
rats rather than mice is known to be inherited. Now, if any slight
innate change of habit or of structure benefited an individual wolf,
it would have the best chance of surviving and of leaving offspring.
Some of its young would probably inherit the same habits or structure,
and by the repetition of this process, a new variety might be formed
which would either supplant or coexist with the parent-form of wolf.
Or, again, the wolves inhabiting a mountainous district, and those
frequenting the lowlands, would naturally be forced to hunt different
prey; and from the continued preservation of the individuals best
fitted for the two sites, two varieties might slowly be formed. These
varieties would cross and blend where they met; but to this subject of
intercrossing we shall soon have to return. I may add, that, according
to Mr. Pierce, there are two varieties of the wolf inhabiting the
Catskill Mountains in the United States, one with a light
greyhound-like form, which pursues deer, and the other more bulky,
with shorter legs, which more frequently attacks the shepherd's
flocks.

Let us now take a more complex case. Certain plants excrete a sweet
juice, apparently for the sake of eliminating something injurious from
their sap: this is effected by glands at the base of the stipules in
some Leguminosae, and at the back of the leaf of the common laurel.
This juice, though small in quantity, is greedily sought by insects.
Let us now suppose a little sweet juice or nectar to be excreted by
the inner bases of the petals of a flower. In this case insects in
seeking the nectar would get dusted with pollen, and would certainly
often transport the pollen from one flower to the stigma of another
flower. The flowers of two distinct individuals of the same species
would thus get crossed; and the act of crossing, we have good reason
to believe (as will hereafter be more fully alluded to), would produce
very vigorous seedlings, which consequently would have the best chance
of flourishing and surviving. Some of these seedlings would probably
inherit the nectar-excreting power. Those individual flowers which had
the largest glands or nectaries, and which excreted most nectar, would
be oftenest visited by insects, and would be oftenest crossed; and so
in the long-run would gain the upper hand. Those flowers, also, which
had their stamens and pistils placed, in relation to the size and
habits of the particular insects which visited them, so as to favour
in any degree the transportal of their pollen from flower to flower,
would likewise be favoured or selected. We might have taken the case
of insects visiting flowers for the sake of collecting pollen instead
of nectar; and as pollen is formed for the sole object of
fertilisation, its destruction appears a simple loss to the plant; yet
if a little pollen were carried, at first occasionally and then
habitually, by the pollen-devouring insects from flower to flower, and
a cross thus effected, although nine-tenths of the pollen were
destroyed, it might still be a great gain to the plant; and those
individuals which produced more and more pollen, and had larger and
larger anthers, would be selected.

When our plant, by this process of the continued preservation or
natural selection of more and more attractive flowers, had been
rendered highly attractive to insects, they would, unintentionally on
their part, regularly carry pollen from flower to flower; and that
they can most effectually do this, I could easily show by many
striking instances. I will give only one--not as a very striking case,
but as likewise illustrating one step in the separation of the sexes
of plants, presently to be alluded to. Some holly-trees bear only male
flowers, which have four stamens producing rather a small quantity of
pollen, and a rudimentary pistil; other holly-trees bear only female
flowers; these have a full-sized pistil, and four stamens with
shrivelled anthers, in which not a grain of pollen can be detected.
Having found a female tree exactly sixty yards from a male tree, I put
the stigmas of twenty flowers, taken from different branches, under
the microscope, and on all, without exception, there were
pollen-grains, and on some a profusion of pollen. As the wind had set
for several days from the female to the male tree, the pollen could
not thus have been carried. The weather had been cold and boisterous,
and therefore not favourable to bees, nevertheless every female flower
which I examined had been effectually fertilised by the bees,
accidentally dusted with pollen, having flown from tree to tree in
search of nectar. But to return to our imaginary case: as soon as the
plant had been rendered so highly attractive to insects that pollen
was regularly carried from flower to flower, another process might
commence. No naturalist doubts the advantage of what has been called
the "physiological division of labour;" hence we may believe that it
would be advantageous to a plant to produce stamens alone in one
flower or on one whole plant, and pistils alone in another flower or
on another plant. In plants under culture and placed under new
conditions of life, sometimes the male organs and sometimes the female
organs become more or less impotent; now if we suppose this to occur
in ever so slight a degree under nature, then as pollen is already
carried regularly from flower to flower, and as a more complete
separation of the sexes of our plant would be advantageous on the
principle of the division of labour, individuals with this tendency
more and more increased, would be continually favoured or selected,
until at last a complete separation of the sexes would be effected.

Let us now turn to the nectar-feeding insects in our imaginary case:
we may suppose the plant of which we have been slowly increasing the
nectar by continued selection, to be a common plant; and that certain
insects depended in main part on its nectar for food. I could give
many facts, showing how anxious bees are to save time; for instance,
their habit of cutting holes and sucking the nectar at the bases of
certain flowers, which they can, with a very little more trouble,
enter by the mouth. Bearing such facts in mind, I can see no reason to
doubt that an accidental deviation in the size and form of the body,
or in the curvature and length of the proboscis, etc., far too slight
to be appreciated by us, might profit a bee or other insect, so that
an individual so characterised would be able to obtain its food more
quickly, and so have a better chance of living and leaving
descendants. Its descendants would probably inherit a tendency to a
similar slight deviation of structure. The tubes of the corollas of
the common red and incarnate clovers (Trifolium pratense and
incarnatum) do not on a hasty glance appear to differ in length; yet
the hive-bee can easily suck the nectar out of the incarnate clover,
but not out of the common red clover, which is visited by humble-bees
alone; so that whole fields of the red clover offer in vain an
abundant supply of precious nectar to the hive-bee. Thus it might be a
great advantage to the hive-bee to have a slightly longer or
differently constructed proboscis. On the other hand, I have found by
experiment that the fertility of clover greatly depends on bees
visiting and moving parts of the corolla, so as to push the pollen on
to the stigmatic surface. Hence, again, if humble-bees were to become
rare in any country, it might be a great advantage to the red clover
to have a shorter or more deeply divided tube to its corolla, so that
the hive-bee could visit its flowers. Thus I can understand how a
flower and a bee might slowly become, either simultaneously or one
after the other, modified and adapted in the most perfect manner to
each other, by the continued preservation of individuals presenting
mutual and slightly favourable deviations of structure.

I am well aware that this doctrine of natural selection, exemplified
in the above imaginary instances, is open to the same objections which
were at first urged against Sir Charles Lyell's noble views on "the
modern changes of the earth, as illustrative of geology;" but we now
very seldom hear the action, for instance, of the coast-waves, called
a trifling and insignificant cause, when applied to the excavation of
gigantic valleys or to the formation of the longest lines of inland
cliffs. Natural selection can act only by the preservation and
accumulation of infinitesimally small inherited modifications, each
profitable to the preserved being; and as modern geology has almost
banished such views as the excavation of a great valley by a single
diluvial wave, so will natural selection, if it be a true principle,
banish the belief of the continued creation of new organic beings, or
of any great and sudden modification in their structure.

ON THE INTERCROSSING OF INDIVIDUALS.

I must here introduce a short digression. In the case of animals and
plants with separated sexes, it is of course obvious that two
individuals must always unite for each birth; but in the case of
hermaphrodites this is far from obvious. Nevertheless I am strongly
inclined to believe that with all hermaphrodites two individuals,
either occasionally or habitually, concur for the reproduction of
their kind. This view, I may add, was first suggested by Andrew
Knight. We shall presently see its importance; but I must here treat
the subject with extreme brevity, though I have the materials prepared
for an ample discussion. All vertebrate animals, all insects, and some
other large groups of animals, pair for each birth. Modern research
has much diminished the number of supposed hermaphrodites, and of real
hermaphrodites a large number pair; that is, two individuals regularly
unite for reproduction, which is all that concerns us. But still there
are many hermaphrodite animals which certainly do not habitually pair,
and a vast majority of plants are hermaphrodites. What reason, it may
be asked, is there for supposing in these cases that two individuals
ever concur in reproduction? As it is impossible here to enter on
details, I must trust to some general considerations alone.

In the first place, I have collected so large a body of facts,
showing, in accordance with the almost universal belief of breeders,
that with animals and plants a cross between different varieties, or
between individuals of the same variety but of another strain, gives
vigour and fertility to the offspring; and on the other hand, that
CLOSE interbreeding diminishes vigour and fertility; that these facts
alone incline me to believe that it is a general law of nature
(utterly ignorant though we be of the meaning of the law) that no
organic being self-fertilises itself for an eternity of generations;
but that a cross with another individual is occasionally--perhaps at
very long intervals--indispensable.

On the belief that this is a law of nature, we can, I think,
understand several large classes of facts, such as the following,
which on any other view are inexplicable. Every hybridizer knows how
unfavourable exposure to wet is to the fertilisation of a flower, yet
what a multitude of flowers have their anthers and stigmas fully
exposed to the weather! but if an occasional cross be indispensable,
the fullest freedom for the entrance of pollen from another individual
will explain this state of exposure, more especially as the plant's
own anthers and pistil generally stand so close together that
self-fertilisation seems almost inevitable. Many flowers, on the other
hand, have their organs of fructification closely enclosed, as in the
great papilionaceous or pea-family; but in several, perhaps in all,
such flowers, there is a very curious adaptation between the structure
of the flower and the manner in which bees suck the nectar; for, in
doing this, they either push the flower's own pollen on the stigma, or
bring pollen from another flower. So necessary are the visits of bees
to papilionaceous flowers, that I have found, by experiments published
elsewhere, that their fertility is greatly diminished if these visits
be prevented. Now, it is scarcely possible that bees should fly from
flower to flower, and not carry pollen from one to the other, to the
great good, as I believe, of the plant. Bees will act like a
camel-hair pencil, and it is quite sufficient just to touch the
anthers of one flower and then the stigma of another with the same
brush to ensure fertilisation; but it must not be supposed that bees
would thus produce a multitude of hybrids between distinct species;
for if you bring on the same brush a plant's own pollen and pollen
from another species, the former will have such a prepotent effect,
that it will invariably and completely destroy, as has been shown by
Gartner, any influence from the foreign pollen.

When the stamens of a flower suddenly spring towards the pistil, or
slowly move one after the other towards it, the contrivance seems
adapted solely to ensure self-fertilisation; and no doubt it is useful
for this end: but, the agency of insects is often required to cause
the stamens to spring forward, as Kolreuter has shown to be the case
with the barberry; and curiously in this very genus, which seems to
have a special contrivance for self-fertilisation, it is well known
that if very closely-allied forms or varieties are planted near each
other, it is hardly possible to raise pure seedlings, so largely do
they naturally cross. In many other cases, far from there being any
aids for self-fertilisation, there are special contrivances, as I
could show from the writings of C. C. Sprengel and from my own
observations, which effectually prevent the stigma receiving pollen
from its own flower: for instance, in Lobelia fulgens, there is a
really beautiful and elaborate contrivance by which every one of the
infinitely numerous pollen-granules are swept out of the conjoined
anthers of each flower, before the stigma of that individual flower is
ready to receive them; and as this flower is never visited, at least
in my garden, by insects, it never sets a seed, though by placing
pollen from one flower on the stigma of another, I raised plenty of
seedlings; and whilst another species of Lobelia growing close by,
which is visited by bees, seeds freely. In very many other cases,
though there be no special mechanical contrivance to prevent the
stigma of a flower receiving its own pollen, yet, as C. C. Sprengel
has shown, and as I can confirm, either the anthers burst before the
stigma is ready for fertilisation, or the stigma is ready before the
pollen of that flower is ready, so that these plants have in fact
separated sexes, and must habitually be crossed. How strange are these
facts! How strange that the pollen and stigmatic surface of the same
flower, though placed so close together, as if for the very purpose of
self-fertilisation, should in so many cases be mutually useless to
each other! How simply are these facts explained on the view of an
occasional cross with a distinct individual being advantageous or
indispensable!

If several varieties of the cabbage, radish, onion, and of some other
plants, be allowed to seed near each other, a large majority, as I
have found, of the seedlings thus raised will turn out mongrels: for
instance, I raised 233 seedling cabbages from some plants of different
varieties growing near each other, and of these only 78 were true to
their kind, and some even of these were not perfectly true. Yet the
pistil of each cabbage-flower is surrounded not only by its own six
stamens, but by those of the many other flowers on the same plant.
How, then, comes it that such a vast number of the seedlings are
mongrelized? I suspect that it must arise from the pollen of a
distinct VARIETY having a prepotent effect over a flower's own pollen;
and that this is part of the general law of good being derived from
the intercrossing of distinct individuals of the same species. When
distinct SPECIES are crossed the case is directly the reverse, for a
plant's own pollen is always prepotent over foreign pollen; but to
this subject we shall return in a future chapter.

In the case of a gigantic tree covered with innumerable flowers, it
may be objected that pollen could seldom be carried from tree to tree,
and at most only from flower to flower on the same tree, and that
flowers on the same tree can be considered as distinct individuals
only in a limited sense. I believe this objection to be valid, but
that nature has largely provided against it by giving to trees a
strong tendency to bear flowers with separated sexes. When the sexes
are separated, although the male and female flowers may be produced on
the same tree, we can see that pollen must be regularly carried from
flower to flower; and this will give a better chance of pollen being
occasionally carried from tree to tree. That trees belonging to all
Orders have their sexes more often separated than other plants, I find
to be the case in this country; and at my request Dr. Hooker tabulated
the trees of New Zealand, and Dr. Asa Gray those of the United States,
and the result was as I anticipated. On the other hand, Dr. Hooker has
recently informed me that he finds that the rule does not hold in
Australia; and I have made these few remarks on the sexes of trees
simply to call attention to the subject.

Turning for a very brief space to animals: on the land there are some
hermaphrodites, as land-mollusca and earth-worms; but these all pair.
As yet I have not found a single case of a terrestrial animal which
fertilises itself. We can understand this remarkable fact, which
offers so strong a contrast with terrestrial plants, on the view of an
occasional cross being indispensable, by considering the medium in
which terrestrial animals live, and the nature of the fertilising
element; for we know of no means, analogous to the action of insects
and of the wind in the case of plants, by which an occasional cross
could be effected with terrestrial animals without the concurrence of
two individuals. Of aquatic animals, there are many self-fertilising
hermaphrodites; but here currents in the water offer an obvious means
for an occasional cross. And, as in the case of flowers, I have as yet
failed, after consultation with one of the highest authorities,
namely, Professor Huxley, to discover a single case of an
hermaphrodite animal with the organs of reproduction so perfectly
enclosed within the body, that access from without and the occasional
influence of a distinct individual can be shown to be physically
impossible. Cirripedes long appeared to me to present a case of very
great difficulty under this point of view; but I have been enabled, by
a fortunate chance, elsewhere to prove that two individuals, though
both are self-fertilising hermaphrodites, do sometimes cross.

It must have struck most naturalists as a strange anomaly that, in the
case of both animals and plants, species of the same family and even
of the same genus, though agreeing closely with each other in almost
their whole organisation, yet are not rarely, some of them
hermaphrodites, and some of them unisexual. But if, in fact, all
hermaphrodites do occasionally intercross with other individuals, the
difference between hermaphrodites and unisexual species, as far as
function is concerned, becomes very small.

From these several considerations and from the many special facts
which I have collected, but which I am not here able to give, I am
strongly inclined to suspect that, both in the vegetable and animal
kingdoms, an occasional intercross with a distinct individual is a law
of nature. I am well aware that there are, on this view, many cases of
difficulty, some of which I am trying to investigate. Finally then, we
may conclude that in many organic beings, a cross between two
individuals is an obvious necessity for each birth; in many others it
occurs perhaps only at long intervals; but in none, as I suspect, can
self-fertilisation go on for perpetuity.

CIRCUMSTANCES FAVOURABLE TO NATURAL SELECTION.

This is an extremely intricate subject. A large amount of inheritable
and diversified variability is favourable, but I believe mere
individual differences suffice for the work. A large number of
individuals, by giving a better chance for the appearance within any
given period of profitable variations, will compensate for a lesser
amount of variability in each individual, and is, I believe, an
extremely important element of success. Though nature grants vast
periods of time for the work of natural selection, she does not grant
an indefinite period; for as all organic beings are striving, it may
be said, to seize on each place in the economy of nature, if any one
species does not become modified and improved in a corresponding
degree with its competitors, it will soon be exterminated.

In man's methodical selection, a breeder selects for some definite
object, and free intercrossing will wholly stop his work. But when
many men, without intending to alter the breed, have a nearly common
standard of perfection, and all try to get and breed from the best
animals, much improvement and modification surely but slowly follow
from this unconscious process of selection, notwithstanding a large
amount of crossing with inferior animals. Thus it will be in nature;
for within a confined area, with some place in its polity not so
perfectly occupied as might be, natural selection will always tend to
preserve all the individuals varying in the right direction, though in
different degrees, so as better to fill up the unoccupied place. But
if the area be large, its several districts will almost certainly
present different conditions of life; and then if natural selection be
modifying and improving a species in the several districts, there will
be intercrossing with the other individuals of the same species on the
confines of each. And in this case the effects of intercrossing can
hardly be counterbalanced by natural selection always tending to
modify all the individuals in each district in exactly the same manner
to the conditions of each; for in a continuous area, the conditions
will generally graduate away insensibly from one district to another.
The intercrossing will most affect those animals which unite for each
birth, which wander much, and which do not breed at a very quick rate.
Hence in animals of this nature, for instance in birds, varieties will
generally be confined to separated countries; and this I believe to be
the case. In hermaphrodite organisms which cross only occasionally,
and likewise in animals which unite for each birth, but which wander
little and which can increase at a very rapid rate, a new and improved
variety might be quickly formed on any one spot, and might there
maintain itself in a body, so that whatever intercrossing took place
would be chiefly between the individuals of the same new variety. A
local variety when once thus formed might subsequently slowly spread
to other districts. On the above principle, nurserymen always prefer
getting seed from a large body of plants of the same variety, as the
chance of intercrossing with other varieties is thus lessened.

Even in the case of slow-breeding animals, which unite for each birth,
we must not overrate the effects of intercrosses in retarding natural
selection; for I can bring a considerable catalogue of facts, showing
that within the same area, varieties of the same animal can long
remain distinct, from haunting different stations, from breeding at
slightly different seasons, or from varieties of the same kind
preferring to pair together.

Intercrossing plays a very important part in nature in keeping the
individuals of the same species, or of the same variety, true and
uniform in character. It will obviously thus act far more efficiently
with those animals which unite for each birth; but I have already
attempted to show that we have reason to believe that occasional
intercrosses take place with all animals and with all plants. Even if
these take place only at long intervals, I am convinced that the young
thus produced will gain so much in vigour and fertility over the
offspring from long-continued self-fertilisation, that they will have
a better chance of surviving and propagating their kind; and thus, in
the long run, the influence of intercrosses, even at rare intervals,
will be great. If there exist organic beings which never intercross,
uniformity of character can be retained amongst them, as long as their
conditions of life remain the same, only through the principle of
inheritance, and through natural selection destroying any which depart
from the proper type; but if their conditions of life change and they
undergo modification, uniformity of character can be given to their
modified offspring, solely by natural selection preserving the same
favourable variations.

Isolation, also, is an important element in the process of natural
selection. In a confined or isolated area, if not very large, the
organic and inorganic conditions of life will generally be in a great
degree uniform; so that natural selection will tend to modify all the
individuals of a varying species throughout the area in the same
manner in relation to the same conditions. Intercrosses, also, with
the individuals of the same species, which otherwise would have
inhabited the surrounding and differently circumstanced districts,
will be prevented. But isolation probably acts more efficiently in
checking the immigration of better adapted organisms, after any
physical change, such as of climate or elevation of the land, etc.;
and thus new places in the natural economy of the country are left
open for the old inhabitants to struggle for, and become adapted to,
through modifications in their structure and constitution. Lastly,
isolation, by checking immigration and consequently competition, will
give time for any new variety to be slowly improved; and this may
sometimes be of importance in the production of new species. If,
however, an isolated area be very small, either from being surrounded
by barriers, or from having very peculiar physical conditions, the
total number of the individuals supported on it will necessarily be
very small; and fewness of individuals will greatly retard the
production of new species through natural selection, by decreasing the
chance of the appearance of favourable variations.

If we turn to nature to test the truth of these remarks, and look at
any small isolated area, such as an oceanic island, although the total
number of the species inhabiting it, will be found to be small, as we
shall see in our chapter on geographical distribution; yet of these
species a very large proportion are endemic,--that is, have been
produced there, and nowhere else. Hence an oceanic island at first
sight seems to have been highly favourable for the production of new
species. But we may thus greatly deceive ourselves, for to ascertain
whether a small isolated area, or a large open area like a continent,
has been most favourable for the production of new organic forms, we
ought to make the comparison within equal times; and this we are
incapable of doing.

Although I do not doubt that isolation is of considerable importance
in the production of new species, on the whole I am inclined to
believe that largeness of area is of more importance, more especially
in the production of species, which will prove capable of enduring for
a long period, and of spreading widely. Throughout a great and open
area, not only will there be a better chance of favourable variations
arising from the large number of individuals of the same species there
supported, but the conditions of life are infinitely complex from the
large number of already existing species; and if some of these many
species become modified and improved, others will have to be improved
in a corresponding degree or they will be exterminated. Each new form,
also, as soon as it has been much improved, will be able to spread
over the open and continuous area, and will thus come into competition
with many others. Hence more new places will be formed, and the
competition to fill them will be more severe, on a large than on a
small and isolated area. Moreover, great areas, though now continuous,
owing to oscillations of level, will often have recently existed in a
broken condition, so that the good effects of isolation will
generally, to a certain extent, have concurred. Finally, I conclude
that, although small isolated areas probably have been in some
respects highly favourable for the production of new species, yet that
the course of modification will generally have been more rapid on
large areas; and what is more important, that the new forms produced
on large areas, which already have been victorious over many
competitors, will be those that will spread most widely, will give
rise to most new varieties and species, and will thus play an
important part in the changing history of the organic world.

We can, perhaps, on these views, understand some facts which will be
again alluded to in our chapter on geographical distribution; for
instance, that the productions of the smaller continent of Australia
have formerly yielded, and apparently are now yielding, before those
of the larger Europaeo-Asiatic area. Thus, also, it is that
continental productions have everywhere become so largely naturalised
on islands. On a small island, the race for life will have been less
severe, and there will have been less modification and less
extermination. Hence, perhaps, it comes that the flora of Madeira,
according to Oswald Heer, resembles the extinct tertiary flora of
Europe. All fresh-water basins, taken together, make a small area
compared with that of the sea or of the land; and, consequently, the
competition between fresh-water productions will have been less severe
than elsewhere; new forms will have been more slowly formed, and old
forms more slowly exterminated. And it is in fresh water that we find
seven genera of Ganoid fishes, remnants of a once preponderant order:
and in fresh water we find some of the most anomalous forms now known
in the world, as the Ornithorhynchus and Lepidosiren, which, like
fossils, connect to a certain extent orders now widely separated in
the natural scale. These anomalous forms may almost be called living
fossils; they have endured to the present day, from having inhabited a
confined area, and from having thus been exposed to less severe
competition.

To sum up the circumstances favourable and unfavourable to natural
selection, as far as the extreme intricacy of the subject permits. I
conclude, looking to the future, that for terrestrial productions a
large continental area, which will probably undergo many oscillations
of level, and which consequently will exist for long periods in a
broken condition, will be the most favourable for the production of
many new forms of life, likely to endure long and to spread widely.
For the area will first have existed as a continent, and the
inhabitants, at this period numerous in individuals and kinds, will
have been subjected to very severe competition. When converted by
subsidence into large separate islands, there will still exist many
individuals of the same species on each island: intercrossing on the
confines of the range of each species will thus be checked: after
physical changes of any kind, immigration will be prevented, so that
new places in the polity of each island will have to be filled up by
modifications of the old inhabitants; and time will be allowed for the
varieties in each to become well modified and perfected. When, by
renewed elevation, the islands shall be re-converted into a
continental area, there will again be severe competition: the most
favoured or improved varieties will be enabled to spread: there will
be much extinction of the less improved forms, and the relative
proportional numbers of the various inhabitants of the renewed
continent will again be changed; and again there will be a fair field
for natural selection to improve still further the inhabitants, and
thus produce new species.

That natural selection will always act with extreme slowness, I fully
admit. Its action depends on there being places in the polity of
nature, which can be better occupied by some of the inhabitants of the
country undergoing modification of some kind. The existence of such
places will often depend on physical changes, which are generally very
slow, and on the immigration of better adapted forms having been
checked. But the action of natural selection will probably still
oftener depend on some of the inhabitants becoming slowly modified;
the mutual relations of many of the other inhabitants being thus
disturbed. Nothing can be effected, unless favourable variations
occur, and variation itself is apparently always a very slow process.
The process will often be greatly retarded by free intercrossing. Many
will exclaim that these several causes are amply sufficient wholly to
stop the action of natural selection. I do not believe so. On the
other hand, I do believe that natural selection will always act very
slowly, often only at long intervals of time, and generally on only a
very few of the inhabitants of the same region at the same time. I
further believe, that this very slow, intermittent action of natural
selection accords perfectly well with what geology tells us of the
rate and manner at which the inhabitants of this world have changed.

Slow though the process of selection may be, if feeble man can do much
by his powers of artificial selection, I can see no limit to the
amount of change, to the beauty and infinite complexity of the
coadaptations between all organic beings, one with another and with
their physical conditions of life, which may be effected in the long
course of time by nature's power of selection.

EXTINCTION.

This subject will be more fully discussed in our chapter on Geology;
but it must be here alluded to from being intimately connected with
natural selection. Natural selection acts solely through the
preservation of variations in some way advantageous, which
consequently endure. But as from the high geometrical powers of
increase of all organic beings, each area is already fully stocked
with inhabitants, it follows that as each selected and favoured form
increases in number, so will the less favoured forms decrease and
become rare. Rarity, as geology tells us, is the precursor to
extinction. We can, also, see that any form represented by few
individuals will, during fluctuations in the seasons or in the number
of its enemies, run a good chance of utter extinction. But we may go
further than this; for as new forms are continually and slowly being
produced, unless we believe that the number of specific forms goes on
perpetually and almost indefinitely increasing, numbers inevitably
must become extinct. That the number of specific forms has not
indefinitely increased, geology shows us plainly; and indeed we can
see reason why they should not have thus increased, for the number of
places in the polity of nature is not indefinitely great,--not that we
have any means of knowing that any one region has as yet got its
maximum of species. Probably no region is as yet fully stocked, for at
the Cape of Good Hope, where more species of plants are crowded
together than in any other quarter of the world, some foreign plants
have become naturalised, without causing, as far as we know, the
extinction of any natives.

Furthermore, the species which are most numerous in individuals will
have the best chance of producing within any given period favourable
variations. We have evidence of this, in the facts given in the second
chapter, showing that it is the common species which afford the
greatest number of recorded varieties, or incipient species. Hence,
rare species will be less quickly modified or improved within any
given period, and they will consequently be beaten in the race for
life by the modified descendants of the commoner species.

From these several considerations I think it inevitably follows, that
as new species in the course of time are formed through natural
selection, others will become rarer and rarer, and finally extinct.
The forms which stand in closest competition with those undergoing
modification and improvement, will naturally suffer most. And we have
seen in the chapter on the Struggle for Existence that it is the most
closely-allied forms,--varieties of the same species, and species of
the same genus or of related genera,--which, from having nearly the
same structure, constitution, and habits, generally come into the
severest competition with each other. Consequently, each new variety
or species, during the progress of its formation, will generally press
hardest on its nearest kindred, and tend to exterminate them. We see
the same process of extermination amongst our domesticated
productions, through the selection of improved forms by man. Many
curious instances could be given showing how quickly new breeds of
cattle, sheep, and other animals, and varieties of flowers, take the
place of older and inferior kinds. In Yorkshire, it is historically
known that the ancient black cattle were displaced by the long-horns,
and that these "were swept away by the short-horns" (I quote the words
of an agricultural writer) "as if by some murderous pestilence."

DIVERGENCE OF CHARACTER.

The principle, which I have designated by this term, is of high
importance on my theory, and explains, as I believe, several important
facts. In the first place, varieties, even strongly-marked ones,
though having somewhat of the character of species--as is shown by the
hopeless doubts in many cases how to rank them--yet certainly differ
from each other far less than do good and distinct species.
Nevertheless, according to my view, varieties are species in the
process of formation, or are, as I have called them, incipient
species. How, then, does the lesser difference between varieties
become augmented into the greater difference between species? That
this does habitually happen, we must infer from most of the
innumerable species throughout nature presenting well-marked
differences; whereas varieties, the supposed prototypes and parents of
future well-marked species, present slight and ill-defined
differences. Mere chance, as we may call it, might cause one variety
to differ in some character from its parents, and the offspring of
this variety again to differ from its parent in the very same
character and in a greater degree; but this alone would never account
for so habitual and large an amount of difference as that between
varieties of the same species and species of the same genus.

As has always been my practice, let us seek light on this head from
our domestic productions. We shall here find something analogous. A
fancier is struck by a pigeon having a slightly shorter beak; another
fancier is struck by a pigeon having a rather longer beak; and on the
acknowledged principle that "fanciers do not and will not admire a
medium standard, but like extremes," they both go on (as has actually
occurred with tumbler-pigeons) choosing and breeding from birds with
longer and longer beaks, or with shorter and shorter beaks. Again, we
may suppose that at an early period one man preferred swifter horses;
another stronger and more bulky horses. The early differences would be
very slight; in the course of time, from the continued selection of
swifter horses by some breeders, and of stronger ones by others, the
differences would become greater, and would be noted as forming two
sub-breeds; finally, after the lapse of centuries, the sub-breeds
would become converted into two well-established and distinct breeds.
As the differences slowly become greater, the inferior animals with
intermediate characters, being neither very swift nor very strong,
will have been neglected, and will have tended to disappear. Here,
then, we see in man's productions the action of what may be called the
principle of divergence, causing differences, at first barely
appreciable, steadily to increase, and the breeds to diverge in
character both from each other and from their common parent.

But how, it may be asked, can any analogous principle apply in nature?
I believe it can and does apply most efficiently, from the simple
circumstance that the more diversified the descendants from any one
species become in structure, constitution, and habits, by so much will
they be better enabled to seize on many and widely diversified places
in the polity of nature, and so be enabled to increase in numbers.

We can clearly see this in the case of animals with simple habits.
Take the case of a carnivorous quadruped, of which the number that can
be supported in any country has long ago arrived at its full average.
If its natural powers of increase be allowed to act, it can succeed in
increasing (the country not undergoing any change in its conditions)
only by its varying descendants seizing on places at present occupied
by other animals: some of them, for instance, being enabled to feed on
new kinds of prey, either dead or alive; some inhabiting new stations,
climbing trees, frequenting water, and some perhaps becoming less
carnivorous. The more diversified in habits and structure the
descendants of our carnivorous animal became, the more places they
would be enabled to occupy. What applies to one animal will apply
throughout all time to all animals--that is, if they vary--for
otherwise natural selection can do nothing. So it will be with plants.
It has been experimentally proved, that if a plot of ground be sown
with one species of grass, and a similar plot be sown with several
distinct genera of grasses, a greater number of plants and a greater
weight of dry herbage can thus be raised. The same has been found to
hold good when first one variety and then several mixed varieties of
wheat have been sown on equal spaces of ground. Hence, if any one
species of grass were to go on varying, and those varieties were
continually selected which differed from each other in at all the same
manner as distinct species and genera of grasses differ from each
other, a greater number of individual plants of this species of grass,
including its modified descendants, would succeed in living on the
same piece of ground. And we well know that each species and each
variety of grass is annually sowing almost countless seeds; and thus,
as it may be said, is striving its utmost to increase its numbers.
Consequently, I cannot doubt that in the course of many thousands of
generations, the most distinct varieties of any one species of grass
would always have the best chance of succeeding and of increasing in
numbers, and thus of supplanting the less distinct varieties; and
varieties, when rendered very distinct from each other, take the rank
of species.

The truth of the principle, that the greatest amount of life can be
supported by great diversification of structure, is seen under many
natural circumstances. In an extremely small area, especially if
freely open to immigration, and where the contest between individual
and individual must be severe, we always find great diversity in its
inhabitants. For instance, I found that a piece of turf, three feet by
four in size, which had been exposed for many years to exactly the
same conditions, supported twenty species of plants, and these
belonged to eighteen genera and to eight orders, which shows how much
these plants differed from each other. So it is with the plants and
insects on small and uniform islets; and so in small ponds of fresh
water. Farmers find that they can raise most food by a rotation of
plants belonging to the most different orders: nature follows what may
be called a simultaneous rotation. Most of the animals and plants
which live close round any small piece of ground, could live on it
(supposing it not to be in any way peculiar in its nature), and may be
said to be striving to the utmost to live there; but, it is seen, that
where they come into the closest competition with each other, the
advantages of diversification of structure, with the accompanying
differences of habit and constitution, determine that the inhabitants,
which thus jostle each other most closely, shall, as a general rule,
belong to what we call different genera and orders.

The same principle is seen in the naturalisation of plants through
man's agency in foreign lands. It might have been expected that the
plants which have succeeded in becoming naturalised in any land would
generally have been closely allied to the indigenes; for these are
commonly looked at as specially created and adapted for their own
country. It might, also, perhaps have been expected that naturalised
plants would have belonged to a few groups more especially adapted to
certain stations in their new homes. But the case is very different;
and Alph. De Candolle has well remarked in his great and admirable
work, that floras gain by naturalisation, proportionally with the
number of the native genera and species, far more in new genera than
in new species. To give a single instance: in the last edition of Dr.
Asa Gray's 'Manual of the Flora of the Northern United States,' 260
naturalised plants are enumerated, and these belong to 162 genera. We
thus see that these naturalised plants are of a highly diversified
nature. They differ, moreover, to a large extent from the indigenes,
for out of the 162 genera, no less than 100 genera are not there
indigenous, and thus a large proportional addition is made to the
genera of these States.

By considering the nature of the plants or animals which have
struggled successfully with the indigenes of any country, and have
there become naturalised, we can gain some crude idea in what manner
some of the natives would have had to be modified, in order to have
gained an advantage over the other natives; and we may, I think, at
least safely infer that diversification of structure, amounting to new
generic differences, would have been profitable to them.

The advantage of diversification in the inhabitants of the same region
is, in fact, the same as that of the physiological division of labour
in the organs of the same individual body--a subject so well
elucidated by Milne Edwards. No physiologist doubts that a stomach by
being adapted to digest vegetable matter alone, or flesh alone, draws
most nutriment from these substances. So in the general economy of any
land, the more widely and perfectly the animals and plants are
diversified for different habits of life, so will a greater number of
individuals be capable of there supporting themselves. A set of
animals, with their organisation but little diversified, could hardly
compete with a set more perfectly diversified in structure. It may be
doubted, for instance, whether the Australian marsupials, which are
divided into groups differing but little from each other, and feebly
representing, as Mr. Waterhouse and others have remarked, our
carnivorous, ruminant, and rodent mammals, could successfully compete
with these well-pronounced orders. In the Australian mammals, we see
the process of diversification in an early and incomplete stage of
development. After the foregoing discussion, which ought to have been
much amplified, we may, I think, assume that the modified descendants
of any one species will succeed by so much the better as they become
more diversified in structure, and are thus enabled to encroach on
places occupied by other beings. Now let us see how this principle of
great benefit being derived from divergence of character, combined
with the principles of natural selection and of extinction, will tend
to act.

The accompanying diagram will aid us in understanding this rather
perplexing subject. Let A to L represent the species of a genus large
in its own country; these species are supposed to resemble each other
in unequal degrees, as is so generally the case in nature, and as is
represented in the diagram by the letters standing at unequal
distances. I have said a large genus, because we have seen in the
second chapter, that on an average more of the species of large genera
vary than of small genera; and the varying species of the large genera
present a greater number of varieties. We have, also, seen that the
species, which are the commonest and the most widely-diffused, vary
more than rare species with restricted ranges. Let (A) be a common,
widely-diffused, and varying species, belonging to a genus large in
its own country. The little fan of diverging dotted lines of unequal
lengths proceeding from (A), may represent its varying offspring. The
variations are supposed to be extremely slight, but of the most
diversified nature; they are not supposed all to appear
simultaneously, but often after long intervals of time; nor are they
all supposed to endure for equal periods. Only those variations which
are in some way profitable will be preserved or naturally selected.
And here the importance of the principle of benefit being derived from
divergence of character comes in; for this will generally lead to the
most different or divergent variations (represented by the outer
dotted lines) being preserved and accumulated by natural selection.
When a dotted line reaches one of the horizontal lines, and is there
marked by a small numbered letter, a sufficient amount of variation is
supposed to have been accumulated to have formed a fairly well-marked
variety, such as would be thought worthy of record in a systematic
work.

The intervals between the horizontal lines in the diagram, may
represent each a thousand generations; but it would have been better
if each had represented ten thousand generations. After a thousand
generations, species (A) is supposed to have produced two fairly
well-marked varieties, namely a1 and m1. These two varieties will
generally continue to be exposed to the same conditions which made
their parents variable, and the tendency to variability is in itself
hereditary, consequently they will tend to vary, and generally to vary
in nearly the same manner as their parents varied. Moreover, these two
varieties, being only slightly modified forms, will tend to inherit
those advantages which made their common parent (A) more numerous than
most of the other inhabitants of the same country; they will likewise
partake of those more general advantages which made the genus to which
the parent-species belonged, a large genus in its own country. And
these circumstances we know to be favourable to the production of new
varieties.

If, then, these two varieties be variable, the most divergent of their
variations will generally be preserved during the next thousand
generations. And after this interval, variety a1 is supposed in the
diagram to have produced variety a2, which will, owing to the
principle of divergence, differ more from (A) than did variety a1.
Variety m1 is supposed to have produced two varieties, namely m2 and
s2, differing from each other, and more considerably from their common
parent (A). We may continue the process by similar steps for any
length of time; some of the varieties, after each thousand
generations, producing only a single variety, but in a more and more
modified condition, some producing two or three varieties, and some
failing to produce any. Thus the varieties or modified descendants,
proceeding from the common parent (A), will generally go on increasing
in number and diverging in character. In the diagram the process is
represented up to the ten-thousandth generation, and under a condensed
and simplified form up to the fourteen-thousandth generation.

But I must here remark that I do not suppose that the process ever
goes on so regularly as is represented in the diagram, though in
itself made somewhat irregular. I am far from thinking that the most
divergent varieties will invariably prevail and multiply: a medium
form may often long endure, and may or may not produce more than one
modified descendant; for natural selection will always act according
to the nature of the places which are either unoccupied or not
perfectly occupied by other beings; and this will depend on infinitely
complex relations. But as a general rule, the more diversified in
structure the descendants from any one species can be rendered, the
more places they will be enabled to seize on, and the more their
modified progeny will be increased. In our diagram the line of
succession is broken at regular intervals by small numbered letters
marking the successive forms which have become sufficiently distinct
to be recorded as varieties. But these breaks are imaginary, and might
have been inserted anywhere, after intervals long enough to have
allowed the accumulation of a considerable amount of divergent
variation.

As all the modified descendants from a common and widely-diffused
species, belonging to a large genus, will tend to partake of the same
advantages which made their parent successful in life, they will
generally go on multiplying in number as well as diverging in
character: this is represented in the diagram by the several divergent
branches proceeding from (A). The modified offspring from the later
and more highly improved branches in the lines of descent, will, it is
probable, often take the place of, and so destroy, the earlier and
less improved branches: this is represented in the diagram by some of
the lower branches not reaching to the upper horizontal lines. In some
cases I do not doubt that the process of modification will be confined
to a single line of descent, and the number of the descendants will
not be increased; although the amount of divergent modification may
have been increased in the successive generations. This case would be
represented in the diagram, if all the lines proceeding from (A) were
removed, excepting that from a1 to a10. In the same way, for instance,
the English race-horse and English pointer have apparently both gone
on slowly diverging in character from their original stocks, without
either having given off any fresh branches or races.

After ten thousand generations, species (A) is supposed to have
produced three forms, a10, f10, and m10, which, from having diverged
in character during the successive generations, will have come to
differ largely, but perhaps unequally, from each other and from their
common parent. If we suppose the amount of change between each
horizontal line in our diagram to be excessively small, these three
forms may still be only well-marked varieties; or they may have
arrived at the doubtful category of sub-species; but we have only to
suppose the steps in the process of modification to be more numerous
or greater in amount, to convert these three forms into well-defined
species: thus the diagram illustrates the steps by which the small
differences distinguishing varieties are increased into the larger
differences distinguishing species. By continuing the same process for
a greater number of generations (as shown in the diagram in a
condensed and simplified manner), we get eight species, marked by the
letters between a14 and m14, all descended from (A). Thus, as I
believe, species are multiplied and genera are formed.

In a large genus it is probable that more than one species would vary.
In the diagram I have assumed that a second species (I) has produced,
by analogous steps, after ten thousand generations, either two
well-marked varieties (w10 and z10) or two species, according to the
amount of change supposed to be represented between the horizontal
lines. After fourteen thousand generations, six new species, marked by
the letters n14 to z14, are supposed to have been produced. In each
genus, the species, which are already extremely different in
character, will generally tend to produce the greatest number of
modified descendants; for these will have the best chance of filling
new and widely different places in the polity of nature: hence in the
diagram I have chosen the extreme species (A), and the nearly extreme
species (I), as those which have largely varied, and have given rise
to new varieties and species. The other nine species (marked by
capital letters) of our original genus, may for a long period continue
transmitting unaltered descendants; and this is shown in the diagram
by the dotted lines not prolonged far upwards from want of space.

But during the process of modification, represented in the diagram,
another of our principles, namely that of extinction, will have played
an important part. As in each fully stocked country natural selection
necessarily acts by the selected form having some advantage in the
struggle for life over other forms, there will be a constant tendency
in the improved descendants of any one species to supplant and
exterminate in each stage of descent their predecessors and their
original parent. For it should be remembered that the competition will
generally be most severe between those forms which are most nearly
related to each other in habits, constitution, and structure. Hence
all the intermediate forms between the earlier and later states, that
is between the less and more improved state of a species, as well as
the original parent-species itself, will generally tend to become
extinct. So it probably will be with many whole collateral lines of
descent, which will be conquered by later and improved lines of
descent. If, however, the modified offspring of a species get into
some distinct country, or become quickly adapted to some quite new
station, in which child and parent do not come into competition, both
may continue to exist.

If then our diagram be assumed to represent a considerable amount of
modification, species (A) and all the earlier varieties will have
become extinct, having been replaced by eight new species (a14 to
m14); and (I) will have been replaced by six (n14 to z14) new species.

But we may go further than this. The original species of our genus
were supposed to resemble each other in unequal degrees, as is so
generally the case in nature; species (A) being more nearly related to
B, C, and D, than to the other species; and species (I) more to G, H,
K, L, than to the others. These two species (A) and (I), were also
supposed to be very common and widely diffused species, so that they
must originally have had some advantage over most of the other species
of the genus. Their modified descendants, fourteen in number at the
fourteen-thousandth generation, will probably have inherited some of
the same advantages: they have also been modified and improved in a
diversified manner at each stage of descent, so as to have become
adapted to many related places in the natural economy of their
country. It seems, therefore, to me extremely probable that they will
have taken the places of, and thus exterminated, not only their
parents (A) and (I), but likewise some of the original species which
were most nearly related to their parents. Hence very few of the
original species will have transmitted offspring to the
fourteen-thousandth generation. We may suppose that only one (F), of
the two species which were least closely related to the other nine
original species, has transmitted descendants to this late stage of
descent.

The new species in our diagram descended from the original eleven
species, will now be fifteen in number. Owing to the divergent
tendency of natural selection, the extreme amount of difference in
character between species a14 and z14 will be much greater than that
between the most different of the original eleven species. The new
species, moreover, will be allied to each other in a widely different
manner. Of the eight descendants from (A) the three marked a14, q14,
p14, will be nearly related from having recently branched off from
a10; b14 and f14, from having diverged at an earlier period from a5,
will be in some degree distinct from the three first-named species;
and lastly, o14, e14, and m14, will be nearly related one to the
other, but from having diverged at the first commencement of the
process of modification, will be widely different from the other five
species, and may constitute a sub-genus or even a distinct genus.

The six descendants from (I) will form two sub-genera or even genera.
But as the original species (I) differed largely from (A), standing
nearly at the extreme points of the original genus, the six
descendants from (I) will, owing to inheritance, differ considerably
from the eight descendants from (A); the two groups, moreover, are
supposed to have gone on diverging in different directions. The
intermediate species, also (and this is a very important
consideration), which connected the original species (A) and (I), have
all become, excepting (F), extinct, and have left no descendants.
Hence the six new species descended from (I), and the eight descended
from (A), will have to be ranked as very distinct genera, or even as
distinct sub-families.

Thus it is, as I believe, that two or more genera are produced by
descent, with modification, from two or more species of the same
genus. And the two or more parent-species are supposed to have
descended from some one species of an earlier genus. In our diagram,
this is indicated by the broken lines, beneath the capital letters,
converging in sub-branches downwards towards a single point; this
point representing a single species, the supposed single parent of our
several new sub-genera and genera.

It is worth while to reflect for a moment on the character of the new
species F14, which is supposed not to have diverged much in character,
but to have retained the form of (F), either unaltered or altered only
in a slight degree. In this case, its affinities to the other fourteen
new species will be of a curious and circuitous nature. Having
descended from a form which stood between the two parent-species (A)
and (I), now supposed to be extinct and unknown, it will be in some
degree intermediate in character between the two groups descended from
these species. But as these two groups have gone on diverging in
character from the type of their parents, the new species (F14) will
not be directly intermediate between them, but rather between types of
the two groups; and every naturalist will be able to bring some such
case before his mind.

In the diagram, each horizontal line has hitherto been supposed to
represent a thousand generations, but each may represent a million or
hundred million generations, and likewise a section of the successive
strata of the earth's crust including extinct remains. We shall, when
we come to our chapter on Geology, have to refer again to this
subject, and I think we shall then see that the diagram throws light
on the affinities of extinct beings, which, though generally belonging
to the same orders, or families, or genera, with those now living, yet
are often, in some degree, intermediate in character between existing
groups; and we can understand this fact, for the extinct species lived
at very ancient epochs when the branching lines of descent had
diverged less.

I see no reason to limit the process of modification, as now
explained, to the formation of genera alone. If, in our diagram, we
suppose the amount of change represented by each successive group of
diverging dotted lines to be very great, the forms marked a14 to p14,
those marked b14 and f14, and those marked o14 to m14, will form three
very distinct genera. We shall also have two very distinct genera
descended from (I) and as these latter two genera, both from continued
divergence of character and from inheritance from a different parent,
will differ widely from the three genera descended from (A), the two
little groups of genera will form two distinct families, or even
orders, according to the amount of divergent modification supposed to
be represented in the diagram. And the two new families, or orders,
will have descended from two species of the original genus; and these
two species are supposed to have descended from one species of a still
more ancient and unknown genus.

We have seen that in each country it is the species of the larger
genera which oftenest present varieties or incipient species. This,
indeed, might have been expected; for as natural selection acts
through one form having some advantage over other forms in the
struggle for existence, it will chiefly act on those which already
have some advantage; and the largeness of any group shows that its
species have inherited from a common ancestor some advantage in
common. Hence, the struggle for the production of new and modified
descendants, will mainly lie between the larger groups, which are all
trying to increase in number. One large group will slowly conquer
another large group, reduce its numbers, and thus lessen its chance of
further variation and improvement. Within the same large group, the
later and more highly perfected sub-groups, from branching out and
seizing on many new places in the polity of Nature, will constantly
tend to supplant and destroy the earlier and less improved sub-groups.
Small and broken groups and sub-groups will finally tend to disappear.
Looking to the future, we can predict that the groups of organic
beings which are now large and triumphant, and which are least broken
up, that is, which as yet have suffered least extinction, will for a
long period continue to increase. But which groups will ultimately
prevail, no man can predict; for we well know that many groups,
formerly most extensively developed, have now become extinct. Looking
still more remotely to the future, we may predict that, owing to the
continued and steady increase of the larger groups, a multitude of
smaller groups will become utterly extinct, and leave no modified
descendants; and consequently that of the species living at any one
period, extremely few will transmit descendants to a remote futurity.
I shall have to return to this subject in the chapter on
Classification, but I may add that on this view of extremely few of
the more ancient species having transmitted descendants, and on the
view of all the descendants of the same species making a class, we can
understand how it is that there exist but very few classes in each
main division of the animal and vegetable kingdoms. Although extremely
few of the most ancient species may now have living and modified
descendants, yet at the most remote geological period, the earth may
have been as well peopled with many species of many genera, families,
orders, and classes, as at the present day.

SUMMARY OF CHAPTER.

If during the long course of ages and under varying conditions of
life, organic beings vary at all in the several parts of their
organisation, and I think this cannot be disputed; if there be, owing
to the high geometrical powers of increase of each species, at some
age, season, or year, a severe struggle for life, and this certainly
cannot be disputed; then, considering the infinite complexity of the
relations of all organic beings to each other and to their conditions
of existence, causing an infinite diversity in structure,
constitution, and habits, to be advantageous to them, I think it would
be a most extraordinary fact if no variation ever had occurred useful
to each being's own welfare, in the same way as so many variations
have occurred useful to man. But if variations useful to any organic
being do occur, assuredly individuals thus characterised will have the
best chance of being preserved in the struggle for life; and from the
strong principle of inheritance they will tend to produce offspring
similarly characterised. This principle of preservation, I have
called, for the sake of brevity, Natural Selection. Natural selection,
on the principle of qualities being inherited at corresponding ages,
can modify the egg, seed, or young, as easily as the adult. Amongst
many animals, sexual selection will give its aid to ordinary
selection, by assuring to the most vigorous and best adapted males the
greatest number of offspring. Sexual selection will also give
characters useful to the males alone, in their struggles with other
males.

Whether natural selection has really thus acted in nature, in
modifying and adapting the various forms of life to their several
conditions and stations, must be judged of by the general tenour and
balance of evidence given in the following chapters. But we already
see how it entails extinction; and how largely extinction has acted in
the world's history, geology plainly declares. Natural selection,
also, leads to divergence of character; for more living beings can be
supported on the same area the more they diverge in structure, habits,
and constitution, of which we see proof by looking at the inhabitants
of any small spot or at naturalised productions. Therefore during the
modification of the descendants of any one species, and during the
incessant struggle of all species to increase in numbers, the more
diversified these descendants become, the better will be their chance
of succeeding in the battle of life. Thus the small differences
distinguishing varieties of the same species, will steadily tend to
increase till they come to equal the greater differences between
species of the same genus, or even of distinct genera.

We have seen that it is the common, the widely-diffused, and
widely-ranging species, belonging to the larger genera, which vary
most; and these will tend to transmit to their modified offspring that
superiority which now makes them dominant in their own countries.
Natural selection, as has just been remarked, leads to divergence of
character and to much extinction of the less improved and intermediate
forms of life. On these principles, I believe, the nature of the
affinities of all organic beings may be explained. It is a truly
wonderful fact--the wonder of which we are apt to overlook from
familiarity--that all animals and all plants throughout all time and
space should be related to each other in group subordinate to group,
in the manner which we everywhere behold--namely, varieties of the
same species most closely related together, species of the same genus
less closely and unequally related together, forming sections and
sub-genera, species of distinct genera much less closely related, and
genera related in different degrees, forming sub-families, families,
orders, sub-classes, and classes. The several subordinate groups in
any class cannot be ranked in a single file, but seem rather to be
clustered round points, and these round other points, and so on in
almost endless cycles. On the view that each species has been
independently created, I can see no explanation of this great fact in
the classification of all organic beings; but, to the best of my
judgment, it is explained through inheritance and the complex action
of natural selection, entailing extinction and divergence of
character, as we have seen illustrated in the diagram.

The affinities of all the beings of the same class have sometimes been
represented by a great tree. I believe this simile largely speaks the
truth. The green and budding twigs may represent existing species; and
those produced during each former year may represent the long
succession of extinct species. At each period of growth all the
growing twigs have tried to branch out on all sides, and to overtop
and kill the surrounding twigs and branches, in the same manner as
species and groups of species have tried to overmaster other species
in the great battle for life. The limbs divided into great branches,
and these into lesser and lesser branches, were themselves once, when
the tree was small, budding twigs; and this connexion of the former
and present buds by ramifying branches may well represent the
classification of all extinct and living species in groups subordinate
to groups. Of the many twigs which flourished when the tree was a mere
bush, only two or three, now grown into great branches, yet survive
and bear all the other branches; so with the species which lived
during long-past geological periods, very few now have living and
modified descendants. From the first growth of the tree, many a limb
and branch has decayed and dropped off; and these lost branches of
various sizes may represent those whole orders, families, and genera
which have now no living representatives, and which are known to us
only from having been found in a fossil state. As we here and there
see a thin straggling branch springing from a fork low down in a tree,
and which by some chance has been favoured and is still alive on its
summit, so we occasionally see an animal like the Ornithorhynchus or
Lepidosiren, which in some small degree connects by its affinities two
large branches of life, and which has apparently been saved from fatal
competition by having inhabited a protected station. As buds give rise
by growth to fresh buds, and these, if vigorous, branch out and
overtop on all sides many a feebler branch, so by generation I believe
it has been with the great Tree of Life, which fills with its dead and
broken branches the crust of the earth, and covers the surface with
its ever branching and beautiful ramifications.


CHAPTER 5. LAWS OF VARIATION.

Effects of external conditions.
Use and disuse, combined with natural selection; organs of flight and
of vision.
Acclimatisation.
Correlation of growth.
Compensation and economy of growth.
False correlations.
Multiple, rudimentary, and lowly organised structures variable.
Parts developed in an unusual manner are highly variable: specific
characters more variable than generic: secondary sexual characters
variable.
Species of the same genus vary in an analogous manner.
Reversions to long lost characters.
Summary.

I have hitherto sometimes spoken as if the variations--so common and
multiform in organic beings under domestication, and in a lesser
degree in those in a state of nature--had been due to chance. This, of
course, is a wholly incorrect expression, but it serves to acknowledge
plainly our ignorance of the cause of each particular variation. Some
authors believe it to be as much the function of the reproductive
system to produce individual differences, or very slight deviations of
structure, as to make the child like its parents. But the much greater
variability, as well as the greater frequency of monstrosities, under
domestication or cultivation, than under nature, leads me to believe
that deviations of structure are in some way due to the nature of the
conditions of life, to which the parents and their more remote
ancestors have been exposed during several generations. I have
remarked in the first chapter--but a long catalogue of facts which
cannot be here given would be necessary to show the truth of the
remark--that the reproductive system is eminently susceptible to
changes in the conditions of life; and to this system being
functionally disturbed in the parents, I chiefly attribute the varying
or plastic condition of the offspring. The male and female sexual
elements seem to be affected before that union takes place which is to
form a new being. In the case of "sporting" plants, the bud, which in
its earliest condition does not apparently differ essentially from an
ovule, is alone affected. But why, because the reproductive system is
disturbed, this or that part should vary more or less, we are
profoundly ignorant. Nevertheless, we can here and there dimly catch a
faint ray of light, and we may feel sure that there must be some cause
for each deviation of structure, however slight.

How much direct effect difference of climate, food, etc., produces on
any being is extremely doubtful. My impression is, that the effect is
extremely small in the case of animals, but perhaps rather more in
that of plants. We may, at least, safely conclude that such influences
cannot have produced the many striking and complex co-adaptations of
structure between one organic being and another, which we see
everywhere throughout nature. Some little influence may be attributed
to climate, food, etc.: thus, E. Forbes speaks confidently that shells
at their southern limit, and when living in shallow water, are more
brightly coloured than those of the same species further north or from
greater depths. Gould believes that birds of the same species are more
brightly coloured under a clear atmosphere, than when living on
islands or near the coast. So with insects, Wollaston is convinced
that residence near the sea affects their colours. Moquin-Tandon gives
a list of plants which when growing near the sea-shore have their
leaves in some degree fleshy, though not elsewhere fleshy. Several
other such cases could be given.

The fact of varieties of one species, when they range into the zone of
habitation of other species, often acquiring in a very slight degree
some of the characters of such species, accords with our view that
species of all kinds are only well-marked and permanent varieties.
Thus the species of shells which are confined to tropical and shallow
seas are generally brighter-coloured than those confined to cold and
deeper seas. The birds which are confined to continents are, according
to Mr. Gould, brighter-coloured than those of islands. The
insect-species confined to sea-coasts, as every collector knows, are
often brassy or lurid. Plants which live exclusively on the sea-side
are very apt to have fleshy leaves. He who believes in the creation of
each species, will have to say that this shell, for instance, was
created with bright colours for a warm sea; but that this other shell
became bright-coloured by variation when it ranged into warmer or
shallower waters.

When a variation is of the slightest use to a being, we cannot tell
how much of it to attribute to the accumulative action of natural
selection, and how much to the conditions of life. Thus, it is well
known to furriers that animals of the same species have thicker and
better fur the more severe the climate is under which they have lived;
but who can tell how much of this difference may be due to the
warmest-clad individuals having been favoured and preserved during
many generations, and how much to the direct action of the severe
climate? for it would appear that climate has some direct action on
the hair of our domestic quadrupeds.

Instances could be given of the same variety being produced under
conditions of life as different as can well be conceived; and, on the
other hand, of different varieties being produced from the same
species under the same conditions. Such facts show how indirectly the
conditions of life must act. Again, innumerable instances are known to
every naturalist of species keeping true, or not varying at all,
although living under the most opposite climates. Such considerations
as these incline me to lay very little weight on the direct action of
the conditions of life. Indirectly, as already remarked, they seem to
play an important part in affecting the reproductive system, and in
thus inducing variability; and natural selection will then accumulate
all profitable variations, however slight, until they become plainly
developed and appreciable by us.

EFFECTS OF USE AND DISUSE.

From the facts alluded to in the first chapter, I think there can be
little doubt that use in our domestic animals strengthens and enlarges
certain parts, and disuse diminishes them; and that such modifications
are inherited. Under free nature, we can have no standard of
comparison, by which to judge of the effects of long-continued use or
disuse, for we know not the parent-forms; but many animals have
structures which can be explained by the effects of disuse. As
Professor Owen has remarked, there is no greater anomaly in nature
than a bird that cannot fly; yet there are several in this state. The
logger-headed duck of South America can only flap along the surface of
the water, and has its wings in nearly the same condition as the
domestic Aylesbury duck. As the larger ground-feeding birds seldom
take flight except to escape danger, I believe that the nearly
wingless condition of several birds, which now inhabit or have lately
inhabited several oceanic islands, tenanted by no beast of prey, has
been caused by disuse. The ostrich indeed inhabits continents and is
exposed to danger from which it cannot escape by flight, but by
kicking it can defend itself from enemies, as well as any of the
smaller quadrupeds. We may imagine that the early progenitor of the
ostrich had habits like those of a bustard, and that as natural
selection increased in successive generations the size and weight of
its body, its legs were used more, and its wings less, until they
became incapable of flight.

Kirby has remarked (and I have observed the same fact) that the
anterior tarsi, or feet, of many male dung-feeding beetles are very
often broken off; he examined seventeen specimens in his own
collection, and not one had even a relic left. In the Onites apelles
the tarsi are so habitually lost, that the insect has been described
as not having them. In some other genera they are present, but in a
rudimentary condition. In the Ateuchus or sacred beetle of the
Egyptians, they are totally deficient. There is not sufficient
evidence to induce us to believe that mutilations are ever inherited;
and I should prefer explaining the entire absence of the anterior
tarsi in Ateuchus, and their rudimentary condition in some other
genera, by the long-continued effects of disuse in their progenitors;
for as the tarsi are almost always lost in many dung-feeding beetles,
they must be lost early in life, and therefore cannot be much used by
these insects.

In some cases we might easily put down to disuse modifications of
structure which are wholly, or mainly, due to natural selection. Mr.
Wollaston has discovered the remarkable fact that 200 beetles, out of
the 550 species inhabiting Madeira, are so far deficient in wings that
they cannot fly; and that of the twenty-nine endemic genera, no less
than twenty-three genera have all their species in this condition!
Several facts, namely, that beetles in many parts of the world are
very frequently blown to sea and perish; that the beetles in Madeira,
as observed by Mr. Wollaston, lie much concealed, until the wind lulls
and the sun shines; that the proportion of wingless beetles is larger
on the exposed Dezertas than in Madeira itself; and especially the
extraordinary fact, so strongly insisted on by Mr. Wollaston, of the
almost entire absence of certain large groups of beetles, elsewhere
excessively numerous, and which groups have habits of life almost
necessitating frequent flight;--these several considerations have made
me believe that the wingless condition of so many Madeira beetles is
mainly due to the action of natural selection, but combined probably
with disuse. For during thousands of successive generations each
individual beetle which flew least, either from its wings having been
ever so little less perfectly developed or from indolent habit, will
have had the best chance of surviving from not being blown out to sea;
and, on the other hand, those beetles which most readily took to
flight will oftenest have been blown to sea and thus have been
destroyed.

The insects in Madeira which are not ground-feeders, and which, as the
flower-feeding coleoptera and lepidoptera, must habitually use their
wings to gain their subsistence, have, as Mr. Wollaston suspects,
their wings not at all reduced, but even enlarged. This is quite
compatible with the action of natural selection. For when a new insect
first arrived on the island, the tendency of natural selection to
enlarge or to reduce the wings, would depend on whether a greater
number of individuals were saved by successfully battling with the
winds, or by giving up the attempt and rarely or never flying. As with
mariners shipwrecked near a coast, it would have been better for the
good swimmers if they had been able to swim still further, whereas it
would have been better for the bad swimmers if they had not been able
to swim at all and had stuck to the wreck.

The eyes of moles and of some burrowing rodents are rudimentary in
size, and in some cases are quite covered up by skin and fur. This
state of the eyes is probably due to gradual reduction from disuse,
but aided perhaps by natural selection. In South America, a burrowing
rodent, the tuco-tuco, or Ctenomys, is even more subterranean in its
habits than the mole; and I was assured by a Spaniard, who had often
caught them, that they were frequently blind; one which I kept alive
was certainly in this condition, the cause, as appeared on dissection,
having been inflammation of the nictitating membrane. As frequent
inflammation of the eyes must be injurious to any animal, and as eyes
are certainly not indispensable to animals with subterranean habits, a
reduction in their size with the adhesion of the eyelids and growth of
fur over them, might in such case be an advantage; and if so, natural
selection would constantly aid the effects of disuse.

It is well known that several animals, belonging to the most different
classes, which inhabit the caves of Styria and of Kentucky, are blind.
In some of the crabs the foot-stalk for the eye remains, though the
eye is gone; the stand for the telescope is there, though the
telescope with its glasses has been lost. As it is difficult to
imagine that eyes, though useless, could be in any way injurious to
animals living in darkness, I attribute their loss wholly to disuse.
In one of the blind animals, namely, the cave-rat, the eyes are of
immense size; and Professor Silliman thought that it regained, after
living some days in the light, some slight power of vision. In the
same manner as in Madeira the wings of some of the insects have been
enlarged, and the wings of others have been reduced by natural
selection aided by use and disuse, so in the case of the cave-rat
natural selection seems to have struggled with the loss of light and
to have increased the size of the eyes; whereas with all the other
inhabitants of the caves, disuse by itself seems to have done its
work.

It is difficult to imagine conditions of life more similar than deep
limestone caverns under a nearly similar climate; so that on the
common view of the blind animals having been separately created for
the American and European caverns, close similarity in their
organisation and affinities might have been expected; but, as Schiodte
and others have remarked, this is not the case, and the cave-insects
of the two continents are not more closely allied than might have been
anticipated from the general resemblance of the other inhabitants of
North America and Europe. On my view we must suppose that American
animals, having ordinary powers of vision, slowly migrated by
successive generations from the outer world into the deeper and deeper
recesses of the Kentucky caves, as did European animals into the caves
of Europe. We have some evidence of this gradation of habit; for, as
Schiodte remarks, "animals not far remote from ordinary forms, prepare
the transition from light to darkness. Next follow those that are
constructed for twilight; and, last of all, those destined for total
darkness." By the time that an animal had reached, after numberless
generations, the deepest recesses, disuse will on this view have more
or less perfectly obliterated its eyes, and natural selection will
often have effected other changes, such as an increase in the length
of the antennae or palpi, as a compensation for blindness.
Notwithstanding such modifications, we might expect still to see in
the cave-animals of America, affinities to the other inhabitants of
that continent, and in those of Europe, to the inhabitants of the
European continent. And this is the case with some of the American
cave-animals, as I hear from Professor Dana; and some of the European
cave-insects are very closely allied to those of the surrounding
country. It would be most difficult to give any rational explanation
of the affinities of the blind cave-animals to the other inhabitants
of the two continents on the ordinary view of their independent
creation. That several of the inhabitants of the caves of the Old and
New Worlds should be closely related, we might expect from the
well-known relationship of most of their other productions. Far from
feeling any surprise that some of the cave-animals should be very
anomalous, as Agassiz has remarked in regard to the blind fish, the
Amblyopsis, and as is the case with the blind Proteus with reference
to the reptiles of Europe, I am only surprised that more wrecks of
ancient life have not been preserved, owing to the less severe
competition to which the inhabitants of these dark abodes will
probably have been exposed.

ACCLIMATISATION.

Habit is hereditary with plants, as in the period of flowering, in the
amount of rain requisite for seeds to germinate, in the time of sleep,
etc., and this leads me to say a few words on acclimatisation. As it
is extremely common for species of the same genus to inhabit very hot
and very cold countries, and as I believe that all the species of the
same genus have descended from a single parent, if this view be
correct, acclimatisation must be readily effected during
long-continued descent. It is notorious that each species is adapted
to the climate of its own home: species from an arctic or even from a
temperate region cannot endure a tropical climate, or conversely. So
again, many succulent plants cannot endure a damp climate. But the
degree of adaptation of species to the climates under which they live
is often overrated. We may infer this from our frequent inability to
predict whether or not an imported plant will endure our climate, and
from the number of plants and animals brought from warmer countries
which here enjoy good health. We have reason to believe that species
in a state of nature are limited in their ranges by the competition of
other organic beings quite as much as, or more than, by adaptation to
particular climates. But whether or not the adaptation be generally
very close, we have evidence, in the case of some few plants, of their
becoming, to a certain extent, naturally habituated to different
temperatures, or becoming acclimatised: thus the pines and
rhododendrons, raised from seed collected by Dr. Hooker from trees
growing at different heights on the Himalaya, were found in this
country to possess different constitutional powers of resisting cold.
Mr. Thwaites informs me that he has observed similar facts in Ceylon,
and analogous observations have been made by Mr. H. C. Watson on
European species of plants brought from the Azores to England. In
regard to animals, several authentic cases could be given of species
within historical times having largely extended their range from
warmer to cooler latitudes, and conversely; but we do not positively
know that these animals were strictly adapted to their native climate,
but in all ordinary cases we assume such to be the case; nor do we
know that they have subsequently become acclimatised to their new
homes.

As I believe that our domestic animals were originally chosen by
uncivilised man because they were useful and bred readily under
confinement, and not because they were subsequently found capable of
far-extended transportation, I think the common and extraordinary
capacity in our domestic animals of not only withstanding the most
different climates but of being perfectly fertile (a far severer test)
under them, may be used as an argument that a large proportion of
other animals, now in a state of nature, could easily be brought to
bear widely different climates. We must not, however, push the
foregoing argument too far, on account of the probable origin of some
of our domestic animals from several wild stocks: the blood, for
instance, of a tropical and arctic wolf or wild dog may perhaps be
mingled in our domestic breeds. The rat and mouse cannot be considered
as domestic animals, but they have been transported by man to many
parts of the world, and now have a far wider range than any other
rodent, living free under the cold climate of Faroe in the north and
of the Falklands in the south, and on many islands in the torrid
zones. Hence I am inclined to look at adaptation to any special
climate as a quality readily grafted on an innate wide flexibility of
constitution, which is common to most animals. On this view, the
capacity of enduring the most different climates by man himself and by
his domestic animals, and such facts as that former species of the
elephant and rhinoceros were capable of enduring a glacial climate,
whereas the living species are now all tropical or sub-tropical in
their habits, ought not to be looked at as anomalies, but merely as
examples of a very common flexibility of constitution, brought, under
peculiar circumstances, into play.

How much of the acclimatisation of species to any peculiar climate is
due to mere habit, and how much to the natural selection of varieties
having different innate constitutions, and how much to both means
combined, is a very obscure question. That habit or custom has some
influence I must believe, both from analogy, and from the incessant
advice given in agricultural works, even in the ancient Encyclopaedias
of China, to be very cautious in transposing animals from one district
to another; for it is not likely that man should have succeeded in
selecting so many breeds and sub-breeds with constitutions specially
fitted for their own districts: the result must, I think, be due to
habit. On the other hand, I can see no reason to doubt that natural
selection will continually tend to preserve those individuals which
are born with constitutions best adapted to their native countries. In
treatises on many kinds of cultivated plants, certain varieties are
said to withstand certain climates better than others: this is very
strikingly shown in works on fruit trees published in the United
States, in which certain varieties are habitually recommended for the
northern, and others for the southern States; and as most of these
varieties are of recent origin, they cannot owe their constitutional
differences to habit. The case of the Jerusalem artichoke, which is
never propagated by seed, and of which consequently new varieties have
not been produced, has even been advanced--for it is now as tender as
ever it was--as proving that acclimatisation cannot be effected! The
case, also, of the kidney-bean has been often cited for a similar
purpose, and with much greater weight; but until some one will sow,
during a score of generations, his kidney-beans so early that a very
large proportion are destroyed by frost, and then collect seed from
the few survivors, with care to prevent accidental crosses, and then
again get seed from these seedlings, with the same precautions, the
experiment cannot be said to have been even tried. Nor let it be
supposed that no differences in the constitution of seedling
kidney-beans ever appear, for an account has been published how much
more hardy some seedlings appeared to be than others.

On the whole, I think we may conclude that habit, use, and disuse,
have, in some cases, played a considerable part in the modification of
the constitution, and of the structure of various organs; but that the
effects of use and disuse have often been largely combined with, and
sometimes overmastered by, the natural selection of innate
differences.

CORRELATION OF GROWTH.

I mean by this expression that the whole organisation is so tied
together during its growth and development, that when slight
variations in any one part occur, and are accumulated through natural
selection, other parts become modified. This is a very important
subject, most imperfectly understood. The most obvious case is, that
modifications accumulated solely for the good of the young or larva,
will, it may safely be concluded, affect the structure of the adult;
in the same manner as any malconformation affecting the early embryo,
seriously affects the whole organisation of the adult. The several
parts of the body which are homologous, and which, at an early
embryonic period, are alike, seem liable to vary in an allied manner:
we see this in the right and left sides of the body varying in the
same manner; in the front and hind legs, and even in the jaws and
limbs, varying together, for the lower jaw is believed to be
homologous with the limbs. These tendencies, I do not doubt, may be
mastered more or less completely by natural selection: thus a family
of stags once existed with an antler only on one side; and if this had
been of any great use to the breed it might probably have been
rendered permanent by natural selection.

Homologous parts, as has been remarked by some authors, tend to
cohere; this is often seen in monstrous plants; and nothing is more
common than the union of homologous parts in normal structures, as the
union of the petals of the corolla into a tube. Hard parts seem to
affect the form of adjoining soft parts; it is believed by some
authors that the diversity in the shape of the pelvis in birds causes
the remarkable diversity in the shape of their kidneys. Others believe
that the shape of the pelvis in the human mother influences by
pressure the shape of the head of the child. In snakes, according to
Schlegel, the shape of the body and the manner of swallowing determine
the position of several of the most important viscera.

The nature of the bond of correlation is very frequently quite
obscure. M. Is. Geoffroy St. Hilaire has forcibly remarked, that
certain malconformations very frequently, and that others rarely
coexist, without our being able to assign any reason. What can be more
singular than the relation between blue eyes and deafness in cats, and
the tortoise-shell colour with the female sex; the feathered feet and
skin between the outer toes in pigeons, and the presence of more or
less down on the young birds when first hatched, with the future
colour of their plumage; or, again, the relation between the hair and
teeth in the naked Turkish dog, though here probably homology comes
into play? With respect to this latter case of correlation, I think it
can hardly be accidental, that if we pick out the two orders of
mammalia which are most abnormal in their dermal coverings, viz.
Cetacea (whales) and Edentata (armadilloes, scaly ant-eaters, etc.),
that these are likewise the most abnormal in their teeth.

I know of no case better adapted to show the importance of the laws of
correlation in modifying important structures, independently of
utility and, therefore, of natural selection, than that of the
difference between the outer and inner flowers in some Compositous and
Umbelliferous plants. Every one knows the difference in the ray and
central florets of, for instance, the daisy, and this difference is
often accompanied with the abortion of parts of the flower. But, in
some Compositous plants, the seeds also differ in shape and sculpture;
and even the ovary itself, with its accessory parts, differs, as has
been described by Cassini. These differences have been attributed by
some authors to pressure, and the shape of the seeds in the
ray-florets in some Compositae countenances this idea; but, in the
case of the corolla of the Umbelliferae, it is by no means, as Dr.
Hooker informs me, in species with the densest heads that the inner
and outer flowers most frequently differ. It might have been thought
that the development of the ray-petals by drawing nourishment from
certain other parts of the flower had caused their abortion; but in
some Compositae there is a difference in the seeds of the outer and
inner florets without any difference in the corolla. Possibly, these
several differences may be connected with some difference in the flow
of nutriment towards the central and external flowers: we know, at
least, that in irregular flowers, those nearest to the axis are
oftenest subject to peloria, and become regular. I may add, as an
instance of this, and of a striking case of correlation, that I have
recently observed in some garden pelargoniums, that the central flower
of the truss often loses the patches of darker colour in the two upper
petals; and that when this occurs, the adherent nectary is quite
aborted; when the colour is absent from only one of the two upper
petals, the nectary is only much shortened.

With respect to the difference in the corolla of the central and
exterior flowers of a head or umbel, I do not feel at all sure that C.
C. Sprengel's idea that the ray-florets serve to attract insects,
whose agency is highly advantageous in the fertilisation of plants of
these two orders, is so far-fetched, as it may at first appear: and if
it be advantageous, natural selection may have come into play. But in
regard to the differences both in the internal and external structure
of the seeds, which are not always correlated with any differences in
the flowers, it seems impossible that they can be in any way
advantageous to the plant: yet in the Umbelliferae these differences
are of such apparent importance--the seeds being in some cases,
according to Tausch, orthospermous in the exterior flowers and
coelospermous in the central flowers,--that the elder De Candolle
founded his main divisions of the order on analogous differences.
Hence we see that modifications of structure, viewed by systematists
as of high value, may be wholly due to unknown laws of correlated
growth, and without being, as far as we can see, of the slightest
service to the species.

We may often falsely attribute to correlation of growth, structures
which are common to whole groups of species, and which in truth are
simply due to inheritance; for an ancient progenitor may have acquired
through natural selection some one modification in structure, and,
after thousands of generations, some other and independent
modification; and these two modifications, having been transmitted to
a whole group of descendants with diverse habits, would naturally be
thought to be correlated in some necessary manner. So, again, I do not
doubt that some apparent correlations, occurring throughout whole
orders, are entirely due to the manner alone in which natural
selection can act. For instance, Alph. De Candolle has remarked that
winged seeds are never found in fruits which do not open: I should
explain the rule by the fact that seeds could not gradually become
winged through natural selection, except in fruits which opened; so
that the individual plants producing seeds which were a little better
fitted to be wafted further, might get an advantage over those
producing seed less fitted for dispersal; and this process could not
possibly go on in fruit which did not open.

The elder Geoffroy and Goethe propounded, at about the same period,
their law of compensation or balancement of growth; or, as Goethe
expressed it, "in order to spend on one side, nature is forced to
economise on the other side." I think this holds true to a certain
extent with our domestic productions: if nourishment flows to one part
or organ in excess, it rarely flows, at least in excess, to another
part; thus it is difficult to get a cow to give much milk and to
fatten readily. The same varieties of the cabbage do not yield
abundant and nutritious foliage and a copious supply of oil-bearing
seeds. When the seeds in our fruits become atrophied, the fruit itself
gains largely in size and quality. In our poultry, a large tuft of
feathers on the head is generally accompanied by a diminished comb,
and a large beard by diminished wattles. With species in a state of
nature it can hardly be maintained that the law is of universal
application; but many good observers, more especially botanists,
believe in its truth. I will not, however, here give any instances,
for I see hardly any way of distinguishing between the effects, on the
one hand, of a part being largely developed through natural selection
and another and adjoining part being reduced by this same process or
by disuse, and, on the other hand, the actual withdrawal of nutriment
from one part owing to the excess of growth in another and adjoining
part.

I suspect, also, that some of the cases of compensation which have
been advanced, and likewise some other facts, may be merged under a
more general principle, namely, that natural selection is continually
trying to economise in every part of the organisation. If under
changed conditions of life a structure before useful becomes less
useful, any diminution, however slight, in its development, will be
seized on by natural selection, for it will profit the individual not
to have its nutriment wasted in building up an useless structure. I
can thus only understand a fact with which I was much struck when
examining cirripedes, and of which many other instances could be
given: namely, that when a cirripede is parasitic within another and
is thus protected, it loses more or less completely its own shell or
carapace. This is the case with the male Ibla, and in a truly
extraordinary manner with the Proteolepas: for the carapace in all
other cirripedes consists of the three highly-important anterior
segments of the head enormously developed, and furnished with great
nerves and muscles; but in the parasitic and protected Proteolepas,
the whole anterior part of the head is reduced to the merest rudiment
attached to the bases of the prehensile antennae. Now the saving of a
large and complex structure, when rendered superfluous by the
parasitic habits of the Proteolepas, though effected by slow steps,
would be a decided advantage to each successive individual of the
species; for in the struggle for life to which every animal is
exposed, each individual Proteolepas would have a better chance of
supporting itself, by less nutriment being wasted in developing a
structure now become useless.

Thus, as I believe, natural selection will always succeed in the long
run in reducing and saving every part of the organisation, as soon as
it is rendered superfluous, without by any means causing some other
part to be largely developed in a corresponding degree. And,
conversely, that natural selection may perfectly well succeed in
largely developing any organ, without requiring as a necessary
compensation the reduction of some adjoining part.

It seems to be a rule, as remarked by Is. Geoffroy St. Hilaire, both
in varieties and in species, that when any part or organ is repeated
many times in the structure of the same individual (as the vertebrae
in snakes, and the stamens in polyandrous flowers) the number is
variable; whereas the number of the same part or organ, when it occurs
in lesser numbers, is constant. The same author and some botanists
have further remarked that multiple parts are also very liable to
variation in structure. Inasmuch as this "vegetative repetition," to
use Professor Owen's expression, seems to be a sign of low
organisation; the foregoing remark seems connected with the very
general opinion of naturalists, that beings low in the scale of nature
are more variable than those which are higher. I presume that lowness
in this case means that the several parts of the organisation have
been but little specialised for particular functions; and as long as
the same part has to perform diversified work, we can perhaps see why
it should remain variable, that is, why natural selection should have
preserved or rejected each little deviation of form less carefully
than when the part has to serve for one special purpose alone. In the
same way that a knife which has to cut all sorts of things may be of
almost any shape; whilst a tool for some particular object had better
be of some particular shape. Natural selection, it should never be
forgotten, can act on each part of each being, solely through and for
its advantage.

Rudimentary parts, it has been stated by some authors, and I believe
with truth, are apt to be highly variable. We shall have to recur to
the general subject of rudimentary and aborted organs; and I will here
only add that their variability seems to be owing to their
uselessness, and therefore to natural selection having no power to
check deviations in their structure. Thus rudimentary parts are left
to the free play of the various laws of growth, to the effects of
long-continued disuse, and to the tendency to reversion.

A PART DEVELOPED IN ANY SPECIES IN AN EXTRAORDINARY DEGREE OR MANNER,
IN COMPARISON WITH THE SAME PART IN ALLIED SPECIES, TENDS TO BE HIGHLY
VARIABLE.

Several years ago I was much struck with a remark, nearly to the above
effect, published by Mr. Waterhouse. I infer also from an observation
made by Professor Owen, with respect to the length of the arms of the
ourang-outang, that he has come to a nearly similar conclusion. It is
hopeless to attempt to convince any one of the truth of this
proposition without giving the long array of facts which I have
collected, and which cannot possibly be here introduced. I can only
state my conviction that it is a rule of high generality. I am aware
of several causes of error, but I hope that I have made due allowance
for them. It should be understood that the rule by no means applies to
any part, however unusually developed, unless it be unusually
developed in comparison with the same part in closely allied species.
Thus, the bat's wing is a most abnormal structure in the class
mammalia; but the rule would not here apply, because there is a whole
group of bats having wings; it would apply only if some one species of
bat had its wings developed in some remarkable manner in comparison
with the other species of the same genus. The rule applies very
strongly in the case of secondary sexual characters, when displayed in
any unusual manner. The term, secondary sexual characters, used by
Hunter, applies to characters which are attached to one sex, but are
not directly connected with the act of reproduction. The rule applies
to males and females; but as females more rarely offer remarkable
secondary sexual characters, it applies more rarely to them. The rule
being so plainly applicable in the case of secondary sexual
characters, may be due to the great variability of these characters,
whether or not displayed in any unusual manner--of which fact I think
there can be little doubt. But that our rule is not confined to
secondary sexual characters is clearly shown in the case of
hermaphrodite cirripedes; and I may here add, that I particularly
attended to Mr. Waterhouse's remark, whilst investigating this Order,
and I am fully convinced that the rule almost invariably holds good
with cirripedes. I shall, in my future work, give a list of the more
remarkable cases; I will here only briefly give one, as it illustrates
the rule in its largest application. The opercular valves of sessile
cirripedes (rock barnacles) are, in every sense of the word, very
important structures, and they differ extremely little even in
different genera; but in the several species of one genus, Pyrgoma,
these valves present a marvellous amount of diversification: the
homologous valves in the different species being sometimes wholly
unlike in shape; and the amount of variation in the individuals of
several of the species is so great, that it is no exaggeration to
state that the varieties differ more from each other in the characters
of these important valves than do other species of distinct genera.

As birds within the same country vary in a remarkably small degree, I
have particularly attended to them, and the rule seems to me certainly
to hold good in this class. I cannot make out that it applies to
plants, and this would seriously have shaken my belief in its truth,
had not the great variability in plants made it particularly difficult
to compare their relative degrees of variability.

When we see any part or organ developed in a remarkable degree or
manner in any species, the fair presumption is that it is of high
importance to that species; nevertheless the part in this case is
eminently liable to variation. Why should this be so? On the view that
each species has been independently created, with all its parts as we
now see them, I can see no explanation. But on the view that groups of
species have descended from other species, and have been modified
through natural selection, I think we can obtain some light. In our
domestic animals, if any part, or the whole animal, be neglected and
no selection be applied, that part (for instance, the comb in the
Dorking fowl) or the whole breed will cease to have a nearly uniform
character. The breed will then be said to have degenerated. In
rudimentary organs, and in those which have been but little
specialised for any particular purpose, and perhaps in polymorphic
groups, we see a nearly parallel natural case; for in such cases
natural selection either has not or cannot come into full play, and
thus the organisation is left in a fluctuating condition. But what
here more especially concerns us is, that in our domestic animals
those points, which at the present time are undergoing rapid change by
continued selection, are also eminently liable to variation. Look at
the breeds of the pigeon; see what a prodigious amount of difference
there is in the beak of the different tumblers, in the beak and wattle
of the different carriers, in the carriage and tail of our fantails,
etc., these being the points now mainly attended to by English
fanciers. Even in the sub-breeds, as in the short-faced tumbler, it is
notoriously difficult to breed them nearly to perfection, and
frequently individuals are born which depart widely from the standard.
There may be truly said to be a constant struggle going on between, on
the one hand, the tendency to reversion to a less modified state, as
well as an innate tendency to further variability of all kinds, and,
on the other hand, the power of steady selection to keep the breed
true. In the long run selection gains the day, and we do not expect to
fail so far as to breed a bird as coarse as a common tumbler from a
good short-faced strain. But as long as selection is rapidly going on,
there may always be expected to be much variability in the structure
undergoing modification. It further deserves notice that these
variable characters, produced by man's selection, sometimes become
attached, from causes quite unknown to us, more to one sex than to the
other, generally to the male sex, as with the wattle of carriers and
the enlarged crop of pouters.

Now let us turn to nature. When a part has been developed in an
extraordinary manner in any one species, compared with the other
species of the same genus, we may conclude that this part has
undergone an extraordinary amount of modification, since the period
when the species branched off from the common progenitor of the genus.
This period will seldom be remote in any extreme degree, as species
very rarely endure for more than one geological period. An
extraordinary amount of modification implies an unusually large and
long-continued amount of variability, which has continually been
accumulated by natural selection for the benefit of the species. But
as the variability of the extraordinarily-developed part or organ has
been so great and long-continued within a period not excessively
remote, we might, as a general rule, expect still to find more
variability in such parts than in other parts of the organisation,
which have remained for a much longer period nearly constant. And
this, I am convinced, is the case. That the struggle between natural
selection on the one hand, and the tendency to reversion and
variability on the other hand, will in the course of time cease; and
that the most abnormally developed organs may be made constant, I can
see no reason to doubt. Hence when an organ, however abnormal it may
be, has been transmitted in approximately the same condition to many
modified descendants, as in the case of the wing of the bat, it must
have existed, according to my theory, for an immense period in nearly
the same state; and thus it comes to be no more variable than any
other structure. It is only in those cases in which the modification
has been comparatively recent and extraordinarily great that we ought
to find the GENERATIVE VARIABILITY, as it may be called, still present
in a high degree. For in this case the variability will seldom as yet
have been fixed by the continued selection of the individuals varying
in the required manner and degree, and by the continued rejection of
those tending to revert to a former and less modified condition.

The principle included in these remarks may be extended. It is
notorious that specific characters are more variable than generic. To
explain by a simple example what is meant. If some species in a large
genus of plants had blue flowers and some had red, the colour would be
only a specific character, and no one would be surprised at one of the
blue species varying into red, or conversely; but if all the species
had blue flowers, the colour would become a generic character, and its
variation would be a more unusual circumstance. I have chosen this
example because an explanation is not in this case applicable, which
most naturalists would advance, namely, that specific characters are
more variable than generic, because they are taken from parts of less
physiological importance than those commonly used for classing genera.
I believe this explanation is partly, yet only indirectly, true; I
shall, however, have to return to this subject in our chapter on
Classification. It would be almost superfluous to adduce evidence in
support of the above statement, that specific characters are more
variable than generic; but I have repeatedly noticed in works on
natural history, that when an author has remarked with surprise that
some IMPORTANT organ or part, which is generally very constant
throughout large groups of species, has DIFFERED considerably in
closely-allied species, that it has, also, been VARIABLE in the
individuals of some of the species. And this fact shows that a
character, which is generally of generic value, when it sinks in value
and becomes only of specific value, often becomes variable, though its
physiological importance may remain the same. Something of the same
kind applies to monstrosities: at least Is. Geoffroy St. Hilaire seems
to entertain no doubt, that the more an organ normally differs in the
different species of the same group, the more subject it is to
individual anomalies.

On the ordinary view of each species having been independently
created, why should that part of the structure, which differs from the
same part in other independently-created species of the same genus, be
more variable than those parts which are closely alike in the several
species? I do not see that any explanation can be given. But on the
view of species being only strongly marked and fixed varieties, we
might surely expect to find them still often continuing to vary in
those parts of their structure which have varied within a moderately
recent period, and which have thus come to differ. Or to state the
case in another manner:--the points in which all the species of a
genus resemble each other, and in which they differ from the species
of some other genus, are called generic characters; and these
characters in common I attribute to inheritance from a common
progenitor, for it can rarely have happened that natural selection
will have modified several species, fitted to more or less
widely-different habits, in exactly the same manner: and as these
so-called generic characters have been inherited from a remote period,
since that period when the species first branched off from their
common progenitor, and subsequently have not varied or come to differ
in any degree, or only in a slight degree, it is not probable that
they should vary at the present day. On the other hand, the points in
which species differ from other species of the same genus, are called
specific characters; and as these specific characters have varied and
come to differ within the period of the branching off of the species
from a common progenitor, it is probable that they should still often
be in some degree variable,--at least more variable than those parts
of the organisation which have for a very long period remained
constant.

In connexion with the present subject, I will make only two other
remarks. I think it will be admitted, without my entering on details,
that secondary sexual characters are very variable; I think it also
will be admitted that species of the same group differ from each other
more widely in their secondary sexual characters, than in other parts
of their organisation; compare, for instance, the amount of difference
between the males of gallinaceous birds, in which secondary sexual
characters are strongly displayed, with the amount of difference
between their females; and the truth of this proposition will be
granted. The cause of the original variability of secondary sexual
characters is not manifest; but we can see why these characters should
not have been rendered as constant and uniform as other parts of the
organisation; for secondary sexual characters have been accumulated by
sexual selection, which is less rigid in its action than ordinary
selection, as it does not entail death, but only gives fewer offspring
to the less favoured males. Whatever the cause may be of the
variability of secondary sexual characters, as they are highly
variable, sexual selection will have had a wide scope for action, and
may thus readily have succeeded in giving to the species of the same
group a greater amount of difference in their sexual characters, than
in other parts of their structure.

It is a remarkable fact, that the secondary sexual differences between
the two sexes of the same species are generally displayed in the very
same parts of the organisation in which the different species of the
same genus differ from each other. Of this fact I will give in
illustration two instances, the first which happen to stand on my
list; and as the differences in these cases are of a very unusual
nature, the relation can hardly be accidental. The same number of
joints in the tarsi is a character generally common to very large
groups of beetles, but in the Engidae, as Westwood has remarked, the
number varies greatly; and the number likewise differs in the two
sexes of the same species: again in fossorial hymenoptera, the manner
of neuration of the wings is a character of the highest importance,
because common to large groups; but in certain genera the neuration
differs in the different species, and likewise in the two sexes of the
same species. This relation has a clear meaning on my view of the
subject: I look at all the species of the same genus as having as
certainly descended from the same progenitor, as have the two sexes of
any one of the species. Consequently, whatever part of the structure
of the common progenitor, or of its early descendants, became
variable; variations of this part would it is highly probable, be
taken advantage of by natural and sexual selection, in order to fit
the several species to their several places in the economy of nature,
and likewise to fit the two sexes of the same species to each other,
or to fit the males and females to different habits of life, or the
males to struggle with other males for the possession of the females.

Finally, then, I conclude that the greater variability of specific
characters, or those which distinguish species from species, than of
generic characters, or those which the species possess in
common;--that the frequent extreme variability of any part which is
developed in a species in an extraordinary manner in comparison with
the same part in its congeners; and the not great degree of
variability in a part, however extraordinarily it may be developed, if
it be common to a whole group of species;--that the great variability
of secondary sexual characters, and the great amount of difference in
these same characters between closely allied species;--that secondary
sexual and ordinary specific differences are generally displayed in
the same parts of the organisation,--are all principles closely
connected together. All being mainly due to the species of the same
group having descended from a common progenitor, from whom they have
inherited much in common,--to parts which have recently and largely
varied being more likely still to go on varying than parts which have
long been inherited and have not varied,--to natural selection having
more or less completely, according to the lapse of time, overmastered
the tendency to reversion and to further variability,--to sexual
selection being less rigid than ordinary selection,--and to variations
in the same parts having been accumulated by natural and sexual
selection, and thus adapted for secondary sexual, and for ordinary
specific purposes.

DISTINCT SPECIES PRESENT ANALOGOUS VARIATIONS; AND A VARIETY OF ONE
SPECIES OFTEN ASSUMES SOME OF THE CHARACTERS OF AN ALLIED SPECIES, OR
REVERTS TO SOME OF THE CHARACTERS OF AN EARLY PROGENITOR.

These propositions will be most readily understood by looking to our
domestic races. The most distinct breeds of pigeons, in countries most
widely apart, present sub-varieties with reversed feathers on the head
and feathers on the feet,--characters not possessed by the aboriginal
rock-pigeon; these then are analogous variations in two or more
distinct races. The frequent presence of fourteen or even sixteen
tail-feathers in the pouter, may be considered as a variation
representing the normal structure of another race, the fantail. I
presume that no one will doubt that all such analogous variations are
due to the several races of the pigeon having inherited from a common
parent the same constitution and tendency to variation, when acted on
by similar unknown influences. In the vegetable kingdom we have a case
of analogous variation, in the enlarged stems, or roots as commonly
called, of the Swedish turnip and Ruta baga, plants which several
botanists rank as varieties produced by cultivation from a common
parent: if this be not so, the case will then be one of analogous
variation in two so-called distinct species; and to these a third may
be added, namely, the common turnip. According to the ordinary view of
each species having been independently created, we should have to
attribute this similarity in the enlarged stems of these three plants,
not to the vera causa of community of descent, and a consequent
tendency to vary in a like manner, but to three separate yet closely
related acts of creation.

With pigeons, however, we have another case, namely, the occasional
appearance in all the breeds, of slaty-blue birds with two black bars
on the wings, a white rump, a bar at the end of the tail, with the
outer feathers externally edged near their bases with white. As all
these marks are characteristic of the parent rock-pigeon, I presume
that no one will doubt that this is a case of reversion, and not of a
new yet analogous variation appearing in the several breeds. We may I
think confidently come to this conclusion, because, as we have seen,
these coloured marks are eminently liable to appear in the crossed
offspring of two distinct and differently coloured breeds; and in this
case there is nothing in the external conditions of life to cause the
reappearance of the slaty-blue, with the several marks, beyond the
influence of the mere act of crossing on the laws of inheritance.

No doubt it is a very surprising fact that characters should reappear
after having been lost for many, perhaps for hundreds of generations.
But when a breed has been crossed only once by some other breed, the
offspring occasionally show a tendency to revert in character to the
foreign breed for many generations--some say, for a dozen or even a
score of generations. After twelve generations, the proportion of
blood, to use a common expression, of any one ancestor, is only 1 in
2048; and yet, as we see, it is generally believed that a tendency to
reversion is retained by this very small proportion of foreign blood.
In a breed which has not been crossed, but in which BOTH parents have
lost some character which their progenitor possessed, the tendency,
whether strong or weak, to reproduce the lost character might be, as
was formerly remarked, for all that we can see to the contrary,
transmitted for almost any number of generations. When a character
which has been lost in a breed, reappears after a great number of
generations, the most probable hypothesis is, not that the offspring
suddenly takes after an ancestor some hundred generations distant, but
that in each successive generation there has been a tendency to
reproduce the character in question, which at last, under unknown
favourable conditions, gains an ascendancy. For instance, it is
probable that in each generation of the barb-pigeon, which produces
most rarely a blue and black-barred bird, there has been a tendency in
each generation in the plumage to assume this colour. This view is
hypothetical, but could be supported by some facts; and I can see no
more abstract improbability in a tendency to produce any character
being inherited for an endless number of generations, than in quite
useless or rudimentary organs being, as we all know them to be, thus
inherited. Indeed, we may sometimes observe a mere tendency to produce
a rudiment inherited: for instance, in the common snapdragon
(Antirrhinum) a rudiment of a fifth stamen so often appears, that this
plant must have an inherited tendency to produce it.

As all the species of the same genus are supposed, on my theory, to
have descended from a common parent, it might be expected that they
would occasionally vary in an analogous manner; so that a variety of
one species would resemble in some of its characters another species;
this other species being on my view only a well-marked and permanent
variety. But characters thus gained would probably be of an
unimportant nature, for the presence of all important characters will
be governed by natural selection, in accordance with the diverse
habits of the species, and will not be left to the mutual action of
the conditions of life and of a similar inherited constitution. It
might further be expected that the species of the same genus would
occasionally exhibit reversions to lost ancestral characters. As,
however, we never know the exact character of the common ancestor of a
group, we could not distinguish these two cases: if, for instance, we
did not know that the rock-pigeon was not feather-footed or
turn-crowned, we could not have told, whether these characters in our
domestic breeds were reversions or only analogous variations; but we
might have inferred that the blueness was a case of reversion, from
the number of the markings, which are correlated with the blue tint,
and which it does not appear probable would all appear together from
simple variation. More especially we might have inferred this, from
the blue colour and marks so often appearing when distinct breeds of
diverse colours are crossed. Hence, though under nature it must
generally be left doubtful, what cases are reversions to an anciently
existing character, and what are new but analogous variations, yet we
ought, on my theory, sometimes to find the varying offspring of a
species assuming characters (either from reversion or from analogous
variation) which already occur in some other members of the same
group. And this undoubtedly is the case in nature.

A considerable part of the difficulty in recognising a variable
species in our systematic works, is due to its varieties mocking, as
it were, some of the other species of the same genus. A considerable
catalogue, also, could be given of forms intermediate between two
other forms, which themselves must be doubtfully ranked as either
varieties or species; and this shows, unless all these forms be
considered as independently created species, that the one in varying
has assumed some of the characters of the other, so as to produce the
intermediate form. But the best evidence is afforded by parts or
organs of an important and uniform nature occasionally varying so as
to acquire, in some degree, the character of the same part or organ in
an allied species. I have collected a long list of such cases; but
here, as before, I lie under a great disadvantage in not being able to
give them. I can only repeat that such cases certainly do occur, and
seem to me very remarkable.

I will, however, give one curious and complex case, not indeed as
affecting any important character, but from occurring in several
species of the same genus, partly under domestication and partly under
nature. It is a case apparently of reversion. The ass not rarely has
very distinct transverse bars on its legs, like those on the legs of a
zebra: it has been asserted that these are plainest in the foal, and
from inquiries which I have made, I believe this to be true. It has
also been asserted that the stripe on each shoulder is sometimes
double. The shoulder stripe is certainly very variable in length and
outline. A white ass, but NOT an albino, has been described without
either spinal or shoulder-stripe; and these stripes are sometimes very
obscure, or actually quite lost, in dark-coloured asses. The koulan of
Pallas is said to have been seen with a double shoulder-stripe. The
hemionus has no shoulder-stripe; but traces of it, as stated by Mr.
Blyth and others, occasionally appear: and I have been informed by
Colonel Poole that the foals of this species are generally striped on
the legs, and faintly on the shoulder. The quagga, though so plainly
barred like a zebra over the body, is without bars on the legs; but
Dr. Gray has figured one specimen with very distinct zebra-like bars
on the hocks.

With respect to the horse, I have collected cases in England of the
spinal stripe in horses of the most distinct breeds, and of ALL
colours; transverse bars on the legs are not rare in duns, mouse-duns,
and in one instance in a chestnut: a faint shoulder-stripe may
sometimes be seen in duns, and I have seen a trace in a bay horse. My
son made a careful examination and sketch for me of a dun Belgian
cart-horse with a double stripe on each shoulder and with leg-stripes;
and a man, whom I can implicitly trust, has examined for me a small
dun Welch pony with THREE short parallel stripes on each shoulder.

In the north-west part of India the Kattywar breed of horses is so
generally striped, that, as I hear from Colonel Poole, who examined
the breed for the Indian Government, a horse without stripes is not
considered as purely-bred. The spine is always striped; the legs are
generally barred; and the shoulder-stripe, which is sometimes double
and sometimes treble, is common; the side of the face, moreover, is
sometimes striped. The stripes are plainest in the foal; and sometimes
quite disappear in old horses. Colonel Poole has seen both gray and
bay Kattywar horses striped when first foaled. I have, also, reason to
suspect, from information given me by Mr. W. W. Edwards, that with the
English race-horse the spinal stripe is much commoner in the foal than
in the full-grown animal. Without here entering on further details, I
may state that I have collected cases of leg and shoulder stripes in
horses of very different breeds, in various countries from Britain to
Eastern China; and from Norway in the north to the Malay Archipelago
in the south. In all parts of the world these stripes occur far
oftenest in duns and mouse-duns; by the term dun a large range of
colour is included, from one between brown and black to a close
approach to cream-colour.

I am aware that Colonel Hamilton Smith, who has written on this
subject, believes that the several breeds of the horse have descended
from several aboriginal species--one of which, the dun, was striped;
and that the above-described appearances are all due to ancient
crosses with the dun stock. But I am not at all satisfied with this
theory, and should be loth to apply it to breeds so distinct as the
heavy Belgian cart-horse, Welch ponies, cobs, the lanky Kattywar race,
etc., inhabiting the most distant parts of the world.

Now let us turn to the effects of crossing the several species of the
horse-genus. Rollin asserts, that the common mule from the ass and
horse is particularly apt to have bars on its legs. I once saw a mule
with its legs so much striped that any one at first would have thought
that it must have been the product of a zebra; and Mr. W. C. Martin,
in his excellent treatise on the horse, has given a figure of a
similar mule. In four coloured drawings, which I have seen, of hybrids
between the ass and zebra, the legs were much more plainly barred than
the rest of the body; and in one of them there was a double
shoulder-stripe. In Lord Moreton's famous hybrid from a chestnut mare
and male quagga, the hybrid, and even the pure offspring subsequently
produced from the mare by a black Arabian sire, were much more plainly
barred across the legs than is even the pure quagga. Lastly, and this
is another most remarkable case, a hybrid has been figured by Dr. Gray
(and he informs me that he knows of a second case) from the ass and
the hemionus; and this hybrid, though the ass seldom has stripes on
its legs and the hemionus has none and has not even a shoulder-stripe,
nevertheless had all four legs barred, and had three short
shoulder-stripes, like those on the dun Welch pony, and even had some
zebra-like stripes on the sides of its face. With respect to this last
fact, I was so convinced that not even a stripe of colour appears from
what would commonly be called an accident, that I was led solely from
the occurrence of the face-stripes on this hybrid from the ass and
hemionus, to ask Colonel Poole whether such face-stripes ever occur in
the eminently striped Kattywar breed of horses, and was, as we have
seen, answered in the affirmative.

What now are we to say to these several facts? We see several very
distinct species of the horse-genus becoming, by simple variation,
striped on the legs like a zebra, or striped on the shoulders like an
ass. In the horse we see this tendency strong whenever a dun tint
appears--a tint which approaches to that of the general colouring of
the other species of the genus. The appearance of the stripes is not
accompanied by any change of form or by any other new character. We
see this tendency to become striped most strongly displayed in hybrids
from between several of the most distinct species. Now observe the
case of the several breeds of pigeons: they are descended from a
pigeon (including two or three sub-species or geographical races) of a
bluish colour, with certain bars and other marks; and when any breed
assumes by simple variation a bluish tint, these bars and other marks
invariably reappear; but without any other change of form or
character. When the oldest and truest breeds of various colours are
crossed, we see a strong tendency for the blue tint and bars and marks
to reappear in the mongrels. I have stated that the most probable
hypothesis to account for the reappearance of very ancient characters,
is--that there is a TENDENCY in the young of each successive
generation to produce the long-lost character, and that this tendency,
from unknown causes, sometimes prevails. And we have just seen that in
several species of the horse-genus the stripes are either plainer or
appear more commonly in the young than in the old. Call the breeds of
pigeons, some of which have bred true for centuries, species; and how
exactly parallel is the case with that of the species of the
horse-genus! For myself, I venture confidently to look back thousands
on thousands of generations, and I see an animal striped like a zebra,
but perhaps otherwise very differently constructed, the common parent
of our domestic horse, whether or not it be descended from one or more
wild stocks, of the ass, the hemionus, quagga, and zebra.

He who believes that each equine species was independently created,
will, I presume, assert that each species has been created with a
tendency to vary, both under nature and under domestication, in this
particular manner, so as often to become striped like other species of
the genus; and that each has been created with a strong tendency, when
crossed with species inhabiting distant quarters of the world, to
produce hybrids resembling in their stripes, not their own parents,
but other species of the genus. To admit this view is, as it seems to
me, to reject a real for an unreal, or at least for an unknown, cause.
It makes the works of God a mere mockery and deception; I would almost
as soon believe with the old and ignorant cosmogonists, that fossil
shells had never lived, but had been created in stone so as to mock
the shells now living on the sea-shore.

SUMMARY.

Our ignorance of the laws of variation is profound. Not in one case
out of a hundred can we pretend to assign any reason why this or that
part differs, more or less, from the same part in the parents. But
whenever we have the means of instituting a comparison, the same laws
appear to have acted in producing the lesser differences between
varieties of the same species, and the greater differences between
species of the same genus. The external conditions of life, as climate
and food, etc., seem to have induced some slight modifications. Habit
in producing constitutional differences, and use in strengthening, and
disuse in weakening and diminishing organs, seem to have been more
potent in their effects. Homologous parts tend to vary in the same
way, and homologous parts tend to cohere. Modifications in hard parts
and in external parts sometimes affect softer and internal parts. When
one part is largely developed, perhaps it tends to draw nourishment
from the adjoining parts; and every part of the structure which can be
saved without detriment to the individual, will be saved. Changes of
structure at an early age will generally affect parts subsequently
developed; and there are very many other correlations of growth, the
nature of which we are utterly unable to understand. Multiple parts
are variable in number and in structure, perhaps arising from such
parts not having been closely specialised to any particular function,
so that their modifications have not been closely checked by natural
selection. It is probably from this same cause that organic beings low
in the scale of nature are more variable than those which have their
whole organisation more specialised, and are higher in the scale.
Rudimentary organs, from being useless, will be disregarded by natural
selection, and hence probably are variable. Specific characters--that
is, the characters which have come to differ since the several species
of the same genus branched off from a common parent--are more variable
than generic characters, or those which have long been inherited, and
have not differed within this same period. In these remarks we have
referred to special parts or organs being still variable, because they
have recently varied and thus come to differ; but we have also seen in
the second Chapter that the same principle applies to the whole
individual; for in a district where many species of any genus are
found--that is, where there has been much former variation and
differentiation, or where the manufactory of new specific forms has
been actively at work--there, on an average, we now find most
varieties or incipient species. Secondary sexual characters are highly
variable, and such characters differ much in the species of the same
group. Variability in the same parts of the organisation has generally
been taken advantage of in giving secondary sexual differences to the
sexes of the same species, and specific differences to the several
species of the same genus. Any part or organ developed to an
extraordinary size or in an extraordinary manner, in comparison with
the same part or organ in the allied species, must have gone through
an extraordinary amount of modification since the genus arose; and
thus we can understand why it should often still be variable in a much
higher degree than other parts; for variation is a long-continued and
slow process, and natural selection will in such cases not as yet have
had time to overcome the tendency to further variability and to
reversion to a less modified state. But when a species with any
extraordinarily-developed organ has become the parent of many modified
descendants--which on my view must be a very slow process, requiring a
long lapse of time--in this case, natural selection may readily have
succeeded in giving a fixed character to the organ, in however
extraordinary a manner it may be developed. Species inheriting nearly
the same constitution from a common parent and exposed to similar
influences will naturally tend to present analogous variations, and
these same species may occasionally revert to some of the characters
of their ancient progenitors. Although new and important modifications
may not arise from reversion and analogous variation, such
modifications will add to the beautiful and harmonious diversity of
nature.

Whatever the cause may be of each slight difference in the offspring
from their parents--and a cause for each must exist--it is the steady
accumulation, through natural selection, of such differences, when
beneficial to the individual, that gives rise to all the more
important modifications of structure, by which the innumerable beings
on the face of this earth are enabled to struggle with each other, and
the best adapted to survive.


CHAPTER 6. DIFFICULTIES ON THEORY.

Difficulties on the theory of descent with modification.
Transitions.
Absence or rarity of transitional varieties.
Transitions in habits of life.
Diversified habits in the same species.
Species with habits widely different from those of their allies.
Organs of extreme perfection.
Means of transition.
Cases of difficulty.
Natura non facit saltum.
Organs of small importance.
Organs not in all cases absolutely perfect.
The law of Unity of Type and of the Conditions of Existence embraced
by the theory of Natural Selection.

Long before having arrived at this part of my work, a crowd of
difficulties will have occurred to the reader. Some of them are so
grave that to this day I can never reflect on them without being
staggered; but, to the best of my judgment, the greater number are
only apparent, and those that are real are not, I think, fatal to my
theory.

These difficulties and objections may be classed under the following
heads:--

Firstly, why, if species have descended from other species by
insensibly fine gradations, do we not everywhere see innumerable
transitional forms? Why is not all nature in confusion instead of the
species being, as we see them, well defined?

Secondly, is it possible that an animal having, for instance, the
structure and habits of a bat, could have been formed by the
modification of some animal with wholly different habits? Can we
believe that natural selection could produce, on the one hand, organs
of trifling importance, such as the tail of a giraffe, which serves as
a fly-flapper, and, on the other hand, organs of such wonderful
structure, as the eye, of which we hardly as yet fully understand the
inimitable perfection?

Thirdly, can instincts be acquired and modified through natural
selection? What shall we say to so marvellous an instinct as that
which leads the bee to make cells, which have practically anticipated
the discoveries of profound mathematicians?

Fourthly, how can we account for species, when crossed, being sterile
and producing sterile offspring, whereas, when varieties are crossed,
their fertility is unimpaired?

The two first heads shall be here discussed--Instinct and Hybridism in
separate chapters.

ON THE ABSENCE OR RARITY OF TRANSITIONAL VARIETIES.

As natural selection acts solely by the preservation of profitable
modifications, each new form will tend in a fully-stocked country to
take the place of, and finally to exterminate, its own less improved
parent or other less-favoured forms with which it comes into
competition. Thus extinction and natural selection will, as we have
seen, go hand in hand. Hence, if we look at each species as descended
from some other unknown form, both the parent and all the transitional
varieties will generally have been exterminated by the very process of
formation and perfection of the new form.

But, as by this theory innumerable transitional forms must have
existed, why do we not find them embedded in countless numbers in the
crust of the earth? It will be much more convenient to discuss this
question in the chapter on the Imperfection of the geological record;
and I will here only state that I believe the answer mainly lies in
the record being incomparably less perfect than is generally supposed;
the imperfection of the record being chiefly due to organic beings not
inhabiting profound depths of the sea, and to their remains being
embedded and preserved to a future age only in masses of sediment
sufficiently thick and extensive to withstand an enormous amount of
future degradation; and such fossiliferous masses can be accumulated
only where much sediment is deposited on the shallow bed of the sea,
whilst it slowly subsides. These contingencies will concur only
rarely, and after enormously long intervals. Whilst the bed of the sea
is stationary or is rising, or when very little sediment is being
deposited, there will be blanks in our geological history. The crust
of the earth is a vast museum; but the natural collections have been
made only at intervals of time immensely remote.

But it may be urged that when several closely-allied species inhabit
the same territory we surely ought to find at the present time many
transitional forms. Let us take a simple case: in travelling from
north to south over a continent, we generally meet at successive
intervals with closely allied or representative species, evidently
filling nearly the same place in the natural economy of the land.
These representative species often meet and interlock; and as the one
becomes rarer and rarer, the other becomes more and more frequent,
till the one replaces the other. But if we compare these species where
they intermingle, they are generally as absolutely distinct from each
other in every detail of structure as are specimens taken from the
metropolis inhabited by each. By my theory these allied species have
descended from a common parent; and during the process of
modification, each has become adapted to the conditions of life of its
own region, and has supplanted and exterminated its original parent
and all the transitional varieties between its past and present
states. Hence we ought not to expect at the present time to meet with
numerous transitional varieties in each region, though they must have
existed there, and may be embedded there in a fossil condition. But in
the intermediate region, having intermediate conditions of life, why
do we not now find closely-linking intermediate varieties? This
difficulty for a long time quite confounded me. But I think it can be
in large part explained.

In the first place we should be extremely cautious in inferring,
because an area is now continuous, that it has been continuous during
a long period. Geology would lead us to believe that almost every
continent has been broken up into islands even during the later
tertiary periods; and in such islands distinct species might have been
separately formed without the possibility of intermediate varieties
existing in the intermediate zones. By changes in the form of the land
and of climate, marine areas now continuous must often have existed
within recent times in a far less continuous and uniform condition
than at present. But I will pass over this way of escaping from the
difficulty; for I believe that many perfectly defined species have
been formed on strictly continuous areas; though I do not doubt that
the formerly broken condition of areas now continuous has played an
important part in the formation of new species, more especially with
freely-crossing and wandering animals.

In looking at species as they are now distributed over a wide area, we
generally find them tolerably numerous over a large territory, then
becoming somewhat abruptly rarer and rarer on the confines, and
finally disappearing. Hence the neutral territory between two
representative species is generally narrow in comparison with the
territory proper to each. We see the same fact in ascending mountains,
and sometimes it is quite remarkable how abruptly, as Alph. De
Candolle has observed, a common alpine species disappears. The same
fact has been noticed by Forbes in sounding the depths of the sea with
the dredge. To those who look at climate and the physical conditions
of life as the all-important elements of distribution, these facts
ought to cause surprise, as climate and height or depth graduate away
insensibly. But when we bear in mind that almost every species, even
in its metropolis, would increase immensely in numbers, were it not
for other competing species; that nearly all either prey on or serve
as prey for others; in short, that each organic being is either
directly or indirectly related in the most important manner to other
organic beings, we must see that the range of the inhabitants of any
country by no means exclusively depends on insensibly changing
physical conditions, but in large part on the presence of other
species, on which it depends, or by which it is destroyed, or with
which it comes into competition; and as these species are already
defined objects (however they may have become so), not blending one
into another by insensible gradations, the range of any one species,
depending as it does on the range of others, will tend to be sharply
defined. Moreover, each species on the confines of its range, where it
exists in lessened numbers, will, during fluctuations in the number of
its enemies or of its prey, or in the seasons, be extremely liable to
utter extermination; and thus its geographical range will come to be
still more sharply defined.

If I am right in believing that allied or representative species, when
inhabiting a continuous area, are generally so distributed that each
has a wide range, with a comparatively narrow neutral territory
between them, in which they become rather suddenly rarer and rarer;
then, as varieties do not essentially differ from species, the same
rule will probably apply to both; and if we in imagination adapt a
varying species to a very large area, we shall have to adapt two
varieties to two large areas, and a third variety to a narrow
intermediate zone. The intermediate variety, consequently, will exist
in lesser numbers from inhabiting a narrow and lesser area; and
practically, as far as I can make out, this rule holds good with
varieties in a state of nature. I have met with striking instances of
the rule in the case of varieties intermediate between well-marked
varieties in the genus Balanus. And it would appear from information
given me by Mr. Watson, Dr. Asa Gray, and Mr. Wollaston, that
generally when varieties intermediate between two other forms occur,
they are much rarer numerically than the forms which they connect.
Now, if we may trust these facts and inferences, and therefore
conclude that varieties linking two other varieties together have
generally existed in lesser numbers than the forms which they connect,
then, I think, we can understand why intermediate varieties should not
endure for very long periods;--why as a general rule they should be
exterminated and disappear, sooner than the forms which they
originally linked together.

For any form existing in lesser numbers would, as already remarked,
run a greater chance of being exterminated than one existing in large
numbers; and in this particular case the intermediate form would be
eminently liable to the inroads of closely allied forms existing on
both sides of it. But a far more important consideration, as I
believe, is that, during the process of further modification, by which
two varieties are supposed on my theory to be converted and perfected
into two distinct species, the two which exist in larger numbers from
inhabiting larger areas, will have a great advantage over the
intermediate variety, which exists in smaller numbers in a narrow and
intermediate zone. For forms existing in larger numbers will always
have a better chance, within any given period, of presenting further
favourable variations for natural selection to seize on, than will the
rarer forms which exist in lesser numbers. Hence, the more common
forms, in the race for life, will tend to beat and supplant the less
common forms, for these will be more slowly modified and improved. It
is the same principle which, as I believe, accounts for the common
species in each country, as shown in the second chapter, presenting on
an average a greater number of well-marked varieties than do the rarer
species. I may illustrate what I mean by supposing three varieties of
sheep to be kept, one adapted to an extensive mountainous region; a
second to a comparatively narrow, hilly tract; and a third to wide
plains at the base; and that the inhabitants are all trying with equal
steadiness and skill to improve their stocks by selection; the chances
in this case will be strongly in favour of the great holders on the
mountains or on the plains improving their breeds more quickly than
the small holders on the intermediate narrow, hilly tract; and
consequently the improved mountain or plain breed will soon take the
place of the less improved hill breed; and thus the two breeds, which
originally existed in greater numbers, will come into close contact
with each other, without the interposition of the supplanted,
intermediate hill-variety.

To sum up, I believe that species come to be tolerably well-defined
objects, and do not at any one period present an inextricable chaos of
varying and intermediate links: firstly, because new varieties are
very slowly formed, for variation is a very slow process, and natural
selection can do nothing until favourable variations chance to occur,
and until a place in the natural polity of the country can be better
filled by some modification of some one or more of its inhabitants.
And such new places will depend on slow changes of climate, or on the
occasional immigration of new inhabitants, and, probably, in a still
more important degree, on some of the old inhabitants becoming slowly
modified, with the new forms thus produced and the old ones acting and
reacting on each other. So that, in any one region and at any one
time, we ought only to see a few species presenting slight
modifications of structure in some degree permanent; and this
assuredly we do see.

Secondly, areas now continuous must often have existed within the
recent period in isolated portions, in which many forms, more
especially amongst the classes which unite for each birth and wander
much, may have separately been rendered sufficiently distinct to rank
as representative species. In this case, intermediate varieties
between the several representative species and their common parent,
must formerly have existed in each broken portion of the land, but
these links will have been supplanted and exterminated during the
process of natural selection, so that they will no longer exist in a
living state.

Thirdly, when two or more varieties have been formed in different
portions of a strictly continuous area, intermediate varieties will,
it is probable, at first have been formed in the intermediate zones,
but they will generally have had a short duration. For these
intermediate varieties will, from reasons already assigned (namely
from what we know of the actual distribution of closely allied or
representative species, and likewise of acknowledged varieties), exist
in the intermediate zones in lesser numbers than the varieties which
they tend to connect. From this cause alone the intermediate varieties
will be liable to accidental extermination; and during the process of
further modification through natural selection, they will almost
certainly be beaten and supplanted by the forms which they connect;
for these from existing in greater numbers will, in the aggregate,
present more variation, and thus be further improved through natural
selection and gain further advantages.

Lastly, looking not to any one time, but to all time, if my theory be
true, numberless intermediate varieties, linking most closely all the
species of the same group together, must assuredly have existed; but
the very process of natural selection constantly tends, as has been so
often remarked, to exterminate the parent forms and the intermediate
links. Consequently evidence of their former existence could be found
only amongst fossil remains, which are preserved, as we shall in a
future chapter attempt to show, in an extremely imperfect and
intermittent record.

ON THE ORIGIN AND TRANSITIONS OF ORGANIC BEINGS WITH PECULIAR HABITS
AND STRUCTURE.

It has been asked by the opponents of such views as I hold, how, for
instance, a land carnivorous animal could have been converted into one
with aquatic habits; for how could the animal in its transitional
state have subsisted? It would be easy to show that within the same
group carnivorous animals exist having every intermediate grade
between truly aquatic and strictly terrestrial habits; and as each
exists by a struggle for life, it is clear that each is well adapted
in its habits to its place in nature. Look at the Mustela vison of
North America, which has webbed feet and which resembles an otter in
its fur, short legs, and form of tail; during summer this animal dives
for and preys on fish, but during the long winter it leaves the frozen
waters, and preys like other polecats on mice and land animals. If a
different case had been taken, and it had been asked how an
insectivorous quadruped could possibly have been converted into a
flying bat, the question would have been far more difficult, and I
could have given no answer. Yet I think such difficulties have very
little weight.

Here, as on other occasions, I lie under a heavy disadvantage, for out
of the many striking cases which I have collected, I can give only one
or two instances of transitional habits and structures in closely
allied species of the same genus; and of diversified habits, either
constant or occasional, in the same species. And it seems to me that
nothing less than a long list of such cases is sufficient to lessen
the difficulty in any particular case like that of the bat.

Look at the family of squirrels; here we have the finest gradation
from animals with their tails only slightly flattened, and from
others, as Sir J. Richardson has remarked, with the posterior part of
their bodies rather wide and with the skin on their flanks rather
full, to the so-called flying squirrels; and flying squirrels have
their limbs and even the base of the tail united by a broad expanse of
skin, which serves as a parachute and allows them to glide through the
air to an astonishing distance from tree to tree. We cannot doubt that
each structure is of use to each kind of squirrel in its own country,
by enabling it to escape birds or beasts of prey, or to collect food
more quickly, or, as there is reason to believe, by lessening the
danger from occasional falls. But it does not follow from this fact
that the structure of each squirrel is the best that it is possible to
conceive under all natural conditions. Let the climate and vegetation
change, let other competing rodents or new beasts of prey immigrate,
or old ones become modified, and all analogy would lead us to believe
that some at least of the squirrels would decrease in numbers or
become exterminated, unless they also became modified and improved in
structure in a corresponding manner. Therefore, I can see no
difficulty, more especially under changing conditions of life, in the
continued preservation of individuals with fuller and fuller
flank-membranes, each modification being useful, each being
propagated, until by the accumulated effects of this process of
natural selection, a perfect so-called flying squirrel was produced.

Now look at the Galeopithecus or flying lemur, which formerly was
falsely ranked amongst bats. It has an extremely wide flank-membrane,
stretching from the corners of the jaw to the tail, and including the
limbs and the elongated fingers: the flank membrane is, also,
furnished with an extensor muscle. Although no graduated links of
structure, fitted for gliding through the air, now connect the
Galeopithecus with the other Lemuridae, yet I can see no difficulty in
supposing that such links formerly existed, and that each had been
formed by the same steps as in the case of the less perfectly gliding
squirrels; and that each grade of structure had been useful to its
possessor. Nor can I see any insuperable difficulty in further
believing it possible that the membrane-connected fingers and fore-arm
of the Galeopithecus might be greatly lengthened by natural selection;
and this, as far as the organs of flight are concerned, would convert
it into a bat. In bats which have the wing-membrane extended from the
top of the shoulder to the tail, including the hind-legs, we perhaps
see traces of an apparatus originally constructed for gliding through
the air rather than for flight.

If about a dozen genera of birds had become extinct or were unknown,
who would have ventured to have surmised that birds might have existed
which used their wings solely as flappers, like the logger-headed duck
(Micropterus of Eyton); as fins in the water and front legs on the
land, like the penguin; as sails, like the ostrich; and functionally
for no purpose, like the Apteryx. Yet the structure of each of these
birds is good for it, under the conditions of life to which it is
exposed, for each has to live by a struggle; but it is not necessarily
the best possible under all possible conditions. It must not be
inferred from these remarks that any of the grades of wing-structure
here alluded to, which perhaps may all have resulted from disuse,
indicate the natural steps by which birds have acquired their perfect
power of flight; but they serve, at least, to show what diversified
means of transition are possible.

Seeing that a few members of such water-breathing classes as the
Crustacea and Mollusca are adapted to live on the land, and seeing
that we have flying birds and mammals, flying insects of the most
diversified types, and formerly had flying reptiles, it is conceivable
that flying-fish, which now glide far through the air, slightly rising
and turning by the aid of their fluttering fins, might have been
modified into perfectly winged animals. If this had been effected, who
would have ever imagined that in an early transitional state they had
been inhabitants of the open ocean, and had used their incipient
organs of flight exclusively, as far as we know, to escape being
devoured by other fish?

When we see any structure highly perfected for any particular habit,
as the wings of a bird for flight, we should bear in mind that animals
displaying early transitional grades of the structure will seldom
continue to exist to the present day, for they will have been
supplanted by the very process of perfection through natural
selection. Furthermore, we may conclude that transitional grades
between structures fitted for very different habits of life will
rarely have been developed at an early period in great numbers and
under many subordinate forms. Thus, to return to our imaginary
illustration of the flying-fish, it does not seem probable that fishes
capable of true flight would have been developed under many
subordinate forms, for taking prey of many kinds in many ways, on the
land and in the water, until their organs of flight had come to a high
stage of perfection, so as to have given them a decided advantage over
other animals in the battle for life. Hence the chance of discovering
species with transitional grades of structure in a fossil condition
will always be less, from their having existed in lesser numbers, than
in the case of species with fully developed structures.

I will now give two or three instances of diversified and of changed
habits in the individuals of the same species. When either case
occurs, it would be easy for natural selection to fit the animal, by
some modification of its structure, for its changed habits, or
exclusively for one of its several different habits. But it is
difficult to tell, and immaterial for us, whether habits generally
change first and structure afterwards; or whether slight modifications
of structure lead to changed habits; both probably often change almost
simultaneously. Of cases of changed habits it will suffice merely to
allude to that of the many British insects which now feed on exotic
plants, or exclusively on artificial substances. Of diversified habits
innumerable instances could be given: I have often watched a tyrant
flycatcher (Saurophagus sulphuratus) in South America, hovering over
one spot and then proceeding to another, like a kestrel, and at other
times standing stationary on the margin of water, and then dashing
like a kingfisher at a fish. In our own country the larger titmouse
(Parus major) may be seen climbing branches, almost like a creeper; it
often, like a shrike, kills small birds by blows on the head; and I
have many times seen and heard it hammering the seeds of the yew on a
branch, and thus breaking them like a nuthatch. In North America the
black bear was seen by Hearne swimming for hours with widely open
mouth, thus catching, like a whale, insects in the water. Even in so
extreme a case as this, if the supply of insects were constant, and if
better adapted competitors did not already exist in the country, I can
see no difficulty in a race of bears being rendered, by natural
selection, more and more aquatic in their structure and habits, with
larger and larger mouths, till a creature was produced as monstrous as
a whale.

As we sometimes see individuals of a species following habits widely
different from those both of their own species and of the other
species of the same genus, we might expect, on my theory, that such
individuals would occasionally have given rise to new species, having
anomalous habits, and with their structure either slightly or
considerably modified from that of their proper type. And such
instances do occur in nature. Can a more striking instance of
adaptation be given than that of a woodpecker for climbing trees and
for seizing insects in the chinks of the bark? Yet in North America
there are woodpeckers which feed largely on fruit, and others with
elongated wings which chase insects on the wing; and on the plains of
La Plata, where not a tree grows, there is a woodpecker, which in
every essential part of its organisation, even in its colouring, in
the harsh tone of its voice, and undulatory flight, told me plainly of
its close blood-relationship to our common species; yet it is a
woodpecker which never climbs a tree!

Petrels are the most aerial and oceanic of birds, yet in the quiet
Sounds of Tierra del Fuego, the Puffinuria berardi, in its general
habits, in its astonishing power of diving, its manner of swimming,
and of flying when unwillingly it takes flight, would be mistaken by
any one for an auk or grebe; nevertheless, it is essentially a petrel,
but with many parts of its organisation profoundly modified. On the
other hand, the acutest observer by examining the dead body of the
water-ouzel would never have suspected its sub-aquatic habits; yet
this anomalous member of the strictly terrestrial thrush family wholly
subsists by diving,--grasping the stones with its feet and using its
wings under water.

He who believes that each being has been created as we now see it,
must occasionally have felt surprise when he has met with an animal
having habits and structure not at all in agreement. What can be
plainer than that the webbed feet of ducks and geese are formed for
swimming? yet there are upland geese with webbed feet which rarely or
never go near the water; and no one except Audubon has seen the
frigate-bird, which has all its four toes webbed, alight on the
surface of the sea. On the other hand, grebes and coots are eminently
aquatic, although their toes are only bordered by membrane. What seems
plainer than that the long toes of grallatores are formed for walking
over swamps and floating plants, yet the water-hen is nearly as
aquatic as the coot; and the landrail nearly as terrestrial as the
quail or partridge. In such cases, and many others could be given,
habits have changed without a corresponding change of structure. The
webbed feet of the upland goose may be said to have become rudimentary
in function, though not in structure. In the frigate-bird, the
deeply-scooped membrane between the toes shows that structure has
begun to change.

He who believes in separate and innumerable acts of creation will say,
that in these cases it has pleased the Creator to cause a being of one
type to take the place of one of another type; but this seems to me
only restating the fact in dignified language. He who believes in the
struggle for existence and in the principle of natural selection, will
acknowledge that every organic being is constantly endeavouring to
increase in numbers; and that if any one being vary ever so little,
either in habits or structure, and thus gain an advantage over some
other inhabitant of the country, it will seize on the place of that
inhabitant, however different it may be from its own place. Hence it
will cause him no surprise that there should be geese and
frigate-birds with webbed feet, either living on the dry land or most
rarely alighting on the water; that there should be long-toed
corncrakes living in meadows instead of in swamps; that there should
be woodpeckers where not a tree grows; that there should be diving
thrushes, and petrels with the habits of auks.

ORGANS OF EXTREME PERFECTION AND COMPLICATION.

To suppose that the eye, with all its inimitable contrivances for
adjusting the focus to different distances, for admitting different
amounts of light, and for the correction of spherical and chromatic
aberration, could have been formed by natural selection, seems, I
freely confess, absurd in the highest possible degree. Yet reason
tells me, that if numerous gradations from a perfect and complex eye
to one very imperfect and simple, each grade being useful to its
possessor, can be shown to exist; if further, the eye does vary ever
so slightly, and the variations be inherited, which is certainly the
case; and if any variation or modification in the organ be ever useful
to an animal under changing conditions of life, then the difficulty of
believing that a perfect and complex eye could be formed by natural
selection, though insuperable by our imagination, can hardly be
considered real. How a nerve comes to be sensitive to light, hardly
concerns us more than how life itself first originated; but I may
remark that several facts make me suspect that any sensitive nerve may
be rendered sensitive to light, and likewise to those coarser
vibrations of the air which produce sound.

In looking for the gradations by which an organ in any species has
been perfected, we ought to look exclusively to its lineal ancestors;
but this is scarcely ever possible, and we are forced in each case to
look to species of the same group, that is to the collateral
descendants from the same original parent-form, in order to see what
gradations are possible, and for the chance of some gradations having
been transmitted from the earlier stages of descent, in an unaltered
or little altered condition. Amongst existing Vertebrata, we find but
a small amount of gradation in the structure of the eye, and from
fossil species we can learn nothing on this head. In this great class
we should probably have to descend far beneath the lowest known
fossiliferous stratum to discover the earlier stages, by which the eye
has been perfected.

In the Articulata we can commence a series with an optic nerve merely
coated with pigment, and without any other mechanism; and from this
low stage, numerous gradations of structure, branching off in two
fundamentally different lines, can be shown to exist, until we reach a
moderately high stage of perfection. In certain crustaceans, for
instance, there is a double cornea, the inner one divided into facets,
within each of which there is a lens-shaped swelling. In other
crustaceans the transparent cones which are coated by pigment, and
which properly act only by excluding lateral pencils of light, are
convex at their upper ends and must act by convergence; and at their
lower ends there seems to be an imperfect vitreous substance. With
these facts, here far too briefly and imperfectly given, which show
that there is much graduated diversity in the eyes of living
crustaceans, and bearing in mind how small the number of living
animals is in proportion to those which have become extinct, I can see
no very great difficulty (not more than in the case of many other
structures) in believing that natural selection has converted the
simple apparatus of an optic nerve merely coated with pigment and
invested by transparent membrane, into an optical instrument as
perfect as is possessed by any member of the great Articulate class.

He who will go thus far, if he find on finishing this treatise that
large bodies of facts, otherwise inexplicable, can be explained by the
theory of descent, ought not to hesitate to go further, and to admit
that a structure even as perfect as the eye of an eagle might be
formed by natural selection, although in this case he does not know
any of the transitional grades. His reason ought to conquer his
imagination; though I have felt the difficulty far too keenly to be
surprised at any degree of hesitation in extending the principle of
natural selection to such startling lengths.

It is scarcely possible to avoid comparing the eye to a telescope. We
know that this instrument has been perfected by the long-continued
efforts of the highest human intellects; and we naturally infer that
the eye has been formed by a somewhat analogous process. But may not
this inference be presumptuous? Have we any right to assume that the
Creator works by intellectual powers like those of man? If we must
compare the eye to an optical instrument, we ought in imagination to
take a thick layer of transparent tissue, with a nerve sensitive to
light beneath, and then suppose every part of this layer to be
continually changing slowly in density, so as to separate into layers
of different densities and thicknesses, placed at different distances
from each other, and with the surfaces of each layer slowly changing
in form. Further we must suppose that there is a power always intently
watching each slight accidental alteration in the transparent layers;
and carefully selecting each alteration which, under varied
circumstances, may in any way, or in any degree, tend to produce a
distincter image. We must suppose each new state of the instrument to
be multiplied by the million; and each to be preserved till a better
be produced, and then the old ones to be destroyed. In living bodies,
variation will cause the slight alterations, generation will multiply
them almost infinitely, and natural selection will pick out with
unerring skill each improvement. Let this process go on for millions
on millions of years; and during each year on millions of individuals
of many kinds; and may we not believe that a living optical instrument
might thus be formed as superior to one of glass, as the works of the
Creator are to those of man?

If it could be demonstrated that any complex organ existed, which
could not possibly have been formed by numerous, successive, slight
modifications, my theory would absolutely break down. But I can find
out no such case. No doubt many organs exist of which we do not know
the transitional grades, more especially if we look to much-isolated
species, round which, according to my theory, there has been much
extinction. Or again, if we look to an organ common to all the members
of a large class, for in this latter case the organ must have been
first formed at an extremely remote period, since which all the many
members of the class have been developed; and in order to discover the
early transitional grades through which the organ has passed, we
should have to look to very ancient ancestral forms, long since become
extinct.

We should be extremely cautious in concluding that an organ could not
have been formed by transitional gradations of some kind. Numerous
cases could be given amongst the lower animals of the same organ
performing at the same time wholly distinct functions; thus the
alimentary canal respires, digests, and excretes in the larva of the
dragon-fly and in the fish Cobites. In the Hydra, the animal may be
turned inside out, and the exterior surface will then digest and the
stomach respire. In such cases natural selection might easily
specialise, if any advantage were thus gained, a part or organ, which
had performed two functions, for one function alone, and thus wholly
change its nature by insensible steps. Two distinct organs sometimes
perform simultaneously the same function in the same individual; to
give one instance, there are fish with gills or branchiae that breathe
the air dissolved in the water, at the same time that they breathe
free air in their swimbladders, this latter organ having a ductus
pneumaticus for its supply, and being divided by highly vascular
partitions. In these cases, one of the two organs might with ease be
modified and perfected so as to perform all the work by itself, being
aided during the process of modification by the other organ; and then
this other organ might be modified for some other and quite distinct
purpose, or be quite obliterated.

The illustration of the swimbladder in fishes is a good one, because
it shows us clearly the highly important fact that an organ originally
constructed for one purpose, namely flotation, may be converted into
one for a wholly different purpose, namely respiration. The
swimbladder has, also, been worked in as an accessory to the auditory
organs of certain fish, or, for I do not know which view is now
generally held, a part of the auditory apparatus has been worked in as
a complement to the swimbladder. All physiologists admit that the
swimbladder is homologous, or "ideally similar," in position and
structure with the lungs of the higher vertebrate animals: hence there
seems to me to be no great difficulty in believing that natural
selection has actually converted a swimbladder into a lung, or organ
used exclusively for respiration.

I can, indeed, hardly doubt that all vertebrate animals having true
lungs have descended by ordinary generation from an ancient prototype,
of which we know nothing, furnished with a floating apparatus or
swimbladder. We can thus, as I infer from Professor Owen's interesting
description of these parts, understand the strange fact that every
particle of food and drink which we swallow has to pass over the
orifice of the trachea, with some risk of falling into the lungs,
notwithstanding the beautiful contrivance by which the glottis is
closed. In the higher Vertebrata the branchiae have wholly
disappeared--the slits on the sides of the neck and the loop-like
course of the arteries still marking in the embryo their former
position. But it is conceivable that the now utterly lost branchiae
might have been gradually worked in by natural selection for some
quite distinct purpose: in the same manner as, on the view entertained
by some naturalists that the branchiae and dorsal scales of Annelids
are homologous with the wings and wing-covers of insects, it is
probable that organs which at a very ancient period served for
respiration have been actually converted into organs of flight.

In considering transitions of organs, it is so important to bear in
mind the probability of conversion from one function to another, that
I will give one more instance. Pedunculated cirripedes have two minute
folds of skin, called by me the ovigerous frena, which serve, through
the means of a sticky secretion, to retain the eggs until they are
hatched within the sack. These cirripedes have no branchiae, the whole
surface of the body and sack, including the small frena, serving for
respiration. The Balanidae or sessile cirripedes, on the other hand,
have no ovigerous frena, the eggs lying loose at the bottom of the
sack, in the well-enclosed shell; but they have large folded
branchiae. Now I think no one will dispute that the ovigerous frena in
the one family are strictly homologous with the branchiae of the other
family; indeed, they graduate into each other. Therefore I do not
doubt that little folds of skin, which originally served as ovigerous
frena, but which, likewise, very slightly aided the act of
respiration, have been gradually converted by natural selection into
branchiae, simply through an increase in their size and the
obliteration of their adhesive glands. If all pedunculated cirripedes
had become extinct, and they have already suffered far more extinction
than have sessile cirripedes, who would ever have imagined that the
branchiae in this latter family had originally existed as organs for
preventing the ova from being washed out of the sack?

Although we must be extremely cautious in concluding that any organ
could not possibly have been produced by successive transitional
gradations, yet, undoubtedly, grave cases of difficulty occur, some of
which will be discussed in my future work.

One of the gravest is that of neuter insects, which are often very
differently constructed from either the males or fertile females; but
this case will be treated of in the next chapter. The electric organs
of fishes offer another case of special difficulty; it is impossible
to conceive by what steps these wondrous organs have been produced;
but, as Owen and others have remarked, their intimate structure
closely resembles that of common muscle; and as it has lately been
shown that Rays have an organ closely analogous to the electric
apparatus, and yet do not, as Matteuchi asserts, discharge any
electricity, we must own that we are far too ignorant to argue that no
transition of any kind is possible.

The electric organs offer another and even more serious difficulty;
for they occur in only about a dozen fishes, of which several are
widely remote in their affinities. Generally when the same organ
appears in several members of the same class, especially if in members
having very different habits of life, we may attribute its presence to
inheritance from a common ancestor; and its absence in some of the
members to its loss through disuse or natural selection. But if the
electric organs had been inherited from one ancient progenitor thus
provided, we might have expected that all electric fishes would have
been specially related to each other. Nor does geology at all lead to
the belief that formerly most fishes had electric organs, which most
of their modified descendants have lost. The presence of luminous
organs in a few insects, belonging to different families and orders,
offers a parallel case of difficulty. Other cases could be given; for
instance in plants, the very curious contrivance of a mass of
pollen-grains, borne on a foot-stalk with a sticky gland at the end,
is the same in Orchis and Asclepias,--genera almost as remote as
possible amongst flowering plants. In all these cases of two very
distinct species furnished with apparently the same anomalous organ,
it should be observed that, although the general appearance and
function of the organ may be the same, yet some fundamental difference
can generally be detected. I am inclined to believe that in nearly the
same way as two men have sometimes independently hit on the very same
invention, so natural selection, working for the good of each being
and taking advantage of analogous variations, has sometimes modified
in very nearly the same manner two parts in two organic beings, which
owe but little of their structure in common to inheritance from the
same ancestor.

Although in many cases it is most difficult to conjecture by what
transitions an organ could have arrived at its present state; yet,
considering that the proportion of living and known forms to the
extinct and unknown is very small, I have been astonished how rarely
an organ can be named, towards which no transitional grade is known to
lead. The truth of this remark is indeed shown by that old canon in
natural history of "Natura non facit saltum." We meet with this
admission in the writings of almost every experienced naturalist; or,
as Milne Edwards has well expressed it, nature is prodigal in variety,
but niggard in innovation. Why, on the theory of Creation, should this
be so? Why should all the parts and organs of many independent beings,
each supposed to have been separately created for its proper place in
nature, be so invariably linked together by graduated steps? Why
should not Nature have taken a leap from structure to structure? On
the theory of natural selection, we can clearly understand why she
should not; for natural selection can act only by taking advantage of
slight successive variations; she can never take a leap, but must
advance by the shortest and slowest steps.

ORGANS OF LITTLE APPARENT IMPORTANCE.

As natural selection acts by life and death,--by the preservation of
individuals with any favourable variation, and by the destruction of
those with any unfavourable deviation of structure,--I have sometimes
felt much difficulty in understanding the origin of simple parts, of
which the importance does not seem sufficient to cause the
preservation of successively varying individuals. I have sometimes
felt as much difficulty, though of a very different kind, on this
head, as in the case of an organ as perfect and complex as the eye.

In the first place, we are much too ignorant in regard to the whole
economy of any one organic being, to say what slight modifications
would be of importance or not. In a former chapter I have given
instances of most trifling characters, such as the down on fruit and
the colour of the flesh, which, from determining the attacks of
insects or from being correlated with constitutional differences,
might assuredly be acted on by natural selection. The tail of the
giraffe looks like an artificially constructed fly-flapper; and it
seems at first incredible that this could have been adapted for its
present purpose by successive slight modifications, each better and
better, for so trifling an object as driving away flies; yet we should
pause before being too positive even in this case, for we know that
the distribution and existence of cattle and other animals in South
America absolutely depends on their power of resisting the attacks of
insects: so that individuals which could by any means defend
themselves from these small enemies, would be able to range into new
pastures and thus gain a great advantage. It is not that the larger
quadrupeds are actually destroyed (except in some rare cases) by the
flies, but they are incessantly harassed and their strength reduced,
so that they are more subject to disease, or not so well enabled in a
coming dearth to search for food, or to escape from beasts of prey.

Organs now of trifling importance have probably in some cases been of
high importance to an early progenitor, and, after having been slowly
perfected at a former period, have been transmitted in nearly the same
state, although now become of very slight use; and any actually
injurious deviations in their structure will always have been checked
by natural selection. Seeing how important an organ of locomotion the
tail is in most aquatic animals, its general presence and use for many
purposes in so many land animals, which in their lungs or modified
swim-bladders betray their aquatic origin, may perhaps be thus
accounted for. A well-developed tail having been formed in an aquatic
animal, it might subsequently come to be worked in for all sorts of
purposes, as a fly-flapper, an organ of prehension, or as an aid in
turning, as with the dog, though the aid must be slight, for the hare,
with hardly any tail, can double quickly enough.

In the second place, we may sometimes attribute importance to
characters which are really of very little importance, and which have
originated from quite secondary causes, independently of natural
selection. We should remember that climate, food, etc., probably have
some little direct influence on the organisation; that characters
reappear from the law of reversion; that correlation of growth will
have had a most important influence in modifying various structures;
and finally, that sexual selection will often have largely modified
the external characters of animals having a will, to give one male an
advantage in fighting with another or in charming the females.
Moreover when a modification of structure has primarily arisen from
the above or other unknown causes, it may at first have been of no
advantage to the species, but may subsequently have been taken
advantage of by the descendants of the species under new conditions of
life and with newly acquired habits.

To give a few instances to illustrate these latter remarks. If green
woodpeckers alone had existed, and we did not know that there were
many black and pied kinds, I dare say that we should have thought that
the green colour was a beautiful adaptation to hide this
tree-frequenting bird from its enemies; and consequently that it was a
character of importance and might have been acquired through natural
selection; as it is, I have no doubt that the colour is due to some
quite distinct cause, probably to sexual selection. A trailing bamboo
in the Malay Archipelago climbs the loftiest trees by the aid of
exquisitely constructed hooks clustered around the ends of the
branches, and this contrivance, no doubt, is of the highest service to
the plant; but as we see nearly similar hooks on many trees which are
not climbers, the hooks on the bamboo may have arisen from unknown
laws of growth, and have been subsequently taken advantage of by the
plant undergoing further modification and becoming a climber. The
naked skin on the head of a vulture is generally looked at as a direct
adaptation for wallowing in putridity; and so it may be, or it may
possibly be due to the direct action of putrid matter; but we should
be very cautious in drawing any such inference, when we see that the
skin on the head of the clean-feeding male turkey is likewise naked.
The sutures in the skulls of young mammals have been advanced as a
beautiful adaptation for aiding parturition, and no doubt they
facilitate, or may be indispensable for this act; but as sutures occur
in the skulls of young birds and reptiles, which have only to escape
from a broken egg, we may infer that this structure has arisen from
the laws of growth, and has been taken advantage of in the parturition
of the higher animals.

We are profoundly ignorant of the causes producing slight and
unimportant variations; and we are immediately made conscious of this
by reflecting on the differences in the breeds of our domesticated
animals in different countries,--more especially in the less civilized
countries where there has been but little artificial selection.
Careful observers are convinced that a damp climate affects the growth
of the hair, and that with the hair the horns are correlated. Mountain
breeds always differ from lowland breeds; and a mountainous country
would probably affect the hind limbs from exercising them more, and
possibly even the form of the pelvis; and then by the law of
homologous variation, the front limbs and even the head would probably
be affected. The shape, also, of the pelvis might affect by pressure
the shape of the head of the young in the womb. The laborious
breathing necessary in high regions would, we have some reason to
believe, increase the size of the chest; and again correlation would
come into play. Animals kept by savages in different countries often
have to struggle for their own subsistence, and would be exposed to a
certain extent to natural selection, and individuals with slightly
different constitutions would succeed best under different climates;
and there is reason to believe that constitution and colour are
correlated. A good observer, also, states that in cattle
susceptibility to the attacks of flies is correlated with colour, as
is the liability to be poisoned by certain plants; so that colour
would be thus subjected to the action of natural selection. But we are
far too ignorant to speculate on the relative importance of the
several known and unknown laws of variation; and I have here alluded
to them only to show that, if we are unable to account for the
characteristic differences of our domestic breeds, which nevertheless
we generally admit to have arisen through ordinary generation, we
ought not to lay too much stress on our ignorance of the precise cause
of the slight analogous differences between species. I might have
adduced for this same purpose the differences between the races of
man, which are so strongly marked; I may add that some little light
can apparently be thrown on the origin of these differences, chiefly
through sexual selection of a particular kind, but without here
entering on copious details my reasoning would appear frivolous.

The foregoing remarks lead me to say a few words on the protest lately
made by some naturalists, against the utilitarian doctrine that every
detail of structure has been produced for the good of its possessor.
They believe that very many structures have been created for beauty in
the eyes of man, or for mere variety. This doctrine, if true, would be
absolutely fatal to my theory. Yet I fully admit that many structures
are of no direct use to their possessors. Physical conditions probably
have had some little effect on structure, quite independently of any
good thus gained. Correlation of growth has no doubt played a most
important part, and a useful modification of one part will often have
entailed on other parts diversified changes of no direct use. So again
characters which formerly were useful, or which formerly had arisen
from correlation of growth, or from other unknown cause, may reappear
from the law of reversion, though now of no direct use. The effects of
sexual selection, when displayed in beauty to charm the females, can
be called useful only in rather a forced sense. But by far the most
important consideration is that the chief part of the organisation of
every being is simply due to inheritance; and consequently, though
each being assuredly is well fitted for its place in nature, many
structures now have no direct relation to the habits of life of each
species. Thus, we can hardly believe that the webbed feet of the
upland goose or of the frigate-bird are of special use to these birds;
we cannot believe that the same bones in the arm of the monkey, in the
fore leg of the horse, in the wing of the bat, and in the flipper of
the seal, are of special use to these animals. We may safely attribute
these structures to inheritance. But to the progenitor of the upland
goose and of the frigate-bird, webbed feet no doubt were as useful as
they now are to the most aquatic of existing birds. So we may believe
that the progenitor of the seal had not a flipper, but a foot with
five toes fitted for walking or grasping; and we may further venture
to believe that the several bones in the limbs of the monkey, horse,
and bat, which have been inherited from a common progenitor, were
formerly of more special use to that progenitor, or its progenitors,
than they now are to these animals having such widely diversified
habits. Therefore we may infer that these several bones might have
been acquired through natural selection, subjected formerly, as now,
to the several laws of inheritance, reversion, correlation of growth,
etc. Hence every detail of structure in every living creature (making
some little allowance for the direct action of physical conditions)
may be viewed, either as having been of special use to some ancestral
form, or as being now of special use to the descendants of this
form--either directly, or indirectly through the complex laws of
growth.

Natural selection cannot possibly produce any modification in any one
species exclusively for the good of another species; though throughout
nature one species incessantly takes advantage of, and profits by, the
structure of another. But natural selection can and does often produce
structures for the direct injury of other species, as we see in the
fang of the adder, and in the ovipositor of the ichneumon, by which
its eggs are deposited in the living bodies of other insects. If it
could be proved that any part of the structure of any one species had
been formed for the exclusive good of another species, it would
annihilate my theory, for such could not have been produced through
natural selection. Although many statements may be found in works on
natural history to this effect, I cannot find even one which seems to
me of any weight. It is admitted that the rattlesnake has a
poison-fang for its own defence and for the destruction of its prey;
but some authors suppose that at the same time this snake is furnished
with a rattle for its own injury, namely, to warn its prey to escape.
I would almost as soon believe that the cat curls the end of its tail
when preparing to spring, in order to warn the doomed mouse. But I
have not space here to enter on this and other such cases.

Natural selection will never produce in a being anything injurious to
itself, for natural selection acts solely by and for the good of each.
No organ will be formed, as Paley has remarked, for the purpose of
causing pain or for doing an injury to its possessor. If a fair
balance be struck between the good and evil caused by each part, each
will be found on the whole advantageous. After the lapse of time,
under changing conditions of life, if any part comes to be injurious,
it will be modified; or if it be not so, the being will become
extinct, as myriads have become extinct.

Natural selection tends only to make each organic being as perfect as,
or slightly more perfect than, the other inhabitants of the same
country with which it has to struggle for existence. And we see that
this is the degree of perfection attained under nature. The endemic
productions of New Zealand, for instance, are perfect one compared
with another; but they are now rapidly yielding before the advancing
legions of plants and animals introduced from Europe. Natural
selection will not produce absolute perfection, nor do we always meet,
as far as we can judge, with this high standard under nature. The
correction for the aberration of light is said, on high authority, not
to be perfect even in that most perfect organ, the eye. If our reason
leads us to admire with enthusiasm a multitude of inimitable
contrivances in nature, this same reason tells us, though we may
easily err on both sides, that some other contrivances are less
perfect. Can we consider the sting of the wasp or of the bee as
perfect, which, when used against many attacking animals, cannot be
withdrawn, owing to the backward serratures, and so inevitably causes
the death of the insect by tearing out its viscera?

If we look at the sting of the bee, as having originally existed in a
remote progenitor as a boring and serrated instrument, like that in so
many members of the same great order, and which has been modified but
not perfected for its present purpose, with the poison originally
adapted to cause galls subsequently intensified, we can perhaps
understand how it is that the use of the sting should so often cause
the insect's own death: for if on the whole the power of stinging be
useful to the community, it will fulfil all the requirements of
natural selection, though it may cause the death of some few members.
If we admire the truly wonderful power of scent by which the males of
many insects find their females, can we admire the production for this
single purpose of thousands of drones, which are utterly useless to
the community for any other end, and which are ultimately slaughtered
by their industrious and sterile sisters? It may be difficult, but we
ought to admire the savage instinctive hatred of the queen-bee, which
urges her instantly to destroy the young queens her daughters as soon
as born, or to perish herself in the combat; for undoubtedly this is
for the good of the community; and maternal love or maternal hatred,
though the latter fortunately is most rare, is all the same to the
inexorable principle of natural selection. If we admire the several
ingenious contrivances, by which the flowers of the orchis and of many
other plants are fertilised through insect agency, can we consider as
equally perfect the elaboration by our fir-trees of dense clouds of
pollen, in order that a few granules may be wafted by a chance breeze
on to the ovules?

SUMMARY OF CHAPTER.

We have in this chapter discussed some of the difficulties and
objections which may be urged against my theory. Many of them are very
grave; but I think that in the discussion light has been thrown on
several facts, which on the theory of independent acts of creation are
utterly obscure. We have seen that species at any one period are not
indefinitely variable, and are not linked together by a multitude of
intermediate gradations, partly because the process of natural
selection will always be very slow, and will act, at any one time,
only on a very few forms; and partly because the very process of
natural selection almost implies the continual supplanting and
extinction of preceding and intermediate gradations. Closely allied
species, now living on a continuous area, must often have been formed
when the area was not continuous, and when the conditions of life did
not insensibly graduate away from one part to another. When two
varieties are formed in two districts of a continuous area, an
intermediate variety will often be formed, fitted for an intermediate
zone; but from reasons assigned, the intermediate variety will usually
exist in lesser numbers than the two forms which it connects;
consequently the two latter, during the course of further
modification, from existing in greater numbers, will have a great
advantage over the less numerous intermediate variety, and will thus
generally succeed in supplanting and exterminating it.

We have seen in this chapter how cautious we should be in concluding
that the most different habits of life could not graduate into each
other; that a bat, for instance, could not have been formed by natural
selection from an animal which at first could only glide through the
air.

We have seen that a species may under new conditions of life change
its habits, or have diversified habits, with some habits very unlike
those of its nearest congeners. Hence we can understand, bearing in
mind that each organic being is trying to live wherever it can live,
how it has arisen that there are upland geese with webbed feet, ground
woodpeckers, diving thrushes, and petrels with the habits of auks.

Although the belief that an organ so perfect as the eye could have
been formed by natural selection, is more than enough to stagger any
one; yet in the case of any organ, if we know of a long series of
gradations in complexity, each good for its possessor, then, under
changing conditions of life, there is no logical impossibility in the
acquirement of any conceivable degree of perfection through natural
selection. In the cases in which we know of no intermediate or
transitional states, we should be very cautious in concluding that
none could have existed, for the homologies of many organs and their
intermediate states show that wonderful metamorphoses in function are
at least possible. For instance, a swim-bladder has apparently been
converted into an air-breathing lung. The same organ having performed
simultaneously very different functions, and then having been
specialised for one function; and two very distinct organs having
performed at the same time the same function, the one having been
perfected whilst aided by the other, must often have largely
facilitated transitions.

We are far too ignorant, in almost every case, to be enabled to assert
that any part or organ is so unimportant for the welfare of a species,
that modifications in its structure could not have been slowly
accumulated by means of natural selection. But we may confidently
believe that many modifications, wholly due to the laws of growth, and
at first in no way advantageous to a species, have been subsequently
taken advantage of by the still further modified descendants of this
species. We may, also, believe that a part formerly of high importance
has often been retained (as the tail of an aquatic animal by its
terrestrial descendants), though it has become of such small
importance that it could not, in its present state, have been acquired
by natural selection,--a power which acts solely by the preservation
of profitable variations in the struggle for life.

Natural selection will produce nothing in one species for the
exclusive good or injury of another; though it may well produce parts,
organs, and excretions highly useful or even indispensable, or highly
injurious to another species, but in all cases at the same time useful
to the owner. Natural selection in each well-stocked country, must act
chiefly through the competition of the inhabitants one with another,
and consequently will produce perfection, or strength in the battle
for life, only according to the standard of that country. Hence the
inhabitants of one country, generally the smaller one, will often
yield, as we see they do yield, to the inhabitants of another and
generally larger country. For in the larger country there will have
existed more individuals, and more diversified forms, and the
competition will have been severer, and thus the standard of
perfection will have been rendered higher. Natural selection will not
necessarily produce absolute perfection; nor, as far as we can judge
by our limited faculties, can absolute perfection be everywhere found.

On the theory of natural selection we can clearly understand the full
meaning of that old canon in natural history, "Natura non facit
saltum." This canon, if we look only to the present inhabitants of the
world, is not strictly correct, but if we include all those of past
times, it must by my theory be strictly true.

It is generally acknowledged that all organic beings have been formed
on two great laws--Unity of Type, and the Conditions of Existence. By
unity of type is meant that fundamental agreement in structure, which
we see in organic beings of the same class, and which is quite
independent of their habits of life. On my theory, unity of type is
explained by unity of descent. The expression of conditions of
existence, so often insisted on by the illustrious Cuvier, is fully
embraced by the principle of natural selection. For natural selection
acts by either now adapting the varying parts of each being to its
organic and inorganic conditions of life; or by having adapted them
during long-past periods of time: the adaptations being aided in some
cases by use and disuse, being slightly affected by the direct action
of the external conditions of life, and being in all cases subjected
to the several laws of growth. Hence, in fact, the law of the
Conditions of Existence is the higher law; as it includes, through the
inheritance of former adaptations, that of Unity of Type.


CHAPTER 7. INSTINCT.

Instincts comparable with habits, but different in their origin.
Instincts graduated.
Aphides and ants.
Instincts variable.
Domestic instincts, their origin.
Natural instincts of the cuckoo, ostrich, and parasitic bees.
Slave-making ants.
Hive-bee, its cell-making instinct.
Difficulties on the theory of the Natural Selection of instincts.
Neuter or sterile insects.
Summary.

The subject of instinct might have been worked into the previous
chapters; but I have thought that it would be more convenient to treat
the subject separately, especially as so wonderful an instinct as that
of the hive-bee making its cells will probably have occurred to many
readers, as a difficulty sufficient to overthrow my whole theory. I
must premise, that I have nothing to do with the origin of the primary
mental powers, any more than I have with that of life itself. We are
concerned only with the diversities of instinct and of the other
mental qualities of animals within the same class.

I will not attempt any definition of instinct. It would be easy to
show that several distinct mental actions are commonly embraced by
this term; but every one understands what is meant, when it is said
that instinct impels the cuckoo to migrate and to lay her eggs in
other birds' nests. An action, which we ourselves should require
experience to enable us to perform, when performed by an animal, more
especially by a very young one, without any experience, and when
performed by many individuals in the same way, without their knowing
for what purpose it is performed, is usually said to be instinctive.
But I could show that none of these characters of instinct are
universal. A little dose, as Pierre Huber expresses it, of judgment or
reason, often comes into play, even in animals very low in the scale
of nature.

Frederick Cuvier and several of the older metaphysicians have compared
instinct with habit. This comparison gives, I think, a remarkably
accurate notion of the frame of mind under which an instinctive action
is performed, but not of its origin. How unconsciously many habitual
actions are performed, indeed not rarely in direct opposition to our
conscious will! yet they may be modified by the will or reason. Habits
easily become associated with other habits, and with certain periods
of time and states of the body. When once acquired, they often remain
constant throughout life. Several other points of resemblance between
instincts and habits could be pointed out. As in repeating a
well-known song, so in instincts, one action follows another by a sort
of rhythm; if a person be interrupted in a song, or in repeating
anything by rote, he is generally forced to go back to recover the
habitual train of thought: so P. Huber found it was with a
caterpillar, which makes a very complicated hammock; for if he took a
caterpillar which had completed its hammock up to, say, the sixth
stage of construction, and put it into a hammock completed up only to
the third stage, the caterpillar simply re-performed the fourth,
fifth, and sixth stages of construction. If, however, a caterpillar
were taken out of a hammock made up, for instance, to the third stage,
and were put into one finished up to the sixth stage, so that much of
its work was already done for it, far from feeling the benefit of
this, it was much embarrassed, and, in order to complete its hammock,
seemed forced to start from the third stage, where it had left off,
and thus tried to complete the already finished work. If we suppose
any habitual action to become inherited--and I think it can be shown
that this does sometimes happen--then the resemblance between what
originally was a habit and an instinct becomes so close as not to be
distinguished. If Mozart, instead of playing the pianoforte at three
years old with wonderfully little practice, had played a tune with no
practice at all, he might truly be said to have done so instinctively.
But it would be the most serious error to suppose that the greater
number of instincts have been acquired by habit in one generation, and
then transmitted by inheritance to succeeding generations. It can be
clearly shown that the most wonderful instincts with which we are
acquainted, namely, those of the hive-bee and of many ants, could not
possibly have been thus acquired.

It will be universally admitted that instincts are as important as
corporeal structure for the welfare of each species, under its present
conditions of life. Under changed conditions of life, it is at least
possible that slight modifications of instinct might be profitable to
a species; and if it can be shown that instincts do vary ever so
little, then I can see no difficulty in natural selection preserving
and continually accumulating variations of instinct to any extent that
may be profitable. It is thus, as I believe, that all the most complex
and wonderful instincts have originated. As modifications of corporeal
structure arise from, and are increased by, use or habit, and are
diminished or lost by disuse, so I do not doubt it has been with
instincts. But I believe that the effects of habit are of quite
subordinate importance to the effects of the natural selection of what
may be called accidental variations of instincts;--that is of
variations produced by the same unknown causes which produce slight
deviations of bodily structure.

No complex instinct can possibly be produced through natural
selection, except by the slow and gradual accumulation of numerous,
slight, yet profitable, variations. Hence, as in the case of corporeal
structures, we ought to find in nature, not the actual transitional
gradations by which each complex instinct has been acquired--for these
could be found only in the lineal ancestors of each species--but we
ought to find in the collateral lines of descent some evidence of such
gradations; or we ought at least to be able to show that gradations of
some kind are possible; and this we certainly can do. I have been
surprised to find, making allowance for the instincts of animals
having been but little observed except in Europe and North America,
and for no instinct being known amongst extinct species, how very
generally gradations, leading to the most complex instincts, can be
discovered. The canon of "Natura non facit saltum" applies with almost
equal force to instincts as to bodily organs. Changes of instinct may
sometimes be facilitated by the same species having different
instincts at different periods of life, or at different seasons of the
year, or when placed under different circumstances, etc.; in which
case either one or the other instinct might be preserved by natural
selection. And such instances of diversity of instinct in the same
species can be shown to occur in nature.

Again as in the case of corporeal structure, and conformably with my
theory, the instinct of each species is good for itself, but has
never, as far as we can judge, been produced for the exclusive good of
others. One of the strongest instances of an animal apparently
performing an action for the sole good of another, with which I am
acquainted, is that of aphides voluntarily yielding their sweet
excretion to ants: that they do so voluntarily, the following facts
show. I removed all the ants from a group of about a dozen aphides on
a dock-plant, and prevented their attendance during several hours.
After this interval, I felt sure that the aphides would want to
excrete. I watched them for some time through a lens, but not one
excreted; I then tickled and stroked them with a hair in the same
manner, as well as I could, as the ants do with their antennae; but
not one excreted. Afterwards I allowed an ant to visit them, and it
immediately seemed, by its eager way of running about, to be well
aware what a rich flock it had discovered; it then began to play with
its antennae on the abdomen first of one aphis and then of another;
and each aphis, as soon as it felt the antennae, immediately lifted up
its abdomen and excreted a limpid drop of sweet juice, which was
eagerly devoured by the ant. Even the quite young aphides behaved in
this manner, showing that the action was instinctive, and not the
result of experience. But as the excretion is extremely viscid, it is
probably a convenience to the aphides to have it removed; and
therefore probably the aphides do not instinctively excrete for the
sole good of the ants. Although I do not believe that any animal in
the world performs an action for the exclusive good of another of a
distinct species, yet each species tries to take advantage of the
instincts of others, as each takes advantage of the weaker bodily
structure of others. So again, in some few cases, certain instincts
cannot be considered as absolutely perfect; but as details on this and
other such points are not indispensable, they may be here passed over.

As some degree of variation in instincts under a state of nature, and
the inheritance of such variations, are indispensable for the action
of natural selection, as many instances as possible ought to have been
here given; but want of space prevents me. I can only assert, that
instincts certainly do vary--for instance, the migratory instinct,
both in extent and direction, and in its total loss. So it is with the
nests of birds, which vary partly in dependence on the situations
chosen, and on the nature and temperature of the country inhabited,
but often from causes wholly unknown to us: Audubon has given several
remarkable cases of differences in nests of the same species in the
northern and southern United States. Fear of any particular enemy is
certainly an instinctive quality, as may be seen in nestling birds,
though it is strengthened by experience, and by the sight of fear of
the same enemy in other animals. But fear of man is slowly acquired,
as I have elsewhere shown, by various animals inhabiting desert
islands; and we may see an instance of this, even in England, in the
greater wildness of all our large birds than of our small birds; for
the large birds have been most persecuted by man. We may safely
attribute the greater wildness of our large birds to this cause; for
in uninhabited islands large birds are not more fearful than small;
and the magpie, so wary in England, is tame in Norway, as is the
hooded crow in Egypt.

That the general disposition of individuals of the same species, born
in a state of nature, is extremely diversified, can be shown by a
multitude of facts. Several cases also, could be given, of occasional
and strange habits in certain species, which might, if advantageous to
the species, give rise, through natural selection, to quite new
instincts. But I am well aware that these general statements, without
facts given in detail, can produce but a feeble effect on the reader's
mind. I can only repeat my assurance, that I do not speak without good
evidence.

The possibility, or even probability, of inherited variations of
instinct in a state of nature will be strengthened by briefly
considering a few cases under domestication. We shall thus also be
enabled to see the respective parts which habit and the selection of
so-called accidental variations have played in modifying the mental
qualities of our domestic animals. A number of curious and authentic
instances could be given of the inheritance of all shades of
disposition and tastes, and likewise of the oddest tricks, associated
with certain frames of mind or periods of time. But let us look to the
familiar case of the several breeds of dogs: it cannot be doubted that
young pointers (I have myself seen a striking instance) will sometimes
point and even back other dogs the very first time that they are taken
out; retrieving is certainly in some degree inherited by retrievers;
and a tendency to run round, instead of at, a flock of sheep, by
shepherd-dogs. I cannot see that these actions, performed without
experience by the young, and in nearly the same manner by each
individual, performed with eager delight by each breed, and without
the end being known,--for the young pointer can no more know that he
points to aid his master, than the white butterfly knows why she lays
her eggs on the leaf of the cabbage,--I cannot see that these actions
differ essentially from true instincts. If we were to see one kind of
wolf, when young and without any training, as soon as it scented its
prey, stand motionless like a statue, and then slowly crawl forward
with a peculiar gait; and another kind of wolf rushing round, instead
of at, a herd of deer, and driving them to a distant point, we should
assuredly call these actions instinctive. Domestic instincts, as they
may be called, are certainly far less fixed or invariable than natural
instincts; but they have been acted on by far less rigorous selection,
and have been transmitted for an incomparably shorter period, under
less fixed conditions of life.

How strongly these domestic instincts, habits, and dispositions are
inherited, and how curiously they become mingled, is well shown when
different breeds of dogs are crossed. Thus it is known that a cross
with a bull-dog has affected for many generations the courage and
obstinacy of greyhounds; and a cross with a greyhound has given to a
whole family of shepherd-dogs a tendency to hunt hares. These domestic
instincts, when thus tested by crossing, resemble natural instincts,
which in a like manner become curiously blended together, and for a
long period exhibit traces of the instincts of either parent: for
example, Le Roy describes a dog, whose great-grandfather was a wolf,
and this dog showed a trace of its wild parentage only in one way, by
not coming in a straight line to his master when called.

Domestic instincts are sometimes spoken of as actions which have
become inherited solely from long-continued and compulsory habit, but
this, I think, is not true. No one would ever have thought of
teaching, or probably could have taught, the tumbler-pigeon to
tumble,--an action which, as I have witnessed, is performed by young
birds, that have never seen a pigeon tumble. We may believe that some
one pigeon showed a slight tendency to this strange habit, and that
the long-continued selection of the best individuals in successive
generations made tumblers what they now are; and near Glasgow there
are house-tumblers, as I hear from Mr. Brent, which cannot fly
eighteen inches high without going head over heels. It may be doubted
whether any one would have thought of training a dog to point, had not
some one dog naturally shown a tendency in this line; and this is
known occasionally to happen, as I once saw in a pure terrier. When
the first tendency was once displayed, methodical selection and the
inherited effects of compulsory training in each successive generation
would soon complete the work; and unconscious selection is still at
work, as each man tries to procure, without intending to improve the
breed, dogs which will stand and hunt best. On the other hand, habit
alone in some cases has sufficed; no animal is more difficult to tame
than the young of the wild rabbit; scarcely any animal is tamer than
the young of the tame rabbit; but I do not suppose that domestic
rabbits have ever been selected for tameness; and I presume that we
must attribute the whole of the inherited change from extreme wildness
to extreme tameness, simply to habit and long-continued close
confinement.

Natural instincts are lost under domestication: a remarkable instance
of this is seen in those breeds of fowls which very rarely or never
become "broody," that is, never wish to sit on their eggs. Familiarity
alone prevents our seeing how universally and largely the minds of our
domestic animals have been modified by domestication. It is scarcely
possible to doubt that the love of man has become instinctive in the
dog. All wolves, foxes, jackals, and species of the cat genus, when
kept tame, are most eager to attack poultry, sheep, and pigs; and this
tendency has been found incurable in dogs which have been brought home
as puppies from countries, such as Tierra del Fuego and Australia,
where the savages do not keep these domestic animals. How rarely, on
the other hand, do our civilised dogs, even when quite young, require
to be taught not to attack poultry, sheep, and pigs! No doubt they
occasionally do make an attack, and are then beaten; and if not cured,
they are destroyed; so that habit, with some degree of selection, has
probably concurred in civilising by inheritance our dogs. On the other
hand, young chickens have lost, wholly by habit, that fear of the dog
and cat which no doubt was originally instinctive in them, in the same
way as it is so plainly instinctive in young pheasants, though reared
under a hen. It is not that chickens have lost all fear, but fear only
of dogs and cats, for if the hen gives the danger-chuckle, they will
run (more especially young turkeys) from under her, and conceal
themselves in the surrounding grass or thickets; and this is evidently
done for the instinctive purpose of allowing, as we see in wild
ground-birds, their mother to fly away. But this instinct retained by
our chickens has become useless under domestication, for the
mother-hen has almost lost by disuse the power of flight.

Hence, we may conclude, that domestic instincts have been acquired and
natural instincts have been lost partly by habit, and partly by man
selecting and accumulating during successive generations, peculiar
mental habits and actions, which at first appeared from what we must
in our ignorance call an accident. In some cases compulsory habit
alone has sufficed to produce such inherited mental changes; in other
cases compulsory habit has done nothing, and all has been the result
of selection, pursued both methodically and unconsciously; but in most
cases, probably, habit and selection have acted together.

We shall, perhaps, best understand how instincts in a state of nature
have become modified by selection, by considering a few cases. I will
select only three, out of the several which I shall have to discuss in
my future work,--namely, the instinct which leads the cuckoo to lay
her eggs in other birds' nests; the slave-making instinct of certain
ants; and the comb-making power of the hive-bee: these two latter
instincts have generally, and most justly, been ranked by naturalists
as the most wonderful of all known instincts.

It is now commonly admitted that the more immediate and final cause of
the cuckoo's instinct is, that she lays her eggs, not daily, but at
intervals of two or three days; so that, if she were to make her own
nest and sit on her own eggs, those first laid would have to be left
for some time unincubated, or there would be eggs and young birds of
different ages in the same nest. If this were the case, the process of
laying and hatching might be inconveniently long, more especially as
she has to migrate at a very early period; and the first hatched young
would probably have to be fed by the male alone. But the American
cuckoo is in this predicament; for she makes her own nest and has eggs
and young successively hatched, all at the same time. It has been
asserted that the American cuckoo occasionally lays her eggs in other
birds' nests; but I hear on the high authority of Dr. Brewer, that
this is a mistake. Nevertheless, I could give several instances of
various birds which have been known occasionally to lay their eggs in
other birds' nests. Now let us suppose that the ancient progenitor of
our European cuckoo had the habits of the American cuckoo; but that
occasionally she laid an egg in another bird's nest. If the old bird
profited by this occasional habit, or if the young were made more
vigorous by advantage having been taken of the mistaken maternal
instinct of another bird, than by their own mother's care, encumbered
as she can hardly fail to be by having eggs and young of different
ages at the same time; then the old birds or the fostered young would
gain an advantage. And analogy would lead me to believe, that the
young thus reared would be apt to follow by inheritance the occasional
and aberrant habit of their mother, and in their turn would be apt to
lay their eggs in other birds' nests, and thus be successful in
rearing their young. By a continued process of this nature, I believe
that the strange instinct of our cuckoo could be, and has been,
generated. I may add that, according to Dr. Gray and to some other
observers, the European cuckoo has not utterly lost all maternal love
and care for her own offspring.

The occasional habit of birds laying their eggs in other birds' nests,
either of the same or of a distinct species, is not very uncommon with
the Gallinaceae; and this perhaps explains the origin of a singular
instinct in the allied group of ostriches. For several hen ostriches,
at least in the case of the American species, unite and lay first a
few eggs in one nest and then in another; and these are hatched by the
males. This instinct may probably be accounted for by the fact of the
hens laying a large number of eggs; but, as in the case of the cuckoo,
at intervals of two or three days. This instinct, however, of the
American ostrich has not as yet been perfected; for a surprising
number of eggs lie strewed over the plains, so that in one day's
hunting I picked up no less than twenty lost and wasted eggs.

Many bees are parasitic, and always lay their eggs in the nests of
bees of other kinds. This case is more remarkable than that of the
cuckoo; for these bees have not only their instincts but their
structure modified in accordance with their parasitic habits; for they
do not possess the pollen-collecting apparatus which would be
necessary if they had to store food for their own young. Some species,
likewise, of Sphegidae (wasp-like insects) are parasitic on other
species; and M. Fabre has lately shown good reason for believing that
although the Tachytes nigra generally makes its own burrow and stores
it with paralysed prey for its own larvae to feed on, yet that when
this insect finds a burrow already made and stored by another sphex,
it takes advantage of the prize, and becomes for the occasion
parasitic. In this case, as with the supposed case of the cuckoo, I
can see no difficulty in natural selection making an occasional habit
permanent, if of advantage to the species, and if the insect whose
nest and stored food are thus feloniously appropriated, be not thus
exterminated.

SLAVE-MAKING INSTINCT.

This remarkable instinct was first discovered in the Formica
(Polyerges) rufescens by Pierre Huber, a better observer even than his
celebrated father. This ant is absolutely dependent on its slaves;
without their aid, the species would certainly become extinct in a
single year. The males and fertile females do no work. The workers or
sterile females, though most energetic and courageous in capturing
slaves, do no other work. They are incapable of making their own
nests, or of feeding their own larvae. When the old nest is found
inconvenient, and they have to migrate, it is the slaves which
determine the migration, and actually carry their masters in their
jaws. So utterly helpless are the masters, that when Huber shut up
thirty of them without a slave, but with plenty of the food which they
like best, and with their larvae and pupae to stimulate them to work,
they did nothing; they could not even feed themselves, and many
perished of hunger. Huber then introduced a single slave (F. fusca),
and she instantly set to work, fed and saved the survivors; made some
cells and tended the larvae, and put all to rights. What can be more
extraordinary than these well-ascertained facts? If we had not known
of any other slave-making ant, it would have been hopeless to have
speculated how so wonderful an instinct could have been perfected.

Formica sanguinea was likewise first discovered by P. Huber to be a
slave-making ant. This species is found in the southern parts of
England, and its habits have been attended to by Mr. F. Smith, of the
British Museum, to whom I am much indebted for information on this and
other subjects. Although fully trusting to the statements of Huber and
Mr. Smith, I tried to approach the subject in a sceptical frame of
mind, as any one may well be excused for doubting the truth of so
extraordinary and odious an instinct as that of making slaves. Hence I
will give the observations which I have myself made, in some little
detail. I opened fourteen nests of F. sanguinea, and found a few
slaves in all. Males and fertile females of the slave-species are
found only in their own proper communities, and have never been
observed in the nests of F. sanguinea. The slaves are black and not
above half the size of their red masters, so that the contrast in
their appearance is very great. When the nest is slightly disturbed,
the slaves occasionally come out, and like their masters are much
agitated and defend the nest: when the nest is much disturbed and the
larvae and pupae are exposed, the slaves work energetically with their
masters in carrying them away to a place of safety. Hence, it is
clear, that the slaves feel quite at home. During the months of June
and July, on three successive years, I have watched for many hours
several nests in Surrey and Sussex, and never saw a slave either leave
or enter a nest. As, during these months, the slaves are very few in
number, I thought that they might behave differently when more
numerous; but Mr. Smith informs me that he has watched the nests at
various hours during May, June and August, both in Surrey and
Hampshire, and has never seen the slaves, though present in large
numbers in August, either leave or enter the nest. Hence he considers
them as strictly household slaves. The masters, on the other hand, may
be constantly seen bringing in materials for the nest, and food of all
kinds. During the present year, however, in the month of July, I came
across a community with an unusually large stock of slaves, and I
observed a few slaves mingled with their masters leaving the nest, and
marching along the same road to a tall Scotch-fir-tree, twenty-five
yards distant, which they ascended together, probably in search of
aphides or cocci. According to Huber, who had ample opportunities for
observation, in Switzerland the slaves habitually work with their
masters in making the nest, and they alone open and close the doors in
the morning and evening; and, as Huber expressly states, their
principal office is to search for aphides. This difference in the
usual habits of the masters and slaves in the two countries, probably
depends merely on the slaves being captured in greater numbers in
Switzerland than in England.

One day I fortunately chanced to witness a migration from one nest to
another, and it was a most interesting spectacle to behold the masters
carefully carrying, as Huber has described, their slaves in their
jaws. Another day my attention was struck by about a score of the
slave-makers haunting the same spot, and evidently not in search of
food; they approached and were vigorously repulsed by an independent
community of the slave species (F. fusca); sometimes as many as three
of these ants clinging to the legs of the slave-making F. sanguinea.
The latter ruthlessly killed their small opponents, and carried their
dead bodies as food to their nest, twenty-nine yards distant; but they
were prevented from getting any pupae to rear as slaves. I then dug up
a small parcel of the pupae of F. fusca from another nest, and put
them down on a bare spot near the place of combat; they were eagerly
seized, and carried off by the tyrants, who perhaps fancied that,
after all, they had been victorious in their late combat.

At the same time I laid on the same place a small parcel of the pupae
of another species, F. flava, with a few of these little yellow ants
still clinging to the fragments of the nest. This species is
sometimes, though rarely, made into slaves, as has been described by
Mr. Smith. Although so small a species, it is very courageous, and I
have seen it ferociously attack other ants. In one instance I found to
my surprise an independent community of F. flava under a stone beneath
a nest of the slave-making F. sanguinea; and when I had accidentally
disturbed both nests, the little ants attacked their big neighbours
with surprising courage. Now I was curious to ascertain whether F.
sanguinea could distinguish the pupae of F. fusca, which they
habitually make into slaves, from those of the little and furious F.
flava, which they rarely capture, and it was evident that they did at
once distinguish them: for we have seen that they eagerly and
instantly seized the pupae of F. fusca, whereas they were much
terrified when they came across the pupae, or even the earth from the
nest of F. flava, and quickly ran away; but in about a quarter of an
hour, shortly after all the little yellow ants had crawled away, they
took heart and carried off the pupae.

One evening I visited another community of F. sanguinea, and found a
number of these ants entering their nest, carrying the dead bodies of
F. fusca (showing that it was not a migration) and numerous pupae. I
traced the returning file burthened with booty, for about forty yards,
to a very thick clump of heath, whence I saw the last individual of F.
sanguinea emerge, carrying a pupa; but I was not able to find the
desolated nest in the thick heath. The nest, however, must have been
close at hand, for two or three individuals of F. fusca were rushing
about in the greatest agitation, and one was perched motionless with
its own pupa in its mouth on the top of a spray of heath over its
ravaged home.

Such are the facts, though they did not need confirmation by me, in
regard to the wonderful instinct of making slaves. Let it be observed
what a contrast the instinctive habits of F. sanguinea present with
those of the F. rufescens. The latter does not build its own nest,
does not determine its own migrations, does not collect food for
itself or its young, and cannot even feed itself: it is absolutely
dependent on its numerous slaves. Formica sanguinea, on the other
hand, possesses much fewer slaves, and in the early part of the summer
extremely few. The masters determine when and where a new nest shall
be formed, and when they migrate, the masters carry the slaves. Both
in Switzerland and England the slaves seem to have the exclusive care
of the larvae, and the masters alone go on slave-making expeditions.
In Switzerland the slaves and masters work together, making and
bringing materials for the nest: both, but chiefly the slaves, tend,
and milk as it may be called, their aphides; and thus both collect
food for the community. In England the masters alone usually leave the
nest to collect building materials and food for themselves, their
slaves and larvae. So that the masters in this country receive much
less service from their slaves than they do in Switzerland.

By what steps the instinct of F. sanguinea originated I will not
pretend to conjecture. But as ants, which are not slave-makers, will,
as I have seen, carry off pupae of other species, if scattered near
their nests, it is possible that pupae originally stored as food might
become developed; and the ants thus unintentionally reared would then
follow their proper instincts, and do what work they could. If their
presence proved useful to the species which had seized them--if it
were more advantageous to this species to capture workers than to
procreate them--the habit of collecting pupae originally for food
might by natural selection be strengthened and rendered permanent for
the very different purpose of raising slaves. When the instinct was
once acquired, if carried out to a much less extent even than in our
British F. sanguinea, which, as we have seen, is less aided by its
slaves than the same species in Switzerland, I can see no difficulty
in natural selection increasing and modifying the instinct--always
supposing each modification to be of use to the species--until an ant
was formed as abjectly dependent on its slaves as is the Formica
rufescens.

CELL-MAKING INSTINCT OF THE HIVE-BEE.

I will not here enter on minute details on this subject, but will
merely give an outline of the conclusions at which I have arrived. He
must be a dull man who can examine the exquisite structure of a comb,
so beautifully adapted to its end, without enthusiastic admiration. We
hear from mathematicians that bees have practically solved a recondite
problem, and have made their cells of the proper shape to hold the
greatest possible amount of honey, with the least possible consumption
of precious wax in their construction. It has been remarked that a
skilful workman, with fitting tools and measures, would find it very
difficult to make cells of wax of the true form, though this is
perfectly effected by a crowd of bees working in a dark hive. Grant
whatever instincts you please, and it seems at first quite
inconceivable how they can make all the necessary angles and planes,
or even perceive when they are correctly made. But the difficulty is
not nearly so great as it at first appears: all this beautiful work
can be shown, I think, to follow from a few very simple instincts.

I was led to investigate this subject by Mr. Waterhouse, who has shown
that the form of the cell stands in close relation to the presence of
adjoining cells; and the following view may, perhaps, be considered
only as a modification of his theory. Let us look to the great
principle of gradation, and see whether Nature does not reveal to us
her method of work. At one end of a short series we have humble-bees,
which use their old cocoons to hold honey, sometimes adding to them
short tubes of wax, and likewise making separate and very irregular
rounded cells of wax. At the other end of the series we have the cells
of the hive-bee, placed in a double layer: each cell, as is well
known, is an hexagonal prism, with the basal edges of its six sides
bevelled so as to join on to a pyramid, formed of three rhombs. These
rhombs have certain angles, and the three which form the pyramidal
base of a single cell on one side of the comb, enter into the
composition of the bases of three adjoining cells on the opposite
side. In the series between the extreme perfection of the cells of the
hive-bee and the simplicity of those of the humble-bee, we have the
cells of the Mexican Melipona domestica, carefully described and
figured by Pierre Huber. The Melipona itself is intermediate in
structure between the hive and humble bee, but more nearly related to
the latter: it forms a nearly regular waxen comb of cylindrical cells,
in which the young are hatched, and, in addition, some large cells of
wax for holding honey. These latter cells are nearly spherical and of
nearly equal sizes, and are aggregated into an irregular mass. But the
important point to notice, is that these cells are always made at that
degree of nearness to each other, that they would have intersected or
broken into each other, if the spheres had been completed; but this is
never permitted, the bees building perfectly flat walls of wax between
the spheres which thus tend to intersect. Hence each cell consists of
an outer spherical portion and of two, three, or more perfectly flat
surfaces, according as the cell adjoins two, three or more other
cells. When one cell comes into contact with three other cells, which,
from the spheres being nearly of the same size, is very frequently and
necessarily the case, the three flat surfaces are united into a
pyramid; and this pyramid, as Huber has remarked, is manifestly a
gross imitation of the three-sided pyramidal basis of the cell of the
hive-bee. As in the cells of the hive-bee, so here, the three plane
surfaces in any one cell necessarily enter into the construction of
three adjoining cells. It is obvious that the Melipona saves wax by
this manner of building; for the flat walls between the adjoining
cells are not double, but are of the same thickness as the outer
spherical portions, and yet each flat portion forms a part of two
cells.

Reflecting on this case, it occurred to me that if the Melipona had
made its spheres at some given distance from each other, and had made
them of equal sizes and had arranged them symmetrically in a double
layer, the resulting structure would probably have been as perfect as
the comb of the hive-bee. Accordingly I wrote to Professor Miller, of
Cambridge, and this geometer has kindly read over the following
statement, drawn up from his information, and tells me that it is
strictly correct:--

If a number of equal spheres be described with their centres placed in
two parallel layers; with the centre of each sphere at the distance of
radius x the square root of 2 or radius x 1.41421 (or at some lesser
distance), from the centres of the six surrounding spheres in the same
layer; and at the same distance from the centres of the adjoining
spheres in the other and parallel layer; then, if planes of
intersection between the several spheres in both layers be formed,
there will result a double layer of hexagonal prisms united together
by pyramidal bases formed of three rhombs; and the rhombs and the
sides of the hexagonal prisms will have every angle identically the
same with the best measurements which have been made of the cells of
the hive-bee.

Hence we may safely conclude that if we could slightly modify the
instincts already possessed by the Melipona, and in themselves not
very wonderful, this bee would make a structure as wonderfully perfect
as that of the hive-bee. We must suppose the Melipona to make her
cells truly spherical, and of equal sizes; and this would not be very
surprising, seeing that she already does so to a certain extent, and
seeing what perfectly cylindrical burrows in wood many insects can
make, apparently by turning round on a fixed point. We must suppose
the Melipona to arrange her cells in level layers, as she already does
her cylindrical cells; and we must further suppose, and this is the
greatest difficulty, that she can somehow judge accurately at what
distance to stand from her fellow-labourers when several are making
their spheres; but she is already so far enabled to judge of distance,
that she always describes her spheres so as to intersect largely; and
then she unites the points of intersection by perfectly flat surfaces.
We have further to suppose, but this is no difficulty, that after
hexagonal prisms have been formed by the intersection of adjoining
spheres in the same layer, she can prolong the hexagon to any length
requisite to hold the stock of honey; in the same way as the rude
humble-bee adds cylinders of wax to the circular mouths of her old
cocoons. By such modifications of instincts in themselves not very
wonderful,--hardly more wonderful than those which guide a bird to
make its nest,--I believe that the hive-bee has acquired, through
natural selection, her inimitable architectural powers.

But this theory can be tested by experiment. Following the example of
Mr. Tegetmeier, I separated two combs, and put between them a long,
thick, square strip of wax: the bees instantly began to excavate
minute circular pits in it; and as they deepened these little pits,
they made them wider and wider until they were converted into shallow
basins, appearing to the eye perfectly true or parts of a sphere, and
of about the diameter of a cell. It was most interesting to me to
observe that wherever several bees had begun to excavate these basins
near together, they had begun their work at such a distance from each
other, that by the time the basins had acquired the above stated width
(i.e. about the width of an ordinary cell), and were in depth about
one sixth of the diameter of the sphere of which they formed a part,
the rims of the basins intersected or broke into each other. As soon
as this occurred, the bees ceased to excavate, and began to build up
flat walls of wax on the lines of intersection between the basins, so
that each hexagonal prism was built upon the festooned edge of a
smooth basin, instead of on the straight edges of a three-sided
pyramid as in the case of ordinary cells.

I then put into the hive, instead of a thick, square piece of wax, a
thin and narrow, knife-edged ridge, coloured with vermilion. The bees
instantly began on both sides to excavate little basins near to each
other, in the same way as before; but the ridge of wax was so thin,
that the bottoms of the basins, if they had been excavated to the same
depth as in the former experiment, would have broken into each other
from the opposite sides. The bees, however, did not suffer this to
happen, and they stopped their excavations in due time; so that the
basins, as soon as they had been a little deepened, came to have flat
bottoms; and these flat bottoms, formed by thin little plates of the
vermilion wax having been left ungnawed, were situated, as far as the
eye could judge, exactly along the planes of imaginary intersection
between the basins on the opposite sides of the ridge of wax. In
parts, only little bits, in other parts, large portions of a rhombic
plate had been left between the opposed basins, but the work, from the
unnatural state of things, had not been neatly performed. The bees
must have worked at very nearly the same rate on the opposite sides of
the ridge of vermilion wax, as they circularly gnawed away and
deepened the basins on both sides, in order to have succeeded in thus
leaving flat plates between the basins, by stopping work along the
intermediate planes or planes of intersection.

Considering how flexible thin wax is, I do not see that there is any
difficulty in the bees, whilst at work on the two sides of a strip of
wax, perceiving when they have gnawed the wax away to the proper
thinness, and then stopping their work. In ordinary combs it has
appeared to me that the bees do not always succeed in working at
exactly the same rate from the opposite sides; for I have noticed
half-completed rhombs at the base of a just-commenced cell, which were
slightly concave on one side, where I suppose that the bees had
excavated too quickly, and convex on the opposed side, where the bees
had worked less quickly. In one well-marked instance, I put the comb
back into the hive, and allowed the bees to go on working for a short
time, and again examined the cell, and I found that the rhombic plate
had been completed, and had become PERFECTLY FLAT: it was absolutely
impossible, from the extreme thinness of the little rhombic plate,
that they could have effected this by gnawing away the convex side;
and I suspect that the bees in such cases stand in the opposed cells
and push and bend the ductile and warm wax (which as I have tried is
easily done) into its proper intermediate plane, and thus flatten it.

From the experiment of the ridge of vermilion wax, we can clearly see
that if the bees were to build for themselves a thin wall of wax, they
could make their cells of the proper shape, by standing at the proper
distance from each other, by excavating at the same rate, and by
endeavouring to make equal spherical hollows, but never allowing the
spheres to break into each other. Now bees, as may be clearly seen by
examining the edge of a growing comb, do make a rough, circumferential
wall or rim all round the comb; and they gnaw into this from the
opposite sides, always working circularly as they deepen each cell.
They do not make the whole three-sided pyramidal base of any one cell
at the same time, but only the one rhombic plate which stands on the
extreme growing margin, or the two plates, as the case may be; and
they never complete the upper edges of the rhombic plates, until the
hexagonal walls are commenced. Some of these statements differ from
those made by the justly celebrated elder Huber, but I am convinced of
their accuracy; and if I had space, I could show that they are
conformable with my theory.

Huber's statement that the very first cell is excavated out of a
little parallel-sided wall of wax, is not, as far as I have seen,
strictly correct; the first commencement having always been a little
hood of wax; but I will not here enter on these details. We see how
important a part excavation plays in the construction of the cells;
but it would be a great error to suppose that the bees cannot build up
a rough wall of wax in the proper position--that is, along the plane
of intersection between two adjoining spheres. I have several
specimens showing clearly that they can do this. Even in the rude
circumferential rim or wall of wax round a growing comb, flexures may
sometimes be observed, corresponding in position to the planes of the
rhombic basal plates of future cells. But the rough wall of wax has in
every case to be finished off, by being largely gnawed away on both
sides. The manner in which the bees build is curious; they always make
the first rough wall from ten to twenty times thicker than the
excessively thin finished wall of the cell, which will ultimately be
left. We shall understand how they work, by supposing masons first to
pile up a broad ridge of cement, and then to begin cutting it away
equally on both sides near the ground, till a smooth, very thin wall
is left in the middle; the masons always piling up the cut-away
cement, and adding fresh cement, on the summit of the ridge. We shall
thus have a thin wall steadily growing upward; but always crowned by a
gigantic coping. From all the cells, both those just commenced and
those completed, being thus crowned by a strong coping of wax, the
bees can cluster and crawl over the comb without injuring the delicate
hexagonal walls, which are only about one four-hundredth of an inch in
thickness; the plates of the pyramidal basis being about twice as
thick. By this singular manner of building, strength is continually
given to the comb, with the utmost ultimate economy of wax.

It seems at first to add to the difficulty of understanding how the
cells are made, that a multitude of bees all work together; one bee
after working a short time at one cell going to another, so that, as
Huber has stated, a score of individuals work even at the commencement
of the first cell. I was able practically to show this fact, by
covering the edges of the hexagonal walls of a single cell, or the
extreme margin of the circumferential rim of a growing comb, with an
extremely thin layer of melted vermilion wax; and I invariably found
that the colour was most delicately diffused by the bees--as
delicately as a painter could have done with his brush--by atoms of
the coloured wax having been taken from the spot on which it had been
placed, and worked into the growing edges of the cells all round. The
work of construction seems to be a sort of balance struck between many
bees, all instinctively standing at the same relative distance from
each other, all trying to sweep equal spheres, and then building up,
or leaving ungnawed, the planes of intersection between these spheres.
It was really curious to note in cases of difficulty, as when two
pieces of comb met at an angle, how often the bees would entirely pull
down and rebuild in different ways the same cell, sometimes recurring
to a shape which they had at first rejected.

When bees have a place on which they can stand in their proper
positions for working,--for instance, on a slip of wood, placed
directly under the middle of a comb growing downwards so that the comb
has to be built over one face of the slip--in this case the bees can
lay the foundations of one wall of a new hexagon, in its strictly
proper place, projecting beyond the other completed cells. It suffices
that the bees should be enabled to stand at their proper relative
distances from each other and from the walls of the last completed
cells, and then, by striking imaginary spheres, they can build up a
wall intermediate between two adjoining spheres; but, as far as I have
seen, they never gnaw away and finish off the angles of a cell till a
large part both of that cell and of the adjoining cells has been
built. This capacity in bees of laying down under certain
circumstances a rough wall in its proper place between two
just-commenced cells, is important, as it bears on a fact, which seems
at first quite subversive of the foregoing theory; namely, that the
cells on the extreme margin of wasp-combs are sometimes strictly
hexagonal; but I have not space here to enter on this subject. Nor
does there seem to me any great difficulty in a single insect (as in
the case of a queen-wasp) making hexagonal cells, if she work
alternately on the inside and outside of two or three cells commenced
at the same time, always standing at the proper relative distance from
the parts of the cells just begun, sweeping spheres or cylinders, and
building up intermediate planes. It is even conceivable that an insect
might, by fixing on a point at which to commence a cell, and then
moving outside, first to one point, and then to five other points, at
the proper relative distances from the central point and from each
other, strike the planes of intersection, and so make an isolated
hexagon: but I am not aware that any such case has been observed; nor
would any good be derived from a single hexagon being built, as in its
construction more materials would be required than for a cylinder.

As natural selection acts only by the accumulation of slight
modifications of structure or instinct, each profitable to the
individual under its conditions of life, it may reasonably be asked,
how a long and graduated succession of modified architectural
instincts, all tending towards the present perfect plan of
construction, could have profited the progenitors of the hive-bee? I
think the answer is not difficult: it is known that bees are often
hard pressed to get sufficient nectar; and I am informed by Mr.
Tegetmeier that it has been experimentally found that no less than
from twelve to fifteen pounds of dry sugar are consumed by a hive of
bees for the secretion of each pound of wax; so that a prodigious
quantity of fluid nectar must be collected and consumed by the bees in
a hive for the secretion of the wax necessary for the construction of
their combs. Moreover, many bees have to remain idle for many days
during the process of secretion. A large store of honey is
indispensable to support a large stock of bees during the winter; and
the security of the hive is known mainly to depend on a large number
of bees being supported. Hence the saving of wax by largely saving
honey must be a most important element of success in any family of
bees. Of course the success of any species of bee may be dependent on
the number of its parasites or other enemies, or on quite distinct
causes, and so be altogether independent of the quantity of honey
which the bees could collect. But let us suppose that this latter
circumstance determined, as it probably often does determine, the
numbers of a humble-bee which could exist in a country; and let us
further suppose that the community lived throughout the winter, and
consequently required a store of honey: there can in this case be no
doubt that it would be an advantage to our humble-bee, if a slight
modification of her instinct led her to make her waxen cells near
together, so as to intersect a little; for a wall in common even to
two adjoining cells, would save some little wax. Hence it would
continually be more and more advantageous to our humble-bee, if she
were to make her cells more and more regular, nearer together, and
aggregated into a mass, like the cells of the Melipona; for in this
case a large part of the bounding surface of each cell would serve to
bound other cells, and much wax would be saved. Again, from the same
cause, it would be advantageous to the Melipona, if she were to make
her cells closer together, and more regular in every way than at
present; for then, as we have seen, the spherical surfaces would
wholly disappear, and would all be replaced by plane surfaces; and the
Melipona would make a comb as perfect as that of the hive-bee. Beyond
this stage of perfection in architecture, natural selection could not
lead; for the comb of the hive-bee, as far as we can see, is
absolutely perfect in economising wax.

Thus, as I believe, the most wonderful of all known instincts, that of
the hive-bee, can be explained by natural selection having taken
advantage of numerous, successive, slight modifications of simpler
instincts; natural selection having by slow degrees, more and more
perfectly, led the bees to sweep equal spheres at a given distance
from each other in a double layer, and to build up and excavate the
wax along the planes of intersection. The bees, of course, no more
knowing that they swept their spheres at one particular distance from
each other, than they know what are the several angles of the
hexagonal prisms and of the basal rhombic plates. The motive power of
the process of natural selection having been economy of wax; that
individual swarm which wasted least honey in the secretion of wax,
having succeeded best, and having transmitted by inheritance its newly
acquired economical instinct to new swarms, which in their turn will
have had the best chance of succeeding in the struggle for existence.

No doubt many instincts of very difficult explanation could be opposed
to the theory of natural selection,--cases, in which we cannot see how
an instinct could possibly have originated; cases, in which no
intermediate gradations are known to exist; cases of instinct of
apparently such trifling importance, that they could hardly have been
acted on by natural selection; cases of instincts almost identically
the same in animals so remote in the scale of nature, that we cannot
account for their similarity by inheritance from a common parent, and
must therefore believe that they have been acquired by independent
acts of natural selection. I will not here enter on these several
cases, but will confine myself to one special difficulty, which at
first appeared to me insuperable, and actually fatal to my whole
theory. I allude to the neuters or sterile females in
insect-communities: for these neuters often differ widely in instinct
and in structure from both the males and fertile females, and yet,
from being sterile, they cannot propagate their kind.

The subject well deserves to be discussed at great length, but I will
here take only a single case, that of working or sterile ants. How the
workers have been rendered sterile is a difficulty; but not much
greater than that of any other striking modification of structure; for
it can be shown that some insects and other articulate animals in a
state of nature occasionally become sterile; and if such insects had
been social, and it had been profitable to the community that a number
should have been annually born capable of work, but incapable of
procreation, I can see no very great difficulty in this being effected
by natural selection. But I must pass over this preliminary
difficulty. The great difficulty lies in the working ants differing
widely from both the males and the fertile females in structure, as in
the shape of the thorax and in being destitute of wings and sometimes
of eyes, and in instinct. As far as instinct alone is concerned, the
prodigious difference in this respect between the workers and the
perfect females, would have been far better exemplified by the
hive-bee. If a working ant or other neuter insect had been an animal
in the ordinary state, I should have unhesitatingly assumed that all
its characters had been slowly acquired through natural selection;
namely, by an individual having been born with some slight profitable
modification of structure, this being inherited by its offspring,
which again varied and were again selected, and so onwards. But with
the working ant we have an insect differing greatly from its parents,
yet absolutely sterile; so that it could never have transmitted
successively acquired modifications of structure or instinct to its
progeny. It may well be asked how is it possible to reconcile this
case with the theory of natural selection?

First, let it be remembered that we have innumerable instances, both
in our domestic productions and in those in a state of nature, of all
sorts of differences of structure which have become correlated to
certain ages, and to either sex. We have differences correlated not
only to one sex, but to that short period alone when the reproductive
system is active, as in the nuptial plumage of many birds, and in the
hooked jaws of the male salmon. We have even slight differences in the
horns of different breeds of cattle in relation to an artificially
imperfect state of the male sex; for oxen of certain breeds have
longer horns than in other breeds, in comparison with the horns of the
bulls or cows of these same breeds. Hence I can see no real difficulty
in any character having become correlated with the sterile condition
of certain members of insect-communities: the difficulty lies in
understanding how such correlated modifications of structure could
have been slowly accumulated by natural selection.

This difficulty, though appearing insuperable, is lessened, or, as I
believe, disappears, when it is remembered that selection may be
applied to the family, as well as to the individual, and may thus gain
the desired end. Thus, a well-flavoured vegetable is cooked, and the
individual is destroyed; but the horticulturist sows seeds of the same
stock, and confidently expects to get nearly the same variety;
breeders of cattle wish the flesh and fat to be well marbled together;
the animal has been slaughtered, but the breeder goes with confidence
to the same family. I have such faith in the powers of selection, that
I do not doubt that a breed of cattle, always yielding oxen with
extraordinarily long horns, could be slowly formed by carefully
watching which individual bulls and cows, when matched, produced oxen
with the longest horns; and yet no one ox could ever have propagated
its kind. Thus I believe it has been with social insects: a slight
modification of structure, or instinct, correlated with the sterile
condition of certain members of the community, has been advantageous
to the community: consequently the fertile males and females of the
same community flourished, and transmitted to their fertile offspring
a tendency to produce sterile members having the same modification.
And I believe that this process has been repeated, until that
prodigious amount of difference between the fertile and sterile
females of the same species has been produced, which we see in many
social insects.

But we have not as yet touched on the climax of the difficulty;
namely, the fact that the neuters of several ants differ, not only
from the fertile females and males, but from each other, sometimes to
an almost incredible degree, and are thus divided into two or even
three castes. The castes, moreover, do not generally graduate into
each other, but are perfectly well defined; being as distinct from
each other, as are any two species of the same genus, or rather as any
two genera of the same family. Thus in Eciton, there are working and
soldier neuters, with jaws and instincts extraordinarily different: in
Cryptocerus, the workers of one caste alone carry a wonderful sort of
shield on their heads, the use of which is quite unknown: in the
Mexican Myrmecocystus, the workers of one caste never leave the nest;
they are fed by the workers of another caste, and they have an
enormously developed abdomen which secretes a sort of honey, supplying
the place of that excreted by the aphides, or the domestic cattle as
they may be called, which our European ants guard or imprison.

It will indeed be thought that I have an overweening confidence in the
principle of natural selection, when I do not admit that such
wonderful and well-established facts at once annihilate my theory. In
the simpler case of neuter insects all of one caste or of the same
kind, which have been rendered by natural selection, as I believe to
be quite possible, different from the fertile males and females,--in
this case, we may safely conclude from the analogy of ordinary
variations, that each successive, slight, profitable modification did
not probably at first appear in all the individual neuters in the same
nest, but in a few alone; and that by the long-continued selection of
the fertile parents which produced most neuters with the profitable
modification, all the neuters ultimately came to have the desired
character. On this view we ought occasionally to find neuter-insects
of the same species, in the same nest, presenting gradations of
structure; and this we do find, even often, considering how few
neuter-insects out of Europe have been carefully examined. Mr. F.
Smith has shown how surprisingly the neuters of several British ants
differ from each other in size and sometimes in colour; and that the
extreme forms can sometimes be perfectly linked together by
individuals taken out of the same nest: I have myself compared perfect
gradations of this kind. It often happens that the larger or the
smaller sized workers are the most numerous; or that both large and
small are numerous, with those of an intermediate size scanty in
numbers. Formica flava has larger and smaller workers, with some of
intermediate size; and, in this species, as Mr. F. Smith has observed,
the larger workers have simple eyes (ocelli), which though small can
be plainly distinguished, whereas the smaller workers have their
ocelli rudimentary. Having carefully dissected several specimens of
these workers, I can affirm that the eyes are far more rudimentary in
the smaller workers than can be accounted for merely by their
proportionally lesser size; and I fully believe, though I dare not
assert so positively, that the workers of intermediate size have their
ocelli in an exactly intermediate condition. So that we here have two
bodies of sterile workers in the same nest, differing not only in
size, but in their organs of vision, yet connected by some few members
in an intermediate condition. I may digress by adding, that if the
smaller workers had been the most useful to the community, and those
males and females had been continually selected, which produced more
and more of the smaller workers, until all the workers had come to be
in this condition; we should then have had a species of ant with
neuters very nearly in the same condition with those of Myrmica. For
the workers of Myrmica have not even rudiments of ocelli, though the
male and female ants of this genus have well-developed ocelli.

I may give one other case: so confidently did I expect to find
gradations in important points of structure between the different
castes of neuters in the same species, that I gladly availed myself of
Mr. F. Smith's offer of numerous specimens from the same nest of the
driver ant (Anomma) of West Africa. The reader will perhaps best
appreciate the amount of difference in these workers, by my giving not
the actual measurements, but a strictly accurate illustration: the
difference was the same as if we were to see a set of workmen building
a house of whom many were five feet four inches high, and many sixteen
feet high; but we must suppose that the larger workmen had heads four
instead of three times as big as those of the smaller men, and jaws
nearly five times as big. The jaws, moreover, of the working ants of
the several sizes differed wonderfully in shape, and in the form and
number of the teeth. But the important fact for us is, that though the
workers can be grouped into castes of different sizes, yet they
graduate insensibly into each other, as does the widely-different
structure of their jaws. I speak confidently on this latter point, as
Mr. Lubbock made drawings for me with the camera lucida of the jaws
which I had dissected from the workers of the several sizes.

With these facts before me, I believe that natural selection, by
acting on the fertile parents, could form a species which should
regularly produce neuters, either all of large size with one form of
jaw, or all of small size with jaws having a widely different
structure; or lastly, and this is our climax of difficulty, one set of
workers of one size and structure, and simultaneously another set of
workers of a different size and structure;--a graduated series having
been first formed, as in the case of the driver ant, and then the
extreme forms, from being the most useful to the community, having
been produced in greater and greater numbers through the natural
selection of the parents which generated them; until none with an
intermediate structure were produced.

Thus, as I believe, the wonderful fact of two distinctly defined
castes of sterile workers existing in the same nest, both widely
different from each other and from their parents, has originated. We
can see how useful their production may have been to a social
community of insects, on the same principle that the division of
labour is useful to civilised man. As ants work by inherited instincts
and by inherited tools or weapons, and not by acquired knowledge and
manufactured instruments, a perfect division of labour could be
effected with them only by the workers being sterile; for had they
been fertile, they would have intercrossed, and their instincts and
structure would have become blended. And nature has, as I believe,
effected this admirable division of labour in the communities of ants,
by the means of natural selection. But I am bound to confess, that,
with all my faith in this principle, I should never have anticipated
that natural selection could have been efficient in so high a degree,
had not the case of these neuter insects convinced me of the fact. I
have, therefore, discussed this case, at some little but wholly
insufficient length, in order to show the power of natural selection,
and likewise because this is by far the most serious special
difficulty, which my theory has encountered. The case, also, is very
interesting, as it proves that with animals, as with plants, any
amount of modification in structure can be effected by the
accumulation of numerous, slight, and as we must call them accidental,
variations, which are in any manner profitable, without exercise or
habit having come into play. For no amount of exercise, or habit, or
volition, in the utterly sterile members of a community could possibly
have affected the structure or instincts of the fertile members, which
alone leave descendants. I am surprised that no one has advanced this
demonstrative case of neuter insects, against the well-known doctrine
of Lamarck.

SUMMARY.

I have endeavoured briefly in this chapter to show that the mental
qualities of our domestic animals vary, and that the variations are
inherited. Still more briefly I have attempted to show that instincts
vary slightly in a state of nature. No one will dispute that instincts
are of the highest importance to each animal. Therefore I can see no
difficulty, under changing conditions of life, in natural selection
accumulating slight modifications of instinct to any extent, in any
useful direction. In some cases habit or use and disuse have probably
come into play. I do not pretend that the facts given in this chapter
strengthen in any great degree my theory; but none of the cases of
difficulty, to the best of my judgment, annihilate it. On the other
hand, the fact that instincts are not always absolutely perfect and
are liable to mistakes;--that no instinct has been produced for the
exclusive good of other animals, but that each animal takes advantage
of the instincts of others;--that the canon in natural history, of
"natura non facit saltum" is applicable to instincts as well as to
corporeal structure, and is plainly explicable on the foregoing views,
but is otherwise inexplicable,--all tend to corroborate the theory of
natural selection.

This theory is, also, strengthened by some few other facts in regard
to instincts; as by that common case of closely allied, but certainly
distinct, species, when inhabiting distant parts of the world and
living under considerably different conditions of life, yet often
retaining nearly the same instincts. For instance, we can understand
on the principle of inheritance, how it is that the thrush of South
America lines its nest with mud, in the same peculiar manner as does
our British thrush: how it is that the male wrens (Troglodytes) of
North America, build "cock-nests," to roost in, like the males of our
distinct Kitty-wrens,--a habit wholly unlike that of any other known
bird. Finally, it may not be a logical deduction, but to my
imagination it is far more satisfactory to look at such instincts as
the young cuckoo ejecting its foster-brothers,--ants making
slaves,--the larvae of ichneumonidae feeding within the live bodies of
caterpillars,--not as specially endowed or created instincts, but as
small consequences of one general law, leading to the advancement of
all organic beings, namely, multiply, vary, let the strongest live and
the weakest die.


CHAPTER 8. HYBRIDISM.

Distinction between the sterility of first crosses and of hybrids.
Sterility various in degree, not universal, affected by close
interbreeding, removed by domestication.
Laws governing the sterility of hybrids.
Sterility not a special endowment, but incidental on other
differences.
Causes of the sterility of first crosses and of hybrids.
Parallelism between the effects of changed conditions of life and
crossing.
Fertility of varieties when crossed and of their mongrel offspring not
universal.
Hybrids and mongrels compared independently of their fertility.
Summary.

The view generally entertained by naturalists is that species, when
intercrossed, have been specially endowed with the quality of
sterility, in order to prevent the confusion of all organic forms.
This view certainly seems at first probable, for species within the
same country could hardly have kept distinct had they been capable of
crossing freely. The importance of the fact that hybrids are very
generally sterile, has, I think, been much underrated by some late
writers. On the theory of natural selection the case is especially
important, inasmuch as the sterility of hybrids could not possibly be
of any advantage to them, and therefore could not have been acquired
by the continued preservation of successive profitable degrees of
sterility. I hope, however, to be able to show that sterility is not a
specially acquired or endowed quality, but is incidental on other
acquired differences.

In treating this subject, two classes of facts, to a large extent
fundamentally different, have generally been confounded together;
namely, the sterility of two species when first crossed, and the
sterility of the hybrids produced from them.

Pure species have of course their organs of reproduction in a perfect
condition, yet when intercrossed they produce either few or no
offspring. Hybrids, on the other hand, have their reproductive organs
functionally impotent, as may be clearly seen in the state of the male
element in both plants and animals; though the organs themselves are
perfect in structure, as far as the microscope reveals. In the first
case the two sexual elements which go to form the embryo are perfect;
in the second case they are either not at all developed, or are
imperfectly developed. This distinction is important, when the cause
of the sterility, which is common to the two cases, has to be
considered. The distinction has probably been slurred over, owing to
the sterility in both cases being looked on as a special endowment,
beyond the province of our reasoning powers.

The fertility of varieties, that is of the forms known or believed to
have descended from common parents, when intercrossed, and likewise
the fertility of their mongrel offspring, is, on my theory, of equal
importance with the sterility of species; for it seems to make a broad
and clear distinction between varieties and species.

First, for the sterility of species when crossed and of their hybrid
offspring. It is impossible to study the several memoirs and works of
those two conscientious and admirable observers, Kolreuter and
Gartner, who almost devoted their lives to this subject, without being
deeply impressed with the high generality of some degree of sterility.
Kolreuter makes the rule universal; but then he cuts the knot, for in
ten cases in which he found two forms, considered by most authors as
distinct species, quite fertile together, he unhesitatingly ranks them
as varieties. Gartner, also, makes the rule equally universal; and he
disputes the entire fertility of Kolreuter's ten cases. But in these
and in many other cases, Gartner is obliged carefully to count the
seeds, in order to show that there is any degree of sterility. He
always compares the maximum number of seeds produced by two species
when crossed and by their hybrid offspring, with the average number
produced by both pure parent-species in a state of nature. But a
serious cause of error seems to me to be here introduced: a plant to
be hybridised must be castrated, and, what is often more important,
must be secluded in order to prevent pollen being brought to it by
insects from other plants. Nearly all the plants experimentised on by
Gartner were potted, and apparently were kept in a chamber in his
house. That these processes are often injurious to the fertility of a
plant cannot be doubted; for Gartner gives in his table about a score
of cases of plants which he castrated, and artificially fertilised
with their own pollen, and (excluding all cases such as the
Leguminosae, in which there is an acknowledged difficulty in the
manipulation) half of these twenty plants had their fertility in some
degree impaired. Moreover, as Gartner during several years repeatedly
crossed the primrose and cowslip, which we have such good reason to
believe to be varieties, and only once or twice succeeded in getting
fertile seed; as he found the common red and blue pimpernels
(Anagallis arvensis and coerulea), which the best botanists rank as
varieties, absolutely sterile together; and as he came to the same
conclusion in several other analogous cases; it seems to me that we
may well be permitted to doubt whether many other species are really
so sterile, when intercrossed, as Gartner believes.

It is certain, on the one hand, that the sterility of various species
when crossed is so different in degree and graduates away so
insensibly, and, on the other hand, that the fertility of pure species
is so easily affected by various circumstances, that for all practical
purposes it is most difficult to say where perfect fertility ends and
sterility begins. I think no better evidence of this can be required
than that the two most experienced observers who have ever lived,
namely, Kolreuter and Gartner, should have arrived at diametrically
opposite conclusions in regard to the very same species. It is also
most instructive to compare--but I have not space here to enter on
details--the evidence advanced by our best botanists on the question
whether certain doubtful forms should be ranked as species or
varieties, with the evidence from fertility adduced by different
hybridisers, or by the same author, from experiments made during
different years. It can thus be shown that neither sterility nor
fertility affords any clear distinction between species and varieties;
but that the evidence from this source graduates away, and is doubtful
in the same degree as is the evidence derived from other
constitutional and structural differences.

In regard to the sterility of hybrids in successive generations;
though Gartner was enabled to rear some hybrids, carefully guarding
them from a cross with either pure parent, for six or seven, and in
one case for ten generations, yet he asserts positively that their
fertility never increased, but generally greatly decreased. I do not
doubt that this is usually the case, and that the fertility often
suddenly decreases in the first few generations. Nevertheless I
believe that in all these experiments the fertility has been
diminished by an independent cause, namely, from close interbreeding.
I have collected so large a body of facts, showing that close
interbreeding lessens fertility, and, on the other hand, that an
occasional cross with a distinct individual or variety increases
fertility, that I cannot doubt the correctness of this almost
universal belief amongst breeders. Hybrids are seldom raised by
experimentalists in great numbers; and as the parent-species, or other
allied hybrids, generally grow in the same garden, the visits of
insects must be carefully prevented during the flowering season: hence
hybrids will generally be fertilised during each generation by their
own individual pollen; and I am convinced that this would be injurious
to their fertility, already lessened by their hybrid origin. I am
strengthened in this conviction by a remarkable statement repeatedly
made by Gartner, namely, that if even the less fertile hybrids be
artificially fertilised with hybrid pollen of the same kind, their
fertility, notwithstanding the frequent ill effects of manipulation,
sometimes decidedly increases, and goes on increasing. Now, in
artificial fertilisation pollen is as often taken by chance (as I know
from my own experience) from the anthers of another flower, as from
the anthers of the flower itself which is to be fertilised; so that a
cross between two flowers, though probably on the same plant, would be
thus effected. Moreover, whenever complicated experiments are in
progress, so careful an observer as Gartner would have castrated his
hybrids, and this would have insured in each generation a cross with
the pollen from a distinct flower, either from the same plant or from
another plant of the same hybrid nature. And thus, the strange fact of
the increase of fertility in the successive generations of
ARTIFICIALLY FERTILISED hybrids may, I believe, be accounted for by
close interbreeding having been avoided.

Now let us turn to the results arrived at by the third most
experienced hybridiser, namely, the Honourable and Reverend W.
Herbert. He is as emphatic in his conclusion that some hybrids are
perfectly fertile--as fertile as the pure parent-species--as are
Kolreuter and Gartner that some degree of sterility between distinct
species is a universal law of nature. He experimentised on some of the
very same species as did Gartner. The difference in their results may,
I think, be in part accounted for by Herbert's great horticultural
skill, and by his having hothouses at his command. Of his many
important statements I will here give only a single one as an example,
namely, that "every ovule in a pod of Crinum capense fertilised by C.
revolutum produced a plant, which (he says) I never saw to occur in a
case of its natural fecundation." So that we here have perfect, or
even more than commonly perfect, fertility in a first cross between
two distinct species.

This case of the Crinum leads me to refer to a most singular fact,
namely, that there are individual plants, as with certain species of
Lobelia, and with all the species of the genus Hippeastrum, which can
be far more easily fertilised by the pollen of another and distinct
species, than by their own pollen. For these plants have been found to
yield seed to the pollen of a distinct species, though quite sterile
with their own pollen, notwithstanding that their own pollen was found
to be perfectly good, for it fertilised distinct species. So that
certain individual plants and all the individuals of certain species
can actually be hybridised much more readily than they can be
self-fertilised! For instance, a bulb of Hippeastrum aulicum produced
four flowers; three were fertilised by Herbert with their own pollen,
and the fourth was subsequently fertilised by the pollen of a compound
hybrid descended from three other and distinct species: the result was
that "the ovaries of the three first flowers soon ceased to grow, and
after a few days perished entirely, whereas the pod impregnated by the
pollen of the hybrid made vigorous growth and rapid progress to
maturity, and bore good seed, which vegetated freely." In a letter to
me, in 1839, Mr. Herbert told me that he had then tried the experiment
during five years, and he continued to try it during several
subsequent years, and always with the same result. This result has,
also, been confirmed by other observers in the case of Hippeastrum
with its sub-genera, and in the case of some other genera, as Lobelia,
Passiflora and Verbascum. Although the plants in these experiments
appeared perfectly healthy, and although both the ovules and pollen of
the same flower were perfectly good with respect to other species, yet
as they were functionally imperfect in their mutual self-action, we
must infer that the plants were in an unnatural state. Nevertheless
these facts show on what slight and mysterious causes the lesser or
greater fertility of species when crossed, in comparison with the same
species when self-fertilised, sometimes depends.

The practical experiments of horticulturists, though not made with
scientific precision, deserve some notice. It is notorious in how
complicated a manner the species of Pelargonium, Fuchsia, Calceolaria,
Petunia, Rhododendron, etc., have been crossed, yet many of these
hybrids seed freely. For instance, Herbert asserts that a hybrid from
Calceolaria integrifolia and plantaginea, species most widely
dissimilar in general habit, "reproduced itself as perfectly as if it
had been a natural species from the mountains of Chile." I have taken
some pains to ascertain the degree of fertility of some of the complex
crosses of Rhododendrons, and I am assured that many of them are
perfectly fertile. Mr. C. Noble, for instance, informs me that he
raises stocks for grafting from a hybrid between Rhododendron Ponticum
and Catawbiense, and that this hybrid "seeds as freely as it is
possible to imagine." Had hybrids, when fairly treated, gone on
decreasing in fertility in each successive generation, as Gartner
believes to be the case, the fact would have been notorious to
nurserymen. Horticulturists raise large beds of the same hybrids, and
such alone are fairly treated, for by insect agency the several
individuals of the same hybrid variety are allowed to freely cross
with each other, and the injurious influence of close interbreeding is
thus prevented. Any one may readily convince himself of the efficiency
of insect-agency by examining the flowers of the more sterile kinds of
hybrid rhododendrons, which produce no pollen, for he will find on
their stigmas plenty of pollen brought from other flowers.

In regard to animals, much fewer experiments have been carefully tried
than with plants. If our systematic arrangements can be trusted, that
is if the genera of animals are as distinct from each other, as are
the genera of plants, then we may infer that animals more widely
separated in the scale of nature can be more easily crossed than in
the case of plants; but the hybrids themselves are, I think, more
sterile. I doubt whether any case of a perfectly fertile hybrid animal
can be considered as thoroughly well authenticated. It should,
however, be borne in mind that, owing to few animals breeding freely
under confinement, few experiments have been fairly tried: for
instance, the canary-bird has been crossed with nine other finches,
but as not one of these nine species breeds freely in confinement, we
have no right to expect that the first crosses between them and the
canary, or that their hybrids, should be perfectly fertile. Again,
with respect to the fertility in successive generations of the more
fertile hybrid animals, I hardly know of an instance in which two
families of the same hybrid have been raised at the same time from
different parents, so as to avoid the ill effects of close
interbreeding. On the contrary, brothers and sisters have usually been
crossed in each successive generation, in opposition to the constantly
repeated admonition of every breeder. And in this case, it is not at
all surprising that the inherent sterility in the hybrids should have
gone on increasing. If we were to act thus, and pair brothers and
sisters in the case of any pure animal, which from any cause had the
least tendency to sterility, the breed would assuredly be lost in a
very few generations.

Although I do not know of any thoroughly well-authenticated cases of
perfectly fertile hybrid animals, I have some reason to believe that
the hybrids from Cervulus vaginalis and Reevesii, and from Phasianus
colchicus with P. torquatus and with P. versicolor are perfectly
fertile. The hybrids from the common and Chinese geese (A. cygnoides),
species which are so different that they are generally ranked in
distinct genera, have often bred in this country with either pure
parent, and in one single instance they have bred inter se. This was
effected by Mr. Eyton, who raised two hybrids from the same parents
but from different hatches; and from these two birds he raised no less
than eight hybrids (grandchildren of the pure geese) from one nest. In
India, however, these cross-bred geese must be far more fertile; for I
am assured by two eminently capable judges, namely Mr. Blyth and Capt.
Hutton, that whole flocks of these crossed geese are kept in various
parts of the country; and as they are kept for profit, where neither
pure parent-species exists, they must certainly be highly fertile.

A doctrine which originated with Pallas, has been largely accepted by
modern naturalists; namely, that most of our domestic animals have
descended from two or more aboriginal species, since commingled by
intercrossing. On this view, the aboriginal species must either at
first have produced quite fertile hybrids, or the hybrids must have
become in subsequent generations quite fertile under domestication.
This latter alternative seems to me the most probable, and I am
inclined to believe in its truth, although it rests on no direct
evidence. I believe, for instance, that our dogs have descended from
several wild stocks; yet, with perhaps the exception of certain
indigenous domestic dogs of South America, all are quite fertile
together; and analogy makes me greatly doubt, whether the several
aboriginal species would at first have freely bred together and have
produced quite fertile hybrids. So again there is reason to believe
that our European and the humped Indian cattle are quite fertile
together; but from facts communicated to me by Mr. Blyth, I think they
must be considered as distinct species. On this view of the origin of
many of our domestic animals, we must either give up the belief of the
almost universal sterility of distinct species of animals when
crossed; or we must look at sterility, not as an indelible
characteristic, but as one capable of being removed by domestication.

Finally, looking to all the ascertained facts on the intercrossing of
plants and animals, it may be concluded that some degree of sterility,
both in first crosses and in hybrids, is an extremely general result;
but that it cannot, under our present state of knowledge, be
considered as absolutely universal.

LAWS GOVERNING THE STERILITY OF FIRST CROSSES AND OF HYBRIDS.

We will now consider a little more in detail the circumstances and
rules governing the sterility of first crosses and of hybrids. Our
chief object will be to see whether or not the rules indicate that
species have specially been endowed with this quality, in order to
prevent their crossing and blending together in utter confusion. The
following rules and conclusions are chiefly drawn up from Gartner's
admirable work on the hybridisation of plants. I have taken much pains
to ascertain how far the rules apply to animals, and considering how
scanty our knowledge is in regard to hybrid animals, I have been
surprised to find how generally the same rules apply to both kingdoms.

It has been already remarked, that the degree of fertility, both of
first crosses and of hybrids, graduates from zero to perfect
fertility. It is surprising in how many curious ways this gradation
can be shown to exist; but only the barest outline of the facts can
here be given. When pollen from a plant of one family is placed on the
stigma of a plant of a distinct family, it exerts no more influence
than so much inorganic dust. From this absolute zero of fertility, the
pollen of different species of the same genus applied to the stigma of
some one species, yields a perfect gradation in the number of seeds
produced, up to nearly complete or even quite complete fertility; and,
as we have seen, in certain abnormal cases, even to an excess of
fertility, beyond that which the plant's own pollen will produce. So
in hybrids themselves, there are some which never have produced, and
probably never would produce, even with the pollen of either pure
parent, a single fertile seed: but in some of these cases a first
trace of fertility may be detected, by the pollen of one of the pure
parent-species causing the flower of the hybrid to wither earlier than
it otherwise would have done; and the early withering of the flower is
well known to be a sign of incipient fertilisation. From this extreme
degree of sterility we have self-fertilised hybrids producing a
greater and greater number of seeds up to perfect fertility.

Hybrids from two species which are very difficult to cross, and which
rarely produce any offspring, are generally very sterile; but the
parallelism between the difficulty of making a first cross, and the
sterility of the hybrids thus produced--two classes of facts which are
generally confounded together--is by no means strict. There are many
cases, in which two pure species can be united with unusual facility,
and produce numerous hybrid-offspring, yet these hybrids are
remarkably sterile. On the other hand, there are species which can be
crossed very rarely, or with extreme difficulty, but the hybrids, when
at last produced, are very fertile. Even within the limits of the same
genus, for instance in Dianthus, these two opposite cases occur.

The fertility, both of first crosses and of hybrids, is more easily
affected by unfavourable conditions, than is the fertility of pure
species. But the degree of fertility is likewise innately variable;
for it is not always the same when the same two species are crossed
under the same circumstances, but depends in part upon the
constitution of the individuals which happen to have been chosen for
the experiment. So it is with hybrids, for their degree of fertility
is often found to differ greatly in the several individuals raised
from seed out of the same capsule and exposed to exactly the same
conditions.

By the term systematic affinity is meant, the resemblance between
species in structure and in constitution, more especially in the
structure of parts which are of high physiological importance and
which differ little in the allied species. Now the fertility of first
crosses between species, and of the hybrids produced from them, is
largely governed by their systematic affinity. This is clearly shown
by hybrids never having been raised between species ranked by
systematists in distinct families; and on the other hand, by very
closely allied species generally uniting with facility. But the
correspondence between systematic affinity and the facility of
crossing is by no means strict. A multitude of cases could be given of
very closely allied species which will not unite, or only with extreme
difficulty; and on the other hand of very distinct species which unite
with the utmost facility. In the same family there may be a genus, as
Dianthus, in which very many species can most readily be crossed; and
another genus, as Silene, in which the most persevering efforts have
failed to produce between extremely close species a single hybrid.
Even within the limits of the same genus, we meet with this same
difference; for instance, the many species of Nicotiana have been more
largely crossed than the species of almost any other genus; but
Gartner found that N. acuminata, which is not a particularly distinct
species, obstinately failed to fertilise, or to be fertilised by, no
less than eight other species of Nicotiana. Very many analogous facts
could be given.

No one has been able to point out what kind, or what amount, of
difference in any recognisable character is sufficient to prevent two
species crossing. It can be shown that plants most widely different in
habit and general appearance, and having strongly marked differences
in every part of the flower, even in the pollen, in the fruit, and in
the cotyledons, can be crossed. Annual and perennial plants, deciduous
and evergreen trees, plants inhabiting different stations and fitted
for extremely different climates, can often be crossed with ease.

By a reciprocal cross between two species, I mean the case, for
instance, of a stallion-horse being first crossed with a female-ass,
and then a male-ass with a mare: these two species may then be said to
have been reciprocally crossed. There is often the widest possible
difference in the facility of making reciprocal crosses. Such cases
are highly important, for they prove that the capacity in any two
species to cross is often completely independent of their systematic
affinity, or of any recognisable difference in their whole
organisation. On the other hand, these cases clearly show that the
capacity for crossing is connected with constitutional differences
imperceptible by us, and confined to the reproductive system. This
difference in the result of reciprocal crosses between the same two
species was long ago observed by Kolreuter. To give an instance:
Mirabilis jalappa can easily be fertilised by the pollen of M.
longiflora, and the hybrids thus produced are sufficiently fertile;
but Kolreuter tried more than two hundred times, during eight
following years, to fertilise reciprocally M. longiflora with the
pollen of M. jalappa, and utterly failed. Several other equally
striking cases could be given. Thuret has observed the same fact with
certain sea-weeds or Fuci. Gartner, moreover, found that this
difference of facility in making reciprocal crosses is extremely
common in a lesser degree. He has observed it even between forms so
closely related (as Matthiola annua and glabra) that many botanists
rank them only as varieties. It is also a remarkable fact, that
hybrids raised from reciprocal crosses, though of course compounded of
the very same two species, the one species having first been used as
the father and then as the mother, generally differ in fertility in a
small, and occasionally in a high degree.

Several other singular rules could be given from Gartner: for
instance, some species have a remarkable power of crossing with other
species; other species of the same genus have a remarkable power of
impressing their likeness on their hybrid offspring; but these two
powers do not at all necessarily go together. There are certain
hybrids which instead of having, as is usual, an intermediate
character between their two parents, always closely resemble one of
them; and such hybrids, though externally so like one of their pure
parent-species, are with rare exceptions extremely sterile. So again
amongst hybrids which are usually intermediate in structure between
their parents, exceptional and abnormal individuals sometimes are
born, which closely resemble one of their pure parents; and these
hybrids are almost always utterly sterile, even when the other hybrids
raised from seed from the same capsule have a considerable degree of
fertility. These facts show how completely fertility in the hybrid is
independent of its external resemblance to either pure parent.

Considering the several rules now given, which govern the fertility of
first crosses and of hybrids, we see that when forms, which must be
considered as good and distinct species, are united, their fertility
graduates from zero to perfect fertility, or even to fertility under
certain conditions in excess. That their fertility, besides being
eminently susceptible to favourable and unfavourable conditions, is
innately variable. That it is by no means always the same in degree in
the first cross and in the hybrids produced from this cross. That the
fertility of hybrids is not related to the degree in which they
resemble in external appearance either parent. And lastly, that the
facility of making a first cross between any two species is not always
governed by their systematic affinity or degree of resemblance to each
other. This latter statement is clearly proved by reciprocal crosses
between the same two species, for according as the one species or the
other is used as the father or the mother, there is generally some
difference, and occasionally the widest possible difference, in the
facility of effecting an union. The hybrids, moreover, produced from
reciprocal crosses often differ in fertility.

Now do these complex and singular rules indicate that species have
been endowed with sterility simply to prevent their becoming
confounded in nature? I think not. For why should the sterility be so
extremely different in degree, when various species are crossed, all
of which we must suppose it would be equally important to keep from
blending together? Why should the degree of sterility be innately
variable in the individuals of the same species? Why should some
species cross with facility, and yet produce very sterile hybrids; and
other species cross with extreme difficulty, and yet produce fairly
fertile hybrids? Why should there often be so great a difference in
the result of a reciprocal cross between the same two species? Why, it
may even be asked, has the production of hybrids been permitted? to
grant to species the special power of producing hybrids, and then to
stop their further propagation by different degrees of sterility, not
strictly related to the facility of the first union between their
parents, seems to be a strange arrangement.

The foregoing rules and facts, on the other hand, appear to me clearly
to indicate that the sterility both of first crosses and of hybrids is
simply incidental or dependent on unknown differences, chiefly in the
reproductive systems, of the species which are crossed. The
differences being of so peculiar and limited a nature, that, in
reciprocal crosses between two species the male sexual element of the
one will often freely act on the female sexual element of the other,
but not in a reversed direction. It will be advisable to explain a
little more fully by an example what I mean by sterility being
incidental on other differences, and not a specially endowed quality.
As the capacity of one plant to be grafted or budded on another is so
entirely unimportant for its welfare in a state of nature, I presume
that no one will suppose that this capacity is a SPECIALLY endowed
quality, but will admit that it is incidental on differences in the
laws of growth of the two plants. We can sometimes see the reason why
one tree will not take on another, from differences in their rate of
growth, in the hardness of their wood, in the period of the flow or
nature of their sap, etc.; but in a multitude of cases we can assign
no reason whatever. Great diversity in the size of two plants, one
being woody and the other herbaceous, one being evergreen and the
other deciduous, and adaptation to widely different climates, does not
always prevent the two grafting together. As in hybridisation, so with
grafting, the capacity is limited by systematic affinity, for no one
has been able to graft trees together belonging to quite distinct
families; and, on the other hand, closely allied species, and
varieties of the same species, can usually, but not invariably, be
grafted with ease. But this capacity, as in hybridisation, is by no
means absolutely governed by systematic affinity. Although many
distinct genera within the same family have been grafted together, in
other cases species of the same genus will not take on each other. The
pear can be grafted far more readily on the quince, which is ranked as
a distinct genus, than on the apple, which is a member of the same
genus. Even different varieties of the pear take with different
degrees of facility on the quince; so do different varieties of the
apricot and peach on certain varieties of the plum.

As Gartner found that there was sometimes an innate difference in
different INDIVIDUALS of the same two species in crossing; so Sagaret
believes this to be the case with different individuals of the same
two species in being grafted together. As in reciprocal crosses, the
facility of effecting an union is often very far from equal, so it
sometimes is in grafting; the common gooseberry, for instance, cannot
be grafted on the currant, whereas the currant will take, though with
difficulty, on the gooseberry.

We have seen that the sterility of hybrids, which have their
reproductive organs in an imperfect condition, is a very different
case from the difficulty of uniting two pure species, which have their
reproductive organs perfect; yet these two distinct cases run to a
certain extent parallel. Something analogous occurs in grafting; for
Thouin found that three species of Robinia, which seeded freely on
their own roots, and which could be grafted with no great difficulty
on another species, when thus grafted were rendered barren. On the
other hand, certain species of Sorbus, when grafted on other species,
yielded twice as much fruit as when on their own roots. We are
reminded by this latter fact of the extraordinary case of Hippeastrum,
Lobelia, etc., which seeded much more freely when fertilised with the
pollen of distinct species, than when self-fertilised with their own
pollen.

We thus see, that although there is a clear and fundamental difference
between the mere adhesion of grafted stocks, and the union of the male
and female elements in the act of reproduction, yet that there is a
rude degree of parallelism in the results of grafting and of crossing
distinct species. And as we must look at the curious and complex laws
governing the facility with which trees can be grafted on each other
as incidental on unknown differences in their vegetative systems, so I
believe that the still more complex laws governing the facility of
first crosses, are incidental on unknown differences, chiefly in their
reproductive systems. These differences, in both cases, follow to a
certain extent, as might have been expected, systematic affinity, by
which every kind of resemblance and dissimilarity between organic
beings is attempted to be expressed. The facts by no means seem to me
to indicate that the greater or lesser difficulty of either grafting
or crossing together various species has been a special endowment;
although in the case of crossing, the difficulty is as important for
the endurance and stability of specific forms, as in the case of
grafting it is unimportant for their welfare.

CAUSES OF THE STERILITY OF FIRST CROSSES AND OF HYBRIDS.

We may now look a little closer at the probable causes of the
sterility of first crosses and of hybrids. These two cases are
fundamentally different, for, as just remarked, in the union of two
pure species the male and female sexual elements are perfect, whereas
in hybrids they are imperfect. Even in first crosses, the greater or
lesser difficulty in effecting a union apparently depends on several
distinct causes. There must sometimes be a physical impossibility in
the male element reaching the ovule, as would be the case with a plant
having a pistil too long for the pollen-tubes to reach the ovarium. It
has also been observed that when pollen of one species is placed on
the stigma of a distantly allied species, though the pollen-tubes
protrude, they do not penetrate the stigmatic surface. Again, the male
element may reach the female element, but be incapable of causing an
embryo to be developed, as seems to have been the case with some of
Thuret's experiments on Fuci. No explanation can be given of these
facts, any more than why certain trees cannot be grafted on others.
Lastly, an embryo may be developed, and then perish at an early
period. This latter alternative has not been sufficiently attended to;
but I believe, from observations communicated to me by Mr. Hewitt, who
has had great experience in hybridising gallinaceous birds, that the
early death of the embryo is a very frequent cause of sterility in
first crosses. I was at first very unwilling to believe in this view;
as hybrids, when once born, are generally healthy and long-lived, as
we see in the case of the common mule. Hybrids, however, are
differently circumstanced before and after birth: when born and living
in a country where their two parents can live, they are generally
placed under suitable conditions of life. But a hybrid partakes of
only half of the nature and constitution of its mother, and therefore
before birth, as long as it is nourished within its mother's womb or
within the egg or seed produced by the mother, it may be exposed to
conditions in some degree unsuitable, and consequently be liable to
perish at an early period; more especially as all very young beings
seem eminently sensitive to injurious or unnatural conditions of life.

In regard to the sterility of hybrids, in which the sexual elements
are imperfectly developed, the case is very different. I have more
than once alluded to a large body of facts, which I have collected,
showing that when animals and plants are removed from their natural
conditions, they are extremely liable to have their reproductive
systems seriously affected. This, in fact, is the great bar to the
domestication of animals. Between the sterility thus superinduced and
that of hybrids, there are many points of similarity. In both cases
the sterility is independent of general health, and is often
accompanied by excess of size or great luxuriance. In both cases, the
sterility occurs in various degrees; in both, the male element is the
most liable to be affected; but sometimes the female more than the
male. In both, the tendency goes to a certain extent with systematic
affinity, for whole groups of animals and plants are rendered impotent
by the same unnatural conditions; and whole groups of species tend to
produce sterile hybrids. On the other hand, one species in a group
will sometimes resist great changes of conditions with unimpaired
fertility; and certain species in a group will produce unusually
fertile hybrids. No one can tell, till he tries, whether any
particular animal will breed under confinement or any plant seed
freely under culture; nor can he tell, till he tries, whether any two
species of a genus will produce more or less sterile hybrids. Lastly,
when organic beings are placed during several generations under
conditions not natural to them, they are extremely liable to vary,
which is due, as I believe, to their reproductive systems having been
specially affected, though in a lesser degree than when sterility
ensues. So it is with hybrids, for hybrids in successive generations
are eminently liable to vary, as every experimentalist has observed.

Thus we see that when organic beings are placed under new and
unnatural conditions, and when hybrids are produced by the unnatural
crossing of two species, the reproductive system, independently of the
general state of health, is affected by sterility in a very similar
manner. In the one case, the conditions of life have been disturbed,
though often in so slight a degree as to be inappreciable by us; in
the other case, or that of hybrids, the external conditions have
remained the same, but the organisation has been disturbed by two
different structures and constitutions having been blended into one.
For it is scarcely possible that two organisations should be
compounded into one, without some disturbance occurring in the
development, or periodical action, or mutual relation of the different
parts and organs one to another, or to the conditions of life. When
hybrids are able to breed inter se, they transmit to their offspring
from generation to generation the same compounded organisation, and
hence we need not be surprised that their sterility, though in some
degree variable, rarely diminishes.

It must, however, be confessed that we cannot understand, excepting on
vague hypotheses, several facts with respect to the sterility of
hybrids; for instance, the unequal fertility of hybrids produced from
reciprocal crosses; or the increased sterility in those hybrids which
occasionally and exceptionally resemble closely either pure parent.
Nor do I pretend that the foregoing remarks go to the root of the
matter: no explanation is offered why an organism, when placed under
unnatural conditions, is rendered sterile. All that I have attempted
to show, is that in two cases, in some respects allied, sterility is
the common result,--in the one case from the conditions of life having
been disturbed, in the other case from the organisation having been
disturbed by two organisations having been compounded into one.

It may seem fanciful, but I suspect that a similar parallelism extends
to an allied yet very different class of facts. It is an old and
almost universal belief, founded, I think, on a considerable body of
evidence, that slight changes in the conditions of life are beneficial
to all living things. We see this acted on by farmers and gardeners in
their frequent exchanges of seed, tubers, etc., from one soil or
climate to another, and back again. During the convalescence of
animals, we plainly see that great benefit is derived from almost any
change in the habits of life. Again, both with plants and animals,
there is abundant evidence, that a cross between very distinct
individuals of the same species, that is between members of different
strains or sub-breeds, gives vigour and fertility to the offspring. I
believe, indeed, from the facts alluded to in our fourth chapter, that
a certain amount of crossing is indispensable even with
hermaphrodites; and that close interbreeding continued during several
generations between the nearest relations, especially if these be kept
under the same conditions of life, always induces weakness and
sterility in the progeny.

Hence it seems that, on the one hand, slight changes in the conditions
of life benefit all organic beings, and on the other hand, that slight
crosses, that is crosses between the males and females of the same
species which have varied and become slightly different, give vigour
and fertility to the offspring. But we have seen that greater changes,
or changes of a particular nature, often render organic beings in some
degree sterile; and that greater crosses, that is crosses between
males and females which have become widely or specifically different,
produce hybrids which are generally sterile in some degree. I cannot
persuade myself that this parallelism is an accident or an illusion.
Both series of facts seem to be connected together by some common but
unknown bond, which is essentially related to the principle of life.

FERTILITY OF VARIETIES WHEN CROSSED, AND OF THEIR MONGREL OFFSPRING.

It may be urged, as a most forcible argument, that there must be some
essential distinction between species and varieties, and that there
must be some error in all the foregoing remarks, inasmuch as
varieties, however much they may differ from each other in external
appearance, cross with perfect facility, and yield perfectly fertile
offspring. I fully admit that this is almost invariably the case. But
if we look to varieties produced under nature, we are immediately
involved in hopeless difficulties; for if two hitherto reputed
varieties be found in any degree sterile together, they are at once
ranked by most naturalists as species. For instance, the blue and red
pimpernel, the primrose and cowslip, which are considered by many of
our best botanists as varieties, are said by Gartner not to be quite
fertile when crossed, and he consequently ranks them as undoubted
species. If we thus argue in a circle, the fertility of all varieties
produced under nature will assuredly have to be granted.

If we turn to varieties, produced, or supposed to have been produced,
under domestication, we are still involved in doubt. For when it is
stated, for instance, that the German Spitz dog unites more easily
than other dogs with foxes, or that certain South American indigenous
domestic dogs do not readily cross with European dogs, the explanation
which will occur to everyone, and probably the true one, is that these
dogs have descended from several aboriginally distinct species.
Nevertheless the perfect fertility of so many domestic varieties,
differing widely from each other in appearance, for instance of the
pigeon or of the cabbage, is a remarkable fact; more especially when
we reflect how many species there are, which, though resembling each
other most closely, are utterly sterile when intercrossed. Several
considerations, however, render the fertility of domestic varieties
less remarkable than at first appears. It can, in the first place, be
clearly shown that mere external dissimilarity between two species
does not determine their greater or lesser degree of sterility when
crossed; and we may apply the same rule to domestic varieties. In the
second place, some eminent naturalists believe that a long course of
domestication tends to eliminate sterility in the successive
generations of hybrids, which were at first only slightly sterile; and
if this be so, we surely ought not to expect to find sterility both
appearing and disappearing under nearly the same conditions of life.
Lastly, and this seems to me by far the most important consideration,
new races of animals and plants are produced under domestication by
man's methodical and unconscious power of selection, for his own use
and pleasure: he neither wishes to select, nor could select, slight
differences in the reproductive system, or other constitutional
differences correlated with the reproductive system. He supplies his
several varieties with the same food; treats them in nearly the same
manner, and does not wish to alter their general habits of life.
Nature acts uniformly and slowly during vast periods of time on the
whole organisation, in any way which may be for each creature's own
good; and thus she may, either directly, or more probably indirectly,
through correlation, modify the reproductive system in the several
descendants from any one species. Seeing this difference in the
process of selection, as carried on by man and nature, we need not be
surprised at some difference in the result.

I have as yet spoken as if the varieties of the same species were
invariably fertile when intercrossed. But it seems to me impossible to
resist the evidence of the existence of a certain amount of sterility
in the few following cases, which I will briefly abstract. The
evidence is at least as good as that from which we believe in the
sterility of a multitude of species. The evidence is, also, derived
from hostile witnesses, who in all other cases consider fertility and
sterility as safe criterions of specific distinction. Gartner kept
during several years a dwarf kind of maize with yellow seeds, and a
tall variety with red seeds, growing near each other in his garden;
and although these plants have separated sexes, they never naturally
crossed. He then fertilised thirteen flowers of the one with the
pollen of the other; but only a single head produced any seed, and
this one head produced only five grains. Manipulation in this case
could not have been injurious, as the plants have separated sexes. No
one, I believe, has suspected that these varieties of maize are
distinct species; and it is important to notice that the hybrid plants
thus raised were themselves PERFECTLY fertile; so that even Gartner
did not venture to consider the two varieties as specifically
distinct.

Girou de Buzareingues crossed three varieties of gourd, which like the
maize has separated sexes, and he asserts that their mutual
fertilisation is by so much the less easy as their differences are
greater. How far these experiments may be trusted, I know not; but the
forms experimentised on, are ranked by Sagaret, who mainly founds his
classification by the test of infertility, as varieties.

The following case is far more remarkable, and seems at first quite
incredible; but it is the result of an astonishing number of
experiments made during many years on nine species of Verbascum, by so
good an observer and so hostile a witness, as Gartner: namely, that
yellow and white varieties of the same species of Verbascum when
intercrossed produce less seed, than do either coloured varieties when
fertilised with pollen from their own coloured flowers. Moreover, he
asserts that when yellow and white varieties of one species are
crossed with yellow and white varieties of a DISTINCT species, more
seed is produced by the crosses between the same coloured flowers,
than between those which are differently coloured. Yet these varieties
of Verbascum present no other difference besides the mere colour of
the flower; and one variety can sometimes be raised from the seed of
the other.

From observations which I have made on certain varieties of hollyhock,
I am inclined to suspect that they present analogous facts.

Kolreuter, whose accuracy has been confirmed by every subsequent
observer, has proved the remarkable fact, that one variety of the
common tobacco is more fertile, when crossed with a widely distinct
species, than are the other varieties. He experimentised on five
forms, which are commonly reputed to be varieties, and which he tested
by the severest trial, namely, by reciprocal crosses, and he found
their mongrel offspring perfectly fertile. But one of these five
varieties, when used either as father or mother, and crossed with the
Nicotiana glutinosa, always yielded hybrids not so sterile as those
which were produced from the four other varieties when crossed with N.
glutinosa. Hence the reproductive system of this one variety must have
been in some manner and in some degree modified.

From these facts; from the great difficulty of ascertaining the
infertility of varieties in a state of nature, for a supposed variety
if infertile in any degree would generally be ranked as species; from
man selecting only external characters in the production of the most
distinct domestic varieties, and from not wishing or being able to
produce recondite and functional differences in the reproductive
system; from these several considerations and facts, I do not think
that the very general fertility of varieties can be proved to be of
universal occurrence, or to form a fundamental distinction between
varieties and species. The general fertility of varieties does not
seem to me sufficient to overthrow the view which I have taken with
respect to the very general, but not invariable, sterility of first
crosses and of hybrids, namely, that it is not a special endowment,
but is incidental on slowly acquired modifications, more especially in
the reproductive systems of the forms which are crossed.

HYBRIDS AND MONGRELS COMPARED, INDEPENDENTLY OF THEIR FERTILITY.

Independently of the question of fertility, the offspring of species
when crossed and of varieties when crossed may be compared in several
other respects. Gartner, whose strong wish was to draw a marked line
of distinction between species and varieties, could find very few and,
as it seems to me, quite unimportant differences between the so-called
hybrid offspring of species, and the so-called mongrel offspring of
varieties. And, on the other hand, they agree most closely in very
many important respects.

I shall here discuss this subject with extreme brevity. The most
important distinction is, that in the first generation mongrels are
more variable than hybrids; but Gartner admits that hybrids from
species which have long been cultivated are often variable in the
first generation; and I have myself seen striking instances of this
fact. Gartner further admits that hybrids between very closely allied
species are more variable than those from very distinct species; and
this shows that the difference in the degree of variability graduates
away. When mongrels and the more fertile hybrids are propagated for
several generations an extreme amount of variability in their
offspring is notorious; but some few cases both of hybrids and
mongrels long retaining uniformity of character could be given. The
variability, however, in the successive generations of mongrels is,
perhaps, greater than in hybrids.

This greater variability of mongrels than of hybrids does not seem to
me at all surprising. For the parents of mongrels are varieties, and
mostly domestic varieties (very few experiments having been tried on
natural varieties), and this implies in most cases that there has been
recent variability; and therefore we might expect that such
variability would often continue and be super-added to that arising
from the mere act of crossing. The slight degree of variability in
hybrids from the first cross or in the first generation, in contrast
with their extreme variability in the succeeding generations, is a
curious fact and deserves attention. For it bears on and corroborates
the view which I have taken on the cause of ordinary variability;
namely, that it is due to the reproductive system being eminently
sensitive to any change in the conditions of life, being thus often
rendered either impotent or at least incapable of its proper function
of producing offspring identical with the parent-form. Now hybrids in
the first generation are descended from species (excluding those long
cultivated) which have not had their reproductive systems in any way
affected, and they are not variable; but hybrids themselves have their
reproductive systems seriously affected, and their descendants are
highly variable.

But to return to our comparison of mongrels and hybrids: Gartner
states that mongrels are more liable than hybrids to revert to either
parent-form; but this, if it be true, is certainly only a difference
in degree. Gartner further insists that when any two species, although
most closely allied to each other, are crossed with a third species,
the hybrids are widely different from each other; whereas if two very
distinct varieties of one species are crossed with another species,
the hybrids do not differ much. But this conclusion, as far as I can
make out, is founded on a single experiment; and seems directly
opposed to the results of several experiments made by Kolreuter.

These alone are the unimportant differences, which Gartner is able to
point out, between hybrid and mongrel plants. On the other hand, the
resemblance in mongrels and in hybrids to their respective parents,
more especially in hybrids produced from nearly related species,
follows according to Gartner the same laws. When two species are
crossed, one has sometimes a prepotent power of impressing its
likeness on the hybrid; and so I believe it to be with varieties of
plants. With animals one variety certainly often has this prepotent
power over another variety. Hybrid plants produced from a reciprocal
cross, generally resemble each other closely; and so it is with
mongrels from a reciprocal cross. Both hybrids and mongrels can be
reduced to either pure parent-form, by repeated crosses in successive
generations with either parent.

These several remarks are apparently applicable to animals; but the
subject is here excessively complicated, partly owing to the existence
of secondary sexual characters; but more especially owing to
prepotency in transmitting likeness running more strongly in one sex
than in the other, both when one species is crossed with another, and
when one variety is crossed with another variety. For instance, I
think those authors are right, who maintain that the ass has a
prepotent power over the horse, so that both the mule and the hinny
more resemble the ass than the horse; but that the prepotency runs
more strongly in the male-ass than in the female, so that the mule,
which is the offspring of the male-ass and mare, is more like an ass,
than is the hinny, which is the offspring of the female-ass and
stallion.

Much stress has been laid by some authors on the supposed fact, that
mongrel animals alone are born closely like one of their parents; but
it can be shown that this does sometimes occur with hybrids; yet I
grant much less frequently with hybrids than with mongrels. Looking to
the cases which I have collected of cross-bred animals closely
resembling one parent, the resemblances seem chiefly confined to
characters almost monstrous in their nature, and which have suddenly
appeared--such as albinism, melanism, deficiency of tail or horns, or
additional fingers and toes; and do not relate to characters which
have been slowly acquired by selection. Consequently, sudden
reversions to the perfect character of either parent would be more
likely to occur with mongrels, which are descended from varieties
often suddenly produced and semi-monstrous in character, than with
hybrids, which are descended from species slowly and naturally
produced. On the whole I entirely agree with Dr. Prosper Lucas, who,
after arranging an enormous body of facts with respect to animals,
comes to the conclusion, that the laws of resemblance of the child to
its parents are the same, whether the two parents differ much or
little from each other, namely in the union of individuals of the same
variety, or of different varieties, or of distinct species.

Laying aside the question of fertility and sterility, in all other
respects there seems to be a general and close similarity in the
offspring of crossed species, and of crossed varieties. If we look at
species as having been specially created, and at varieties as having
been produced by secondary laws, this similarity would be an
astonishing fact. But it harmonises perfectly with the view that there
is no essential distinction between species and varieties.

SUMMARY OF CHAPTER.

First crosses between forms sufficiently distinct to be ranked as
species, and their hybrids, are very generally, but not universally,
sterile. The sterility is of all degrees, and is often so slight that
the two most careful experimentalists who have ever lived, have come
to diametrically opposite conclusions in ranking forms by this test.
The sterility is innately variable in individuals of the same species,
and is eminently susceptible of favourable and unfavourable
conditions. The degree of sterility does not strictly follow
systematic affinity, but is governed by several curious and complex
laws. It is generally different, and sometimes widely different, in
reciprocal crosses between the same two species. It is not always
equal in degree in a first cross and in the hybrid produced from this
cross.

In the same manner as in grafting trees, the capacity of one species
or variety to take on another, is incidental on generally unknown
differences in their vegetative systems, so in crossing, the greater
or less facility of one species to unite with another, is incidental
on unknown differences in their reproductive systems. There is no more
reason to think that species have been specially endowed with various
degrees of sterility to prevent them crossing and blending in nature,
than to think that trees have been specially endowed with various and
somewhat analogous degrees of difficulty in being grafted together in
order to prevent them becoming inarched in our forests.

The sterility of first crosses between pure species, which have their
reproductive systems perfect, seems to depend on several
circumstances; in some cases largely on the early death of the embryo.
The sterility of hybrids, which have their reproductive systems
imperfect, and which have had this system and their whole organisation
disturbed by being compounded of two distinct species, seems closely
allied to that sterility which so frequently affects pure species,
when their natural conditions of life have been disturbed. This view
is supported by a parallelism of another kind;--namely, that the
crossing of forms only slightly different is favourable to the vigour
and fertility of their offspring; and that slight changes in the
conditions of life are apparently favourable to the vigour and
fertility of all organic beings. It is not surprising that the degree
of difficulty in uniting two species, and the degree of sterility of
their hybrid-offspring should generally correspond, though due to
distinct causes; for both depend on the amount of difference of some
kind between the species which are crossed. Nor is it surprising that
the facility of effecting a first cross, the fertility of the hybrids
produced, and the capacity of being grafted together--though this
latter capacity evidently depends on widely different
circumstances--should all run, to a certain extent, parallel with the
systematic affinity of the forms which are subjected to experiment;
for systematic affinity attempts to express all kinds of resemblance
between all species.

First crosses between forms known to be varieties, or sufficiently
alike to be considered as varieties, and their mongrel offspring, are
very generally, but not quite universally, fertile. Nor is this nearly
general and perfect fertility surprising, when we remember how liable
we are to argue in a circle with respect to varieties in a state of
nature; and when we remember that the greater number of varieties have
been produced under domestication by the selection of mere external
differences, and not of differences in the reproductive system. In all
other respects, excluding fertility, there is a close general
resemblance between hybrids and mongrels. Finally, then, the facts
briefly given in this chapter do not seem to me opposed to, but even
rather to support the view, that there is no fundamental distinction
between species and varieties.


CHAPTER 9. ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.

On the absence of intermediate varieties at the present day.
On the nature of extinct intermediate varieties; on their number.
On the vast lapse of time, as inferred from the rate of deposition and
of denudation.
On the poorness of our palaeontological collections.
On the intermittence of geological formations.
On the absence of intermediate varieties in any one formation.
On the sudden appearance of groups of species.
On their sudden appearance in the lowest known fossiliferous strata.

In the sixth chapter I enumerated the chief objections which might be
justly urged against the views maintained in this volume. Most of them
have now been discussed. One, namely the distinctness of specific
forms, and their not being blended together by innumerable
transitional links, is a very obvious difficulty. I assigned reasons
why such links do not commonly occur at the present day, under the
circumstances apparently most favourable for their presence, namely on
an extensive and continuous area with graduated physical conditions. I
endeavoured to show, that the life of each species depends in a more
important manner on the presence of other already defined organic
forms, than on climate; and, therefore, that the really governing
conditions of life do not graduate away quite insensibly like heat or
moisture. I endeavoured, also, to show that intermediate varieties,
from existing in lesser numbers than the forms which they connect,
will generally be beaten out and exterminated during the course of
further modification and improvement. The main cause, however, of
innumerable intermediate links not now occurring everywhere throughout
nature depends on the very process of natural selection, through which
new varieties continually take the places of and exterminate their
parent-forms. But just in proportion as this process of extermination
has acted on an enormous scale, so must the number of intermediate
varieties, which have formerly existed on the earth, be truly
enormous. Why then is not every geological formation and every stratum
full of such intermediate links? Geology assuredly does not reveal any
such finely graduated organic chain; and this, perhaps, is the most
obvious and gravest objection which can be urged against my theory.
The explanation lies, as I believe, in the extreme imperfection of the
geological record.

In the first place it should always be borne in mind what sort of
intermediate forms must, on my theory, have formerly existed. I have
found it difficult, when looking at any two species, to avoid
picturing to myself, forms DIRECTLY intermediate between them. But
this is a wholly false view; we should always look for forms
intermediate between each species and a common but unknown progenitor;
and the progenitor will generally have differed in some respects from
all its modified descendants. To give a simple illustration: the
fantail and pouter pigeons have both descended from the rock-pigeon;
if we possessed all the intermediate varieties which have ever
existed, we should have an extremely close series between both and the
rock-pigeon; but we should have no varieties directly intermediate
between the fantail and pouter; none, for instance, combining a tail
somewhat expanded with a crop somewhat enlarged, the characteristic
features of these two breeds. These two breeds, moreover, have become
so much modified, that if we had no historical or indirect evidence
regarding their origin, it would not have been possible to have
determined from a mere comparison of their structure with that of the
rock-pigeon, whether they had descended from this species or from some
other allied species, such as C. oenas.

So with natural species, if we look to forms very distinct, for
instance to the horse and tapir, we have no reason to suppose that
links ever existed directly intermediate between them, but between
each and an unknown common parent. The common parent will have had in
its whole organisation much general resemblance to the tapir and to
the horse; but in some points of structure may have differed
considerably from both, even perhaps more than they differ from each
other. Hence in all such cases, we should be unable to recognise the
parent-form of any two or more species, even if we closely compared
the structure of the parent with that of its modified descendants,
unless at the same time we had a nearly perfect chain of the
intermediate links.

It is just possible by my theory, that one of two living forms might
have descended from the other; for instance, a horse from a tapir; and
in this case DIRECT intermediate links will have existed between them.
But such a case would imply that one form had remained for a very long
period unaltered, whilst its descendants had undergone a vast amount
of change; and the principle of competition between organism and
organism, between child and parent, will render this a very rare
event; for in all cases the new and improved forms of life will tend
to supplant the old and unimproved forms.

By the theory of natural selection all living species have been
connected with the parent-species of each genus, by differences not
greater than we see between the varieties of the same species at the
present day; and these parent-species, now generally extinct, have in
their turn been similarly connected with more ancient species; and so
on backwards, always converging to the common ancestor of each great
class. So that the number of intermediate and transitional links,
between all living and extinct species, must have been inconceivably
great. But assuredly, if this theory be true, such have lived upon
this earth.

ON THE LAPSE OF TIME.

Independently of our not finding fossil remains of such infinitely
numerous connecting links, it may be objected, that time will not have
sufficed for so great an amount of organic change, all changes having
been effected very slowly through natural selection. It is hardly
possible for me even to recall to the reader, who may not be a
practical geologist, the facts leading the mind feebly to comprehend
the lapse of time. He who can read Sir Charles Lyell's grand work on
the Principles of Geology, which the future historian will recognise
as having produced a revolution in natural science, yet does not admit
how incomprehensibly vast have been the past periods of time, may at
once close this volume. Not that it suffices to study the Principles
of Geology, or to read special treatises by different observers on
separate formations, and to mark how each author attempts to give an
inadequate idea of the duration of each formation or even each
stratum. A man must for years examine for himself great piles of
superimposed strata, and watch the sea at work grinding down old rocks
and making fresh sediment, before he can hope to comprehend anything
of the lapse of time, the monuments of which we see around us.

It is good to wander along lines of sea-coast, when formed of
moderately hard rocks, and mark the process of degradation. The tides
in most cases reach the cliffs only for a short time twice a day, and
the waves eat into them only when they are charged with sand or
pebbles; for there is reason to believe that pure water can effect
little or nothing in wearing away rock. At last the base of the cliff
is undermined, huge fragments fall down, and these remaining fixed,
have to be worn away, atom by atom, until reduced in size they can be
rolled about by the waves, and then are more quickly ground into
pebbles, sand, or mud. But how often do we see along the bases of
retreating cliffs rounded boulders, all thickly clothed by marine
productions, showing how little they are abraded and how seldom they
are rolled about! Moreover, if we follow for a few miles any line of
rocky cliff, which is undergoing degradation, we find that it is only
here and there, along a short length or round a promontory, that the
cliffs are at the present time suffering. The appearance of the
surface and the vegetation show that elsewhere years have elapsed
since the waters washed their base.

He who most closely studies the action of the sea on our shores, will,
I believe, be most deeply impressed with the slowness with which rocky
coasts are worn away. The observations on this head by Hugh Miller,
and by that excellent observer Mr. Smith of Jordan Hill, are most
impressive. With the mind thus impressed, let any one examine beds of
conglomerate many thousand feet in thickness, which, though probably
formed at a quicker rate than many other deposits, yet, from being
formed of worn and rounded pebbles, each of which bears the stamp of
time, are good to show how slowly the mass has been accumulated. Let
him remember Lyell's profound remark, that the thickness and extent of
sedimentary formations are the result and measure of the degradation
which the earth's crust has elsewhere suffered. And what an amount of
degradation is implied by the sedimentary deposits of many countries!
Professor Ramsay has given me the maximum thickness, in most cases
from actual measurement, in a few cases from estimate, of each
formation in different parts of Great Britain; and this is the
result:--

                                                 Feet
Palaeozoic strata (not including igneous beds)..57,154.
Secondary strata................................13,190.
Tertiary strata..................................2,240.

--making altogether 72,584 feet; that is, very nearly thirteen and
three-quarters British miles. Some of these formations, which are
represented in England by thin beds, are thousands of feet in
thickness on the Continent. Moreover, between each successive
formation, we have, in the opinion of most geologists, enormously long
blank periods. So that the lofty pile of sedimentary rocks in Britain,
gives but an inadequate idea of the time which has elapsed during
their accumulation; yet what time this must have consumed! Good
observers have estimated that sediment is deposited by the great
Mississippi river at the rate of only 600 feet in a hundred thousand
years. This estimate may be quite erroneous; yet, considering over
what wide spaces very fine sediment is transported by the currents of
the sea, the process of accumulation in any one area must be extremely
slow.

But the amount of denudation which the strata have in many places
suffered, independently of the rate of accumulation of the degraded
matter, probably offers the best evidence of the lapse of time. I
remember having been much struck with the evidence of denudation, when
viewing volcanic islands, which have been worn by the waves and pared
all round into perpendicular cliffs of one or two thousand feet in
height; for the gentle slope of the lava-streams, due to their
formerly liquid state, showed at a glance how far the hard, rocky beds
had once extended into the open ocean. The same story is still more
plainly told by faults,--those great cracks along which the strata
have been upheaved on one side, or thrown down on the other, to the
height or depth of thousands of feet; for since the crust cracked, the
surface of the land has been so completely planed down by the action
of the sea, that no trace of these vast dislocations is externally
visible.

The Craven fault, for instance, extends for upwards of 30 miles, and
along this line the vertical displacement of the strata has varied
from 600 to 3000 feet. Professor Ramsay has published an account of a
downthrow in Anglesea of 2300 feet; and he informs me that he fully
believes there is one in Merionethshire of 12,000 feet; yet in these
cases there is nothing on the surface to show such prodigious
movements; the pile of rocks on the one or other side having been
smoothly swept away. The consideration of these facts impresses my
mind almost in the same manner as does the vain endeavour to grapple
with the idea of eternity.

I am tempted to give one other case, the well-known one of the
denudation of the Weald. Though it must be admitted that the
denudation of the Weald has been a mere trifle, in comparison with
that which has removed masses of our palaeozoic strata, in parts ten
thousand feet in thickness, as shown in Professor Ramsay's masterly
memoir on this subject. Yet it is an admirable lesson to stand on the
North Downs and to look at the distant South Downs; for, remembering
that at no great distance to the west the northern and southern
escarpments meet and close, one can safely picture to oneself the
great dome of rocks which must have covered up the Weald within so
limited a period as since the latter part of the Chalk formation. The
distance from the northern to the southern Downs is about 22 miles,
and the thickness of the several formations is on an average about
1100 feet, as I am informed by Professor Ramsay. But if, as some
geologists suppose, a range of older rocks underlies the Weald, on the
flanks of which the overlying sedimentary deposits might have
accumulated in thinner masses than elsewhere, the above estimate would
be erroneous; but this source of doubt probably would not greatly
affect the estimate as applied to the western extremity of the
district. If, then, we knew the rate at which the sea commonly wears
away a line of cliff of any given height, we could measure the time
requisite to have denuded the Weald. This, of course, cannot be done;
but we may, in order to form some crude notion on the subject, assume
that the sea would eat into cliffs 500 feet in height at the rate of
one inch in a century. This will at first appear much too small an
allowance; but it is the same as if we were to assume a cliff one yard
in height to be eaten back along a whole line of coast at the rate of
one yard in nearly every twenty-two years. I doubt whether any rock,
even as soft as chalk, would yield at this rate excepting on the most
exposed coasts; though no doubt the degradation of a lofty cliff would
be more rapid from the breakage of the fallen fragments. On the other
hand, I do not believe that any line of coast, ten or twenty miles in
length, ever suffers degradation at the same time along its whole
indented length; and we must remember that almost all strata contain
harder layers or nodules, which from long resisting attrition form a
breakwater at the base. Hence, under ordinary circumstances, I
conclude that for a cliff 500 feet in height, a denudation of one inch
per century for the whole length would be an ample allowance. At this
rate, on the above data, the denudation of the Weald must have
required 306,662,400 years; or say three hundred million years.

The action of fresh water on the gently inclined Wealden district,
when upraised, could hardly have been great, but it would somewhat
reduce the above estimate. On the other hand, during oscillations of
level, which we know this area has undergone, the surface may have
existed for millions of years as land, and thus have escaped the
action of the sea: when deeply submerged for perhaps equally long
periods, it would, likewise, have escaped the action of the
coast-waves. So that in all probability a far longer period than 300
million years has elapsed since the latter part of the Secondary
period.

I have made these few remarks because it is highly important for us to
gain some notion, however imperfect, of the lapse of years. During
each of these years, over the whole world, the land and the water has
been peopled by hosts of living forms. What an infinite number of
generations, which the mind cannot grasp, must have succeeded each
other in the long roll of years! Now turn to our richest geological
museums, and what a paltry display we behold!

ON THE POORNESS OF OUR PALAEONTOLOGICAL COLLECTIONS.

That our palaeontological collections are very imperfect, is admitted
by every one. The remark of that admirable palaeontologist, the late
Edward Forbes, should not be forgotten, namely, that numbers of our
fossil species are known and named from single and often broken
specimens, or from a few specimens collected on some one spot. Only a
small portion of the surface of the earth has been geologically
explored, and no part with sufficient care, as the important
discoveries made every year in Europe prove. No organism wholly soft
can be preserved. Shells and bones will decay and disappear when left
on the bottom of the sea, where sediment is not accumulating. I
believe we are continually taking a most erroneous view, when we
tacitly admit to ourselves that sediment is being deposited over
nearly the whole bed of the sea, at a rate sufficiently quick to embed
and preserve fossil remains. Throughout an enormously large proportion
of the ocean, the bright blue tint of the water bespeaks its purity.
The many cases on record of a formation conformably covered, after an
enormous interval of time, by another and later formation, without the
underlying bed having suffered in the interval any wear and tear, seem
explicable only on the view of the bottom of the sea not rarely lying
for ages in an unaltered condition. The remains which do become
embedded, if in sand or gravel, will when the beds are upraised
generally be dissolved by the percolation of rain-water. I suspect
that but few of the very many animals which live on the beach between
high and low watermark are preserved. For instance, the several
species of the Chthamalinae (a sub-family of sessile cirripedes) coat
the rocks all over the world in infinite numbers: they are all
strictly littoral, with the exception of a single Mediterranean
species, which inhabits deep water and has been found fossil in
Sicily, whereas not one other species has hitherto been found in any
tertiary formation: yet it is now known that the genus Chthamalus
existed during the chalk period. The molluscan genus Chiton offers a
partially analogous case.

With respect to the terrestrial productions which lived during the
Secondary and Palaeozoic periods, it is superfluous to state that our
evidence from fossil remains is fragmentary in an extreme degree. For
instance, not a land shell is known belonging to either of these vast
periods, with one exception discovered by Sir C. Lyell in the
carboniferous strata of North America. In regard to mammiferous
remains, a single glance at the historical table published in the
Supplement to Lyell's Manual, will bring home the truth, how
accidental and rare is their preservation, far better than pages of
detail. Nor is their rarity surprising, when we remember how large a
proportion of the bones of tertiary mammals have been discovered
either in caves or in lacustrine deposits; and that not a cave or true
lacustrine bed is known belonging to the age of our secondary or
palaeozoic formations.

But the imperfection in the geological record mainly results from
another and more important cause than any of the foregoing; namely,
from the several formations being separated from each other by wide
intervals of time. When we see the formations tabulated in written
works, or when we follow them in nature, it is difficult to avoid
believing that they are closely consecutive. But we know, for
instance, from Sir R. Murchison's great work on Russia, what wide gaps
there are in that country between the superimposed formations; so it
is in North America, and in many other parts of the world. The most
skilful geologist, if his attention had been exclusively confined to
these large territories, would never have suspected that during the
periods which were blank and barren in his own country, great piles of
sediment, charged with new and peculiar forms of life, had elsewhere
been accumulated. And if in each separate territory, hardly any idea
can be formed of the length of time which has elapsed between the
consecutive formations, we may infer that this could nowhere be
ascertained. The frequent and great changes in the mineralogical
composition of consecutive formations, generally implying great
changes in the geography of the surrounding lands, whence the sediment
has been derived, accords with the belief of vast intervals of time
having elapsed between each formation.

But we can, I think, see why the geological formations of each region
are almost invariably intermittent; that is, have not followed each
other in close sequence. Scarcely any fact struck me more when
examining many hundred miles of the South American coasts, which have
been upraised several hundred feet within the recent period, than the
absence of any recent deposits sufficiently extensive to last for even
a short geological period. Along the whole west coast, which is
inhabited by a peculiar marine fauna, tertiary beds are so scantily
developed, that no record of several successive and peculiar marine
faunas will probably be preserved to a distant age. A little
reflection will explain why along the rising coast of the western side
of South America, no extensive formations with recent or tertiary
remains can anywhere be found, though the supply of sediment must for
ages have been great, from the enormous degradation of the coast-rocks
and from muddy streams entering the sea. The explanation, no doubt,
is, that the littoral and sub-littoral deposits are continually worn
away, as soon as they are brought up by the slow and gradual rising of
the land within the grinding action of the coast-waves.

We may, I think, safely conclude that sediment must be accumulated in
extremely thick, solid, or extensive masses, in order to withstand the
incessant action of the waves, when first upraised and during
subsequent oscillations of level. Such thick and extensive
accumulations of sediment may be formed in two ways; either, in
profound depths of the sea, in which case, judging from the researches
of E. Forbes, we may conclude that the bottom will be inhabited by
extremely few animals, and the mass when upraised will give a most
imperfect record of the forms of life which then existed; or, sediment
may be accumulated to any thickness and extent over a shallow bottom,
if it continue slowly to subside. In this latter case, as long as the
rate of subsidence and supply of sediment nearly balance each other,
the sea will remain shallow and favourable for life, and thus a
fossiliferous formation thick enough, when upraised, to resist any
amount of degradation, may be formed.

I am convinced that all our ancient formations, which are rich in
fossils, have thus been formed during subsidence. Since publishing my
views on this subject in 1845, I have watched the progress of Geology,
and have been surprised to note how author after author, in treating
of this or that great formation, has come to the conclusion that it
was accumulated during subsidence. I may add, that the only ancient
tertiary formation on the west coast of South America, which has been
bulky enough to resist such degradation as it has as yet suffered, but
which will hardly last to a distant geological age, was certainly
deposited during a downward oscillation of level, and thus gained
considerable thickness.

All geological facts tell us plainly that each area has undergone
numerous slow oscillations of level, and apparently these oscillations
have affected wide spaces. Consequently formations rich in fossils and
sufficiently thick and extensive to resist subsequent degradation, may
have been formed over wide spaces during periods of subsidence, but
only where the supply of sediment was sufficient to keep the sea
shallow and to embed and preserve the remains before they had time to
decay. On the other hand, as long as the bed of the sea remained
stationary, THICK deposits could not have been accumulated in the
shallow parts, which are the most favourable to life. Still less could
this have happened during the alternate periods of elevation; or, to
speak more accurately, the beds which were then accumulated will have
been destroyed by being upraised and brought within the limits of the
coast-action.

Thus the geological record will almost necessarily be rendered
intermittent. I feel much confidence in the truth of these views, for
they are in strict accordance with the general principles inculcated
by Sir C. Lyell; and E. Forbes independently arrived at a similar
conclusion.

One remark is here worth a passing notice. During periods of elevation
the area of the land and of the adjoining shoal parts of the sea will
be increased, and new stations will often be formed;--all
circumstances most favourable, as previously explained, for the
formation of new varieties and species; but during such periods there
will generally be a blank in the geological record. On the other hand,
during subsidence, the inhabited area and number of inhabitants will
decrease (excepting the productions on the shores of a continent when
first broken up into an archipelago), and consequently during
subsidence, though there will be much extinction, fewer new varieties
or species will be formed; and it is during these very periods of
subsidence, that our great deposits rich in fossils have been
accumulated. Nature may almost be said to have guarded against the
frequent discovery of her transitional or linking forms.

From the foregoing considerations it cannot be doubted that the
geological record, viewed as a whole, is extremely imperfect; but if
we confine our attention to any one formation, it becomes more
difficult to understand, why we do not therein find closely graduated
varieties between the allied species which lived at its commencement
and at its close. Some cases are on record of the same species
presenting distinct varieties in the upper and lower parts of the same
formation, but, as they are rare, they may be here passed over.
Although each formation has indisputably required a vast number of
years for its deposition, I can see several reasons why each should
not include a graduated series of links between the species which then
lived; but I can by no means pretend to assign due proportional weight
to the following considerations.

Although each formation may mark a very long lapse of years, each
perhaps is short compared with the period requisite to change one
species into another. I am aware that two palaeontologists, whose
opinions are worthy of much deference, namely Bronn and Woodward, have
concluded that the average duration of each formation is twice or
thrice as long as the average duration of specific forms. But
insuperable difficulties, as it seems to me, prevent us coming to any
just conclusion on this head. When we see a species first appearing in
the middle of any formation, it would be rash in the extreme to infer
that it had not elsewhere previously existed. So again when we find a
species disappearing before the uppermost layers have been deposited,
it would be equally rash to suppose that it then became wholly
extinct. We forget how small the area of Europe is compared with the
rest of the world; nor have the several stages of the same formation
throughout Europe been correlated with perfect accuracy.

With marine animals of all kinds, we may safely infer a large amount
of migration during climatal and other changes; and when we see a
species first appearing in any formation, the probability is that it
only then first immigrated into that area. It is well known, for
instance, that several species appeared somewhat earlier in the
palaeozoic beds of North America than in those of Europe; time having
apparently been required for their migration from the American to the
European seas. In examining the latest deposits of various quarters of
the world, it has everywhere been noted, that some few still existing
species are common in the deposit, but have become extinct in the
immediately surrounding sea; or, conversely, that some are now
abundant in the neighbouring sea, but are rare or absent in this
particular deposit. It is an excellent lesson to reflect on the
ascertained amount of migration of the inhabitants of Europe during
the Glacial period, which forms only a part of one whole geological
period; and likewise to reflect on the great changes of level, on the
inordinately great change of climate, on the prodigious lapse of time,
all included within this same glacial period. Yet it may be doubted
whether in any quarter of the world, sedimentary deposits, INCLUDING
FOSSIL REMAINS, have gone on accumulating within the same area during
the whole of this period. It is not, for instance, probable that
sediment was deposited during the whole of the glacial period near the
mouth of the Mississippi, within that limit of depth at which marine
animals can flourish; for we know what vast geographical changes
occurred in other parts of America during this space of time. When
such beds as were deposited in shallow water near the mouth of the
Mississippi during some part of the glacial period shall have been
upraised, organic remains will probably first appear and disappear at
different levels, owing to the migration of species and to
geographical changes. And in the distant future, a geologist examining
these beds, might be tempted to conclude that the average duration of
life of the embedded fossils had been less than that of the glacial
period, instead of having been really far greater, that is extending
from before the glacial epoch to the present day.

In order to get a perfect gradation between two forms in the upper and
lower parts of the same formation, the deposit must have gone on
accumulating for a very long period, in order to have given sufficient
time for the slow process of variation; hence the deposit will
generally have to be a very thick one; and the species undergoing
modification will have had to live on the same area throughout this
whole time. But we have seen that a thick fossiliferous formation can
only be accumulated during a period of subsidence; and to keep the
depth approximately the same, which is necessary in order to enable
the same species to live on the same space, the supply of sediment
must nearly have counterbalanced the amount of subsidence. But this
same movement of subsidence will often tend to sink the area whence
the sediment is derived, and thus diminish the supply whilst the
downward movement continues. In fact, this nearly exact balancing
between the supply of sediment and the amount of subsidence is
probably a rare contingency; for it has been observed by more than one
palaeontologist, that very thick deposits are usually barren of
organic remains, except near their upper or lower limits.

It would seem that each separate formation, like the whole pile of
formations in any country, has generally been intermittent in its
accumulation. When we see, as is so often the case, a formation
composed of beds of different mineralogical composition, we may
reasonably suspect that the process of deposition has been much
interrupted, as a change in the currents of the sea and a supply of
sediment of a different nature will generally have been due to
geographical changes requiring much time. Nor will the closest
inspection of a formation give any idea of the time which its
deposition has consumed. Many instances could be given of beds only a
few feet in thickness, representing formations, elsewhere thousands of
feet in thickness, and which must have required an enormous period for
their accumulation; yet no one ignorant of this fact would have
suspected the vast lapse of time represented by the thinner formation.
Many cases could be given of the lower beds of a formation having been
upraised, denuded, submerged, and then re-covered by the upper beds of
the same formation,--facts, showing what wide, yet easily overlooked,
intervals have occurred in its accumulation. In other cases we have
the plainest evidence in great fossilised trees, still standing
upright as they grew, of many long intervals of time and changes of
level during the process of deposition, which would never even have
been suspected, had not the trees chanced to have been preserved:
thus, Messrs. Lyell and Dawson found carboniferous beds 1400 feet
thick in Nova Scotia, with ancient root-bearing strata, one above the
other, at no less than sixty-eight different levels. Hence, when the
same species occur at the bottom, middle, and top of a formation, the
probability is that they have not lived on the same spot during the
whole period of deposition, but have disappeared and reappeared,
perhaps many times, during the same geological period. So that if such
species were to undergo a considerable amount of modification during
any one geological period, a section would not probably include all
the fine intermediate gradations which must on my theory have existed
between them, but abrupt, though perhaps very slight, changes of form.

It is all-important to remember that naturalists have no golden rule
by which to distinguish species and varieties; they grant some little
variability to each species, but when they meet with a somewhat
greater amount of difference between any two forms, they rank both as
species, unless they are enabled to connect them together by close
intermediate gradations. And this from the reasons just assigned we
can seldom hope to effect in any one geological section. Supposing B
and C to be two species, and a third, A, to be found in an underlying
bed; even if A were strictly intermediate between B and C, it would
simply be ranked as a third and distinct species, unless at the same
time it could be most closely connected with either one or both forms
by intermediate varieties. Nor should it be forgotten, as before
explained, that A might be the actual progenitor of B and C, and yet
might not at all necessarily be strictly intermediate between them in
all points of structure. So that we might obtain the parent-species
and its several modified descendants from the lower and upper beds of
a formation, and unless we obtained numerous transitional gradations,
we should not recognise their relationship, and should consequently be
compelled to rank them all as distinct species.

It is notorious on what excessively slight differences many
palaeontologists have founded their species; and they do this the more
readily if the specimens come from different sub-stages of the same
formation. Some experienced conchologists are now sinking many of the
very fine species of D'Orbigny and others into the rank of varieties;
and on this view we do find the kind of evidence of change which on my
theory we ought to find. Moreover, if we look to rather wider
intervals, namely, to distinct but consecutive stages of the same
great formation, we find that the embedded fossils, though almost
universally ranked as specifically different, yet are far more closely
allied to each other than are the species found in more widely
separated formations; but to this subject I shall have to return in
the following chapter.

One other consideration is worth notice: with animals and plants that
can propagate rapidly and are not highly locomotive, there is reason
to suspect, as we have formerly seen, that their varieties are
generally at first local; and that such local varieties do not spread
widely and supplant their parent-forms until they have been modified
and perfected in some considerable degree. According to this view, the
chance of discovering in a formation in any one country all the early
stages of transition between any two forms, is small, for the
successive changes are supposed to have been local or confined to some
one spot. Most marine animals have a wide range; and we have seen that
with plants it is those which have the widest range, that oftenest
present varieties; so that with shells and other marine animals, it is
probably those which have had the widest range, far exceeding the
limits of the known geological formations of Europe, which have
oftenest given rise, first to local varieties and ultimately to new
species; and this again would greatly lessen the chance of our being
able to trace the stages of transition in any one geological
formation.

It should not be forgotten, that at the present day, with perfect
specimens for examination, two forms can seldom be connected by
intermediate varieties and thus proved to be the same species, until
many specimens have been collected from many places; and in the case
of fossil species this could rarely be effected by palaeontologists.
We shall, perhaps, best perceive the improbability of our being
enabled to connect species by numerous, fine, intermediate, fossil
links, by asking ourselves whether, for instance, geologists at some
future period will be able to prove, that our different breeds of
cattle, sheep, horses, and dogs have descended from a single stock or
from several aboriginal stocks; or, again, whether certain sea-shells
inhabiting the shores of North America, which are ranked by some
conchologists as distinct species from their European representatives,
and by other conchologists as only varieties, are really varieties or
are, as it is called, specifically distinct. This could be effected
only by the future geologist discovering in a fossil state numerous
intermediate gradations; and such success seems to me improbable in
the highest degree.

Geological research, though it has added numerous species to existing
and extinct genera, and has made the intervals between some few groups
less wide than they otherwise would have been, yet has done scarcely
anything in breaking down the distinction between species, by
connecting them together by numerous, fine, intermediate varieties;
and this not having been effected, is probably the gravest and most
obvious of all the many objections which may be urged against my
views. Hence it will be worth while to sum up the foregoing remarks,
under an imaginary illustration. The Malay Archipelago is of about the
size of Europe from the North Cape to the Mediterranean, and from
Britain to Russia; and therefore equals all the geological formations
which have been examined with any accuracy, excepting those of the
United States of America. I fully agree with Mr. Godwin-Austen, that
the present condition of the Malay Archipelago, with its numerous
large islands separated by wide and shallow seas, probably represents
the former state of Europe, when most of our formations were
accumulating. The Malay Archipelago is one of the richest regions of
the whole world in organic beings; yet if all the species were to be
collected which have ever lived there, how imperfectly would they
represent the natural history of the world!

But we have every reason to believe that the terrestrial productions
of the archipelago would be preserved in an excessively imperfect
manner in the formations which we suppose to be there accumulating. I
suspect that not many of the strictly littoral animals, or of those
which lived on naked submarine rocks, would be embedded; and those
embedded in gravel or sand, would not endure to a distant epoch.
Wherever sediment did not accumulate on the bed of the sea, or where
it did not accumulate at a sufficient rate to protect organic bodies
from decay, no remains could be preserved.

In our archipelago, I believe that fossiliferous formations could be
formed of sufficient thickness to last to an age, as distant in
futurity as the secondary formations lie in the past, only during
periods of subsidence. These periods of subsidence would be separated
from each other by enormous intervals, during which the area would be
either stationary or rising; whilst rising, each fossiliferous
formation would be destroyed, almost as soon as accumulated, by the
incessant coast-action, as we now see on the shores of South America.
During the periods of subsidence there would probably be much
extinction of life; during the periods of elevation, there would be
much variation, but the geological record would then be least perfect.

It may be doubted whether the duration of any one great period of
subsidence over the whole or part of the archipelago, together with a
contemporaneous accumulation of sediment, would EXCEED the average
duration of the same specific forms; and these contingencies are
indispensable for the preservation of all the transitional gradations
between any two or more species. If such gradations were not fully
preserved, transitional varieties would merely appear as so many
distinct species. It is, also, probable that each great period of
subsidence would be interrupted by oscillations of level, and that
slight climatal changes would intervene during such lengthy periods;
and in these cases the inhabitants of the archipelago would have to
migrate, and no closely consecutive record of their modifications
could be preserved in any one formation.

Very many of the marine inhabitants of the archipelago now range
thousands of miles beyond its confines; and analogy leads me to
believe that it would be chiefly these far-ranging species which would
oftenest produce new varieties; and the varieties would at first
generally be local or confined to one place, but if possessed of any
decided advantage, or when further modified and improved, they would
slowly spread and supplant their parent-forms. When such varieties
returned to their ancient homes, as they would differ from their
former state, in a nearly uniform, though perhaps extremely slight
degree, they would, according to the principles followed by many
palaeontologists, be ranked as new and distinct species.

If then, there be some degree of truth in these remarks, we have no
right to expect to find in our geological formations, an infinite
number of those fine transitional forms, which on my theory assuredly
have connected all the past and present species of the same group into
one long and branching chain of life. We ought only to look for a few
links, some more closely, some more distantly related to each other;
and these links, let them be ever so close, if found in different
stages of the same formation, would, by most palaeontologists, be
ranked as distinct species. But I do not pretend that I should ever
have suspected how poor a record of the mutations of life, the best
preserved geological section presented, had not the difficulty of our
not discovering innumerable transitional links between the species
which appeared at the commencement and close of each formation,
pressed so hardly on my theory.

ON THE SUDDEN APPEARANCE OF WHOLE GROUPS OF ALLIED SPECIES.

The abrupt manner in which whole groups of species suddenly appear in
certain formations, has been urged by several palaeontologists, for
instance, by Agassiz, Pictet, and by none more forcibly than by
Professor Sedgwick, as a fatal objection to the belief in the
transmutation of species. If numerous species, belonging to the same
genera or families, have really started into life all at once, the
fact would be fatal to the theory of descent with slow modification
through natural selection. For the development of a group of forms,
all of which have descended from some one progenitor, must have been
an extremely slow process; and the progenitors must have lived long
ages before their modified descendants. But we continually over-rate
the perfection of the geological record, and falsely infer, because
certain genera or families have not been found beneath a certain
stage, that they did not exist before that stage. We continually
forget how large the world is, compared with the area over which our
geological formations have been carefully examined; we forget that
groups of species may elsewhere have long existed and have slowly
multiplied before they invaded the ancient archipelagoes of Europe and
of the United States. We do not make due allowance for the enormous
intervals of time, which have probably elapsed between our consecutive
formations,--longer perhaps in some cases than the time required for
the accumulation of each formation. These intervals will have given
time for the multiplication of species from some one or some few
parent-forms; and in the succeeding formation such species will appear
as if suddenly created.

I may here recall a remark formerly made, namely that it might require
a long succession of ages to adapt an organism to some new and
peculiar line of life, for instance to fly through the air; but that
when this had been effected, and a few species had thus acquired a
great advantage over other organisms, a comparatively short time would
be necessary to produce many divergent forms, which would be able to
spread rapidly and widely throughout the world.

I will now give a few examples to illustrate these remarks; and to
show how liable we are to error in supposing that whole groups of
species have suddenly been produced. I may recall the well-known fact
that in geological treatises, published not many years ago, the great
class of mammals was always spoken of as having abruptly come in at
the commencement of the tertiary series. And now one of the richest
known accumulations of fossil mammals belongs to the middle of the
secondary series; and one true mammal has been discovered in the new
red sandstone at nearly the commencement of this great series. Cuvier
used to urge that no monkey occurred in any tertiary stratum; but now
extinct species have been discovered in India, South America, and in
Europe even as far back as the eocene stage. The most striking case,
however, is that of the Whale family; as these animals have huge
bones, are marine, and range over the world, the fact of not a single
bone of a whale having been discovered in any secondary formation,
seemed fully to justify the belief that this great and distinct order
had been suddenly produced in the interval between the latest
secondary and earliest tertiary formation. But now we may read in the
Supplement to Lyell's 'Manual,' published in 1858, clear evidence of
the existence of whales in the upper greensand, some time before the
close of the secondary period.

I may give another instance, which from having passed under my own
eyes has much struck me. In a memoir on Fossil Sessile Cirripedes, I
have stated that, from the number of existing and extinct tertiary
species; from the extraordinary abundance of the individuals of many
species all over the world, from the Arctic regions to the equator,
inhabiting various zones of depths from the upper tidal limits to 50
fathoms; from the perfect manner in which specimens are preserved in
the oldest tertiary beds; from the ease with which even a fragment of
a valve can be recognised; from all these circumstances, I inferred
that had sessile cirripedes existed during the secondary periods, they
would certainly have been preserved and discovered; and as not one
species had been discovered in beds of this age, I concluded that this
great group had been suddenly developed at the commencement of the
tertiary series. This was a sore trouble to me, adding as I thought
one more instance of the abrupt appearance of a great group of
species. But my work had hardly been published, when a skilful
palaeontologist, M. Bosquet, sent me a drawing of a perfect specimen
of an unmistakeable sessile cirripede, which he had himself extracted
from the chalk of Belgium. And, as if to make the case as striking as
possible, this sessile cirripede was a Chthamalus, a very common,
large, and ubiquitous genus, of which not one specimen has as yet been
found even in any tertiary stratum. Hence we now positively know that
sessile cirripedes existed during the secondary period; and these
cirripedes might have been the progenitors of our many tertiary and
existing species.

The case most frequently insisted on by palaeontologists of the
apparently sudden appearance of a whole group of species, is that of
the teleostean fishes, low down in the Chalk period. This group
includes the large majority of existing species. Lately, Professor
Pictet has carried their existence one sub-stage further back; and
some palaeontologists believe that certain much older fishes, of which
the affinities are as yet imperfectly known, are really teleostean.
Assuming, however, that the whole of them did appear, as Agassiz
believes, at the commencement of the chalk formation, the fact would
certainly be highly remarkable; but I cannot see that it would be an
insuperable difficulty on my theory, unless it could likewise be shown
that the species of this group appeared suddenly and simultaneously
throughout the world at this same period. It is almost superfluous to
remark that hardly any fossil-fish are known from south of the
equator; and by running through Pictet's Palaeontology it will be seen
that very few species are known from several formations in Europe.
Some few families of fish now have a confined range; the teleostean
fish might formerly have had a similarly confined range, and after
having been largely developed in some one sea, might have spread
widely. Nor have we any right to suppose that the seas of the world
have always been so freely open from south to north as they are at
present. Even at this day, if the Malay Archipelago were converted
into land, the tropical parts of the Indian Ocean would form a large
and perfectly enclosed basin, in which any great group of marine
animals might be multiplied; and here they would remain confined,
until some of the species became adapted to a cooler climate, and were
enabled to double the southern capes of Africa or Australia, and thus
reach other and distant seas.

From these and similar considerations, but chiefly from our ignorance
of the geology of other countries beyond the confines of Europe and
the United States; and from the revolution in our palaeontological
ideas on many points, which the discoveries of even the last dozen
years have effected, it seems to me to be about as rash in us to
dogmatize on the succession of organic beings throughout the world, as
it would be for a naturalist to land for five minutes on some one
barren point in Australia, and then to discuss the number and range of
its productions.

ON THE SUDDEN APPEARANCE OF GROUPS OF ALLIED SPECIES IN THE LOWEST
KNOWN FOSSILIFEROUS STRATA.

There is another and allied difficulty, which is much graver. I allude
to the manner in which numbers of species of the same group, suddenly
appear in the lowest known fossiliferous rocks. Most of the arguments
which have convinced me that all the existing species of the same
group have descended from one progenitor, apply with nearly equal
force to the earliest known species. For instance, I cannot doubt that
all the Silurian trilobites have descended from some one crustacean,
which must have lived long before the Silurian age, and which probably
differed greatly from any known animal. Some of the most ancient
Silurian animals, as the Nautilus, Lingula, etc., do not differ much
from living species; and it cannot on my theory be supposed, that
these old species were the progenitors of all the species of the
orders to which they belong, for they do not present characters in any
degree intermediate between them. If, moreover, they had been the
progenitors of these orders, they would almost certainly have been
long ago supplanted and exterminated by their numerous and improved
descendants.

Consequently, if my theory be true, it is indisputable that before the
lowest Silurian stratum was deposited, long periods elapsed, as long
as, or probably far longer than, the whole interval from the Silurian
age to the present day; and that during these vast, yet quite unknown,
periods of time, the world swarmed with living creatures.

To the question why we do not find records of these vast primordial
periods, I can give no satisfactory answer. Several of the most
eminent geologists, with Sir R. Murchison at their head, are convinced
that we see in the organic remains of the lowest Silurian stratum the
dawn of life on this planet. Other highly competent judges, as Lyell
and the late E. Forbes, dispute this conclusion. We should not forget
that only a small portion of the world is known with accuracy. M.
Barrande has lately added another and lower stage to the Silurian
system, abounding with new and peculiar species. Traces of life have
been detected in the Longmynd beds beneath Barrande's so-called
primordial zone. The presence of phosphatic nodules and bituminous
matter in some of the lowest azoic rocks, probably indicates the
former existence of life at these periods. But the difficulty of
understanding the absence of vast piles of fossiliferous strata, which
on my theory no doubt were somewhere accumulated before the Silurian
epoch, is very great. If these most ancient beds had been wholly worn
away by denudation, or obliterated by metamorphic action, we ought to
find only small remnants of the formations next succeeding them in
age, and these ought to be very generally in a metamorphosed
condition. But the descriptions which we now possess of the Silurian
deposits over immense territories in Russia and in North America, do
not support the view, that the older a formation is, the more it has
suffered the extremity of denudation and metamorphism.

The case at present must remain inexplicable; and may be truly urged
as a valid argument against the views here entertained. To show that
it may hereafter receive some explanation, I will give the following
hypothesis. From the nature of the organic remains, which do not
appear to have inhabited profound depths, in the several formations of
Europe and of the United States; and from the amount of sediment,
miles in thickness, of which the formations are composed, we may infer
that from first to last large islands or tracts of land, whence the
sediment was derived, occurred in the neighbourhood of the existing
continents of Europe and North America. But we do not know what was
the state of things in the intervals between the successive
formations; whether Europe and the United States during these
intervals existed as dry land, or as a submarine surface near land, on
which sediment was not deposited, or again as the bed of an open and
unfathomable sea.

Looking to the existing oceans, which are thrice as extensive as the
land, we see them studded with many islands; but not one oceanic
island is as yet known to afford even a remnant of any palaeozoic or
secondary formation. Hence we may perhaps infer, that during the
palaeozoic and secondary periods, neither continents nor continental
islands existed where our oceans now extend; for had they existed
there, palaeozoic and secondary formations would in all probability
have been accumulated from sediment derived from their wear and tear;
and would have been at least partially upheaved by the oscillations of
level, which we may fairly conclude must have intervened during these
enormously long periods. If then we may infer anything from these
facts, we may infer that where our oceans now extend, oceans have
extended from the remotest period of which we have any record; and on
the other hand, that where continents now exist, large tracts of land
have existed, subjected no doubt to great oscillations of level, since
the earliest silurian period. The coloured map appended to my volume
on Coral Reefs, led me to conclude that the great oceans are still
mainly areas of subsidence, the great archipelagoes still areas of
oscillations of level, and the continents areas of elevation. But have
we any right to assume that things have thus remained from eternity?
Our continents seem to have been formed by a preponderance, during
many oscillations of level, of the force of elevation; but may not the
areas of preponderant movement have changed in the lapse of ages? At a
period immeasurably antecedent to the silurian epoch, continents may
have existed where oceans are now spread out; and clear and open
oceans may have existed where our continents now stand. Nor should we
be justified in assuming that if, for instance, the bed of the Pacific
Ocean were now converted into a continent, we should there find
formations older than the silurian strata, supposing such to have been
formerly deposited; for it might well happen that strata which had
subsided some miles nearer to the centre of the earth, and which had
been pressed on by an enormous weight of superincumbent water, might
have undergone far more metamorphic action than strata which have
always remained nearer to the surface. The immense areas in some parts
of the world, for instance in South America, of bare metamorphic
rocks, which must have been heated under great pressure, have always
seemed to me to require some special explanation; and we may perhaps
believe that we see in these large areas, the many formations long
anterior to the silurian epoch in a completely metamorphosed
condition.

The several difficulties here discussed, namely our not finding in the
successive formations infinitely numerous transitional links between
the many species which now exist or have existed; the sudden manner in
which whole groups of species appear in our European formations; the
almost entire absence, as at present known, of fossiliferous
formations beneath the Silurian strata, are all undoubtedly of the
gravest nature. We see this in the plainest manner by the fact that
all the most eminent palaeontologists, namely Cuvier, Owen, Agassiz,
Barrande, Falconer, E. Forbes, etc., and all our greatest geologists,
as Lyell, Murchison, Sedgwick, etc., have unanimously, often
vehemently, maintained the immutability of species. But I have reason
to believe that one great authority, Sir Charles Lyell, from further
reflexion entertains grave doubts on this subject. I feel how rash it
is to differ from these great authorities, to whom, with others, we
owe all our knowledge. Those who think the natural geological record
in any degree perfect, and who do not attach much weight to the facts
and arguments of other kinds given in this volume, will undoubtedly at
once reject my theory. For my part, following out Lyell's metaphor, I
look at the natural geological record, as a history of the world
imperfectly kept, and written in a changing dialect; of this history
we possess the last volume alone, relating only to two or three
countries. Of this volume, only here and there a short chapter has
been preserved; and of each page, only here and there a few lines.
Each word of the slowly-changing language, in which the history is
supposed to be written, being more or less different in the
interrupted succession of chapters, may represent the apparently
abruptly changed forms of life, entombed in our consecutive, but
widely separated formations. On this view, the difficulties above
discussed are greatly diminished, or even disappear.


CHAPTER 10. ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS.

On the slow and successive appearance of new species.
On their different rates of change.
Species once lost do not reappear.
Groups of species follow the same general rules in their appearance
and disappearance as do single species.
On Extinction.
On simultaneous changes in the forms of life throughout the world.
On the affinities of extinct species to each other and to living
species.
On the state of development of ancient forms.
On the succession of the same types within the same areas.
Summary of preceding and present chapters.

Let us now see whether the several facts and rules relating to the
geological succession of organic beings, better accord with the common
view of the immutability of species, or with that of their slow and
gradual modification, through descent and natural selection.

New species have appeared very slowly, one after another, both on the
land and in the waters. Lyell has shown that it is hardly possible to
resist the evidence on this head in the case of the several tertiary
stages; and every year tends to fill up the blanks between them, and
to make the percentage system of lost and new forms more gradual. In
some of the most recent beds, though undoubtedly of high antiquity if
measured by years, only one or two species are lost forms, and only
one or two are new forms, having here appeared for the first time,
either locally, or, as far as we know, on the face of the earth. If we
may trust the observations of Philippi in Sicily, the successive
changes in the marine inhabitants of that island have been many and
most gradual. The secondary formations are more broken; but, as Bronn
has remarked, neither the appearance nor disappearance of their many
now extinct species has been simultaneous in each separate formation.

Species of different genera and classes have not changed at the same
rate, or in the same degree. In the oldest tertiary beds a few living
shells may still be found in the midst of a multitude of extinct
forms. Falconer has given a striking instance of a similar fact, in an
existing crocodile associated with many strange and lost mammals and
reptiles in the sub-Himalayan deposits. The Silurian Lingula differs
but little from the living species of this genus; whereas most of the
other Silurian Molluscs and all the Crustaceans have changed greatly.
The productions of the land seem to change at a quicker rate than
those of the sea, of which a striking instance has lately been
observed in Switzerland. There is some reason to believe that
organisms, considered high in the scale of nature, change more quickly
than those that are low: though there are exceptions to this rule. The
amount of organic change, as Pictet has remarked, does not strictly
correspond with the succession of our geological formations; so that
between each two consecutive formations, the forms of life have seldom
changed in exactly the same degree. Yet if we compare any but the most
closely related formations, all the species will be found to have
undergone some change. When a species has once disappeared from the
face of the earth, we have reason to believe that the same identical
form never reappears. The strongest apparent exception to this latter
rule, is that of the so-called "colonies" of M. Barrande, which
intrude for a period in the midst of an older formation, and then
allow the pre-existing fauna to reappear; but Lyell's explanation,
namely, that it is a case of temporary migration from a distinct
geographical province, seems to me satisfactory.

These several facts accord well with my theory. I believe in no fixed
law of development, causing all the inhabitants of a country to change
abruptly, or simultaneously, or to an equal degree. The process of
modification must be extremely slow. The variability of each species
is quite independent of that of all others. Whether such variability
be taken advantage of by natural selection, and whether the variations
be accumulated to a greater or lesser amount, thus causing a greater
or lesser amount of modification in the varying species, depends on
many complex contingencies,--on the variability being of a beneficial
nature, on the power of intercrossing, on the rate of breeding, on the
slowly changing physical conditions of the country, and more
especially on the nature of the other inhabitants with which the
varying species comes into competition. Hence it is by no means
surprising that one species should retain the same identical form much
longer than others; or, if changing, that it should change less. We
see the same fact in geographical distribution; for instance, in the
land-shells and coleopterous insects of Madeira having come to differ
considerably from their nearest allies on the continent of Europe,
whereas the marine shells and birds have remained unaltered. We can
perhaps understand the apparently quicker rate of change in
terrestrial and in more highly organised productions compared with
marine and lower productions, by the more complex relations of the
higher beings to their organic and inorganic conditions of life, as
explained in a former chapter. When many of the inhabitants of a
country have become modified and improved, we can understand, on the
principle of competition, and on that of the many all-important
relations of organism to organism, that any form which does not become
in some degree modified and improved, will be liable to be
exterminated. Hence we can see why all the species in the same region
do at last, if we look to wide enough intervals of time, become
modified; for those which do not change will become extinct.

In members of the same class the average amount of change, during long
and equal periods of time, may, perhaps, be nearly the same; but as
the accumulation of long-enduring fossiliferous formations depends on
great masses of sediment having been deposited on areas whilst
subsiding, our formations have been almost necessarily accumulated at
wide and irregularly intermittent intervals; consequently the amount
of organic change exhibited by the fossils embedded in consecutive
formations is not equal. Each formation, on this view, does not mark a
new and complete act of creation, but only an occasional scene, taken
almost at hazard, in a slowly changing drama.

We can clearly understand why a species when once lost should never
reappear, even if the very same conditions of life, organic and
inorganic, should recur. For though the offspring of one species might
be adapted (and no doubt this has occurred in innumerable instances)
to fill the exact place of another species in the economy of nature,
and thus supplant it; yet the two forms--the old and the new--would
not be identically the same; for both would almost certainly inherit
different characters from their distinct progenitors. For instance, it
is just possible, if our fantail-pigeons were all destroyed, that
fanciers, by striving during long ages for the same object, might make
a new breed hardly distinguishable from our present fantail; but if
the parent rock-pigeon were also destroyed, and in nature we have
every reason to believe that the parent-form will generally be
supplanted and exterminated by its improved offspring, it is quite
incredible that a fantail, identical with the existing breed, could be
raised from any other species of pigeon, or even from the other
well-established races of the domestic pigeon, for the newly-formed
fantail would be almost sure to inherit from its new progenitor some
slight characteristic differences.

Groups of species, that is, genera and families, follow the same
general rules in their appearance and disappearance as do single
species, changing more or less quickly, and in a greater or lesser
degree. A group does not reappear after it has once disappeared; or
its existence, as long as it lasts, is continuous. I am aware that
there are some apparent exceptions to this rule, but the exceptions
are surprisingly few, so few, that E. Forbes, Pictet, and Woodward
(though all strongly opposed to such views as I maintain) admit its
truth; and the rule strictly accords with my theory. For as all the
species of the same group have descended from some one species, it is
clear that as long as any species of the group have appeared in the
long succession of ages, so long must its members have continuously
existed, in order to have generated either new and modified or the
same old and unmodified forms. Species of the genus Lingula, for
instance, must have continuously existed by an unbroken succession of
generations, from the lowest Silurian stratum to the present day.

We have seen in the last chapter that the species of a group sometimes
falsely appear to have come in abruptly; and I have attempted to give
an explanation of this fact, which if true would have been fatal to my
views. But such cases are certainly exceptional; the general rule
being a gradual increase in number, till the group reaches its
maximum, and then, sooner or later, it gradually decreases. If the
number of the species of a genus, or the number of the genera of a
family, be represented by a vertical line of varying thickness,
crossing the successive geological formations in which the species are
found, the line will sometimes falsely appear to begin at its lower
end, not in a sharp point, but abruptly; it then gradually thickens
upwards, sometimes keeping for a space of equal thickness, and
ultimately thins out in the upper beds, marking the decrease and final
extinction of the species. This gradual increase in number of the
species of a group is strictly conformable with my theory; as the
species of the same genus, and the genera of the same family, can
increase only slowly and progressively; for the process of
modification and the production of a number of allied forms must be
slow and gradual,--one species giving rise first to two or three
varieties, these being slowly converted into species, which in their
turn produce by equally slow steps other species, and so on, like the
branching of a great tree from a single stem, till the group becomes
large.

ON EXTINCTION.

We have as yet spoken only incidentally of the disappearance of
species and of groups of species. On the theory of natural selection
the extinction of old forms and the production of new and improved
forms are intimately connected together. The old notion of all the
inhabitants of the earth having been swept away at successive periods
by catastrophes, is very generally given up, even by those geologists,
as Elie de Beaumont, Murchison, Barrande, etc., whose general views
would naturally lead them to this conclusion. On the contrary, we have
every reason to believe, from the study of the tertiary formations,
that species and groups of species gradually disappear, one after
another, first from one spot, then from another, and finally from the
world. Both single species and whole groups of species last for very
unequal periods; some groups, as we have seen, having endured from the
earliest known dawn of life to the present day; some having
disappeared before the close of the palaeozoic period. No fixed law
seems to determine the length of time during which any single species
or any single genus endures. There is reason to believe that the
complete extinction of the species of a group is generally a slower
process than their production: if the appearance and disappearance of
a group of species be represented, as before, by a vertical line of
varying thickness, the line is found to taper more gradually at its
upper end, which marks the progress of extermination, than at its
lower end, which marks the first appearance and increase in numbers of
the species. In some cases, however, the extermination of whole groups
of beings, as of ammonites towards the close of the secondary period,
has been wonderfully sudden.

The whole subject of the extinction of species has been involved in
the most gratuitous mystery. Some authors have even supposed that as
the individual has a definite length of life, so have species a
definite duration. No one I think can have marvelled more at the
extinction of species, than I have done. When I found in La Plata the
tooth of a horse embedded with the remains of Mastodon, Megatherium,
Toxodon, and other extinct monsters, which all co-existed with still
living shells at a very late geological period, I was filled with
astonishment; for seeing that the horse, since its introduction by the
Spaniards into South America, has run wild over the whole country and
has increased in numbers at an unparalleled rate, I asked myself what
could so recently have exterminated the former horse under conditions
of life apparently so favourable. But how utterly groundless was my
astonishment! Professor Owen soon perceived that the tooth, though so
like that of the existing horse, belonged to an extinct species. Had
this horse been still living, but in some degree rare, no naturalist
would have felt the least surprise at its rarity; for rarity is the
attribute of a vast number of species of all classes, in all
countries. If we ask ourselves why this or that species is rare, we
answer that something is unfavourable in its conditions of life; but
what that something is, we can hardly ever tell. On the supposition of
the fossil horse still existing as a rare species, we might have felt
certain from the analogy of all other mammals, even of the
slow-breeding elephant, and from the history of the naturalisation of
the domestic horse in South America, that under more favourable
conditions it would in a very few years have stocked the whole
continent. But we could not have told what the unfavourable conditions
were which checked its increase, whether some one or several
contingencies, and at what period of the horse's life, and in what
degree, they severally acted. If the conditions had gone on, however
slowly, becoming less and less favourable, we assuredly should not
have perceived the fact, yet the fossil horse would certainly have
become rarer and rarer, and finally extinct;--its place being seized
on by some more successful competitor.

It is most difficult always to remember that the increase of every
living being is constantly being checked by unperceived injurious
agencies; and that these same unperceived agencies are amply
sufficient to cause rarity, and finally extinction. We see in many
cases in the more recent tertiary formations, that rarity precedes
extinction; and we know that this has been the progress of events with
those animals which have been exterminated, either locally or wholly,
through man's agency. I may repeat what I published in 1845, namely,
that to admit that species generally become rare before they become
extinct--to feel no surprise at the rarity of a species, and yet to
marvel greatly when it ceases to exist, is much the same as to admit
that sickness in the individual is the forerunner of death--to feel no
surprise at sickness, but when the sick man dies, to wonder and to
suspect that he died by some unknown deed of violence.

The theory of natural selection is grounded on the belief that each
new variety, and ultimately each new species, is produced and
maintained by having some advantage over those with which it comes
into competition; and the consequent extinction of less-favoured forms
almost inevitably follows. It is the same with our domestic
productions: when a new and slightly improved variety has been raised,
it at first supplants the less improved varieties in the same
neighbourhood; when much improved it is transported far and near, like
our short-horn cattle, and takes the place of other breeds in other
countries. Thus the appearance of new forms and the disappearance of
old forms, both natural and artificial, are bound together. In certain
flourishing groups, the number of new specific forms which have been
produced within a given time is probably greater than that of the old
forms which have been exterminated; but we know that the number of
species has not gone on indefinitely increasing, at least during the
later geological periods, so that looking to later times we may
believe that the production of new forms has caused the extinction of
about the same number of old forms.

The competition will generally be most severe, as formerly explained
and illustrated by examples, between the forms which are most like
each other in all respects. Hence the improved and modified
descendants of a species will generally cause the extermination of the
parent-species; and if many new forms have been developed from any one
species, the nearest allies of that species, i.e. the species of the
same genus, will be the most liable to extermination. Thus, as I
believe, a number of new species descended from one species, that is a
new genus, comes to supplant an old genus, belonging to the same
family. But it must often have happened that a new species belonging
to some one group will have seized on the place occupied by a species
belonging to a distinct group, and thus caused its extermination; and
if many allied forms be developed from the successful intruder, many
will have to yield their places; and it will generally be allied
forms, which will suffer from some inherited inferiority in common.
But whether it be species belonging to the same or to a distinct
class, which yield their places to other species which have been
modified and improved, a few of the sufferers may often long be
preserved, from being fitted to some peculiar line of life, or from
inhabiting some distant and isolated station, where they have escaped
severe competition. For instance, a single species of Trigonia, a
great genus of shells in the secondary formations, survives in the
Australian seas; and a few members of the great and almost extinct
group of Ganoid fishes still inhabit our fresh waters. Therefore the
utter extinction of a group is generally, as we have seen, a slower
process than its production.

With respect to the apparently sudden extermination of whole families
or orders, as of Trilobites at the close of the palaeozoic period and
of Ammonites at the close of the secondary period, we must remember
what has been already said on the probable wide intervals of time
between our consecutive formations; and in these intervals there may
have been much slow extermination. Moreover, when by sudden
immigration or by unusually rapid development, many species of a new
group have taken possession of a new area, they will have exterminated
in a correspondingly rapid manner many of the old inhabitants; and the
forms which thus yield their places will commonly be allied, for they
will partake of some inferiority in common.

Thus, as it seems to me, the manner in which single species and whole
groups of species become extinct, accords well with the theory of
natural selection. We need not marvel at extinction; if we must
marvel, let it be at our presumption in imagining for a moment that we
understand the many complex contingencies, on which the existence of
each species depends. If we forget for an instant, that each species
tends to increase inordinately, and that some check is always in
action, yet seldom perceived by us, the whole economy of nature will
be utterly obscured. Whenever we can precisely say why this species is
more abundant in individuals than that; why this species and not
another can be naturalised in a given country; then, and not till
then, we may justly feel surprise why we cannot account for the
extinction of this particular species or group of species.

ON THE FORMS OF LIFE CHANGING ALMOST SIMULTANEOUSLY THROUGHOUT THE
WORLD.

Scarcely any palaeontological discovery is more striking than the
fact, that the forms of life change almost simultaneously throughout
the world. Thus our European Chalk formation can be recognised in many
distant parts of the world, under the most different climates, where
not a fragment of the mineral chalk itself can be found; namely, in
North America, in equatorial South America, in Tierra del Fuego, at
the Cape of Good Hope, and in the peninsula of India. For at these
distant points, the organic remains in certain beds present an
unmistakeable degree of resemblance to those of the Chalk. It is not
that the same species are met with; for in some cases not one species
is identically the same, but they belong to the same families, genera,
and sections of genera, and sometimes are similarly characterised in
such trifling points as mere superficial sculpture. Moreover other
forms, which are not found in the Chalk of Europe, but which occur in
the formations either above or below, are similarly absent at these
distant points of the world. In the several successive palaeozoic
formations of Russia, Western Europe and North America, a similar
parallelism in the forms of life has been observed by several authors:
so it is, according to Lyell, with the several European and North
American tertiary deposits. Even if the few fossil species which are
common to the Old and New Worlds be kept wholly out of view, the
general parallelism in the successive forms of life, in the stages of
the widely separated palaeozoic and tertiary periods, would still be
manifest, and the several formations could be easily correlated.

These observations, however, relate to the marine inhabitants of
distant parts of the world: we have not sufficient data to judge
whether the productions of the land and of fresh water change at
distant points in the same parallel manner. We may doubt whether they
have thus changed: if the Megatherium, Mylodon, Macrauchenia, and
Toxodon had been brought to Europe from La Plata, without any
information in regard to their geological position, no one would have
suspected that they had coexisted with still living sea-shells; but as
these anomalous monsters coexisted with the Mastodon and Horse, it
might at least have been inferred that they had lived during one of
the latter tertiary stages.

When the marine forms of life are spoken of as having changed
simultaneously throughout the world, it must not be supposed that this
expression relates to the same thousandth or hundred-thousandth year,
or even that it has a very strict geological sense; for if all the
marine animals which live at the present day in Europe, and all those
that lived in Europe during the pleistocene period (an enormously
remote period as measured by years, including the whole glacial
epoch), were to be compared with those now living in South America or
in Australia, the most skilful naturalist would hardly be able to say
whether the existing or the pleistocene inhabitants of Europe
resembled most closely those of the southern hemisphere. So, again,
several highly competent observers believe that the existing
productions of the United States are more closely related to those
which lived in Europe during certain later tertiary stages, than to
those which now live here; and if this be so, it is evident that
fossiliferous beds deposited at the present day on the shores of North
America would hereafter be liable to be classed with somewhat older
European beds. Nevertheless, looking to a remotely future epoch, there
can, I think, be little doubt that all the more modern MARINE
formations, namely, the upper pliocene, the pleistocene and strictly
modern beds, of Europe, North and South America, and Australia, from
containing fossil remains in some degree allied, and from not
including those forms which are only found in the older underlying
deposits, would be correctly ranked as simultaneous in a geological
sense.

The fact of the forms of life changing simultaneously, in the above
large sense, at distant parts of the world, has greatly struck those
admirable observers, MM. de Verneuil and d'Archiac. After referring to
the parallelism of the palaeozoic forms of life in various parts of
Europe, they add, "If struck by this strange sequence, we turn our
attention to North America, and there discover a series of analogous
phenomena, it will appear certain that all these modifications of
species, their extinction, and the introduction of new ones, cannot be
owing to mere changes in marine currents or other causes more or less
local and temporary, but depend on general laws which govern the whole
animal kingdom." M. Barrande has made forcible remarks to precisely
the same effect. It is, indeed, quite futile to look to changes of
currents, climate, or other physical conditions, as the cause of these
great mutations in the forms of life throughout the world, under the
most different climates. We must, as Barrande has remarked, look to
some special law. We shall see this more clearly when we treat of the
present distribution of organic beings, and find how slight is the
relation between the physical conditions of various countries, and the
nature of their inhabitants.

This great fact of the parallel succession of the forms of life
throughout the world, is explicable on the theory of natural
selection. New species are formed by new varieties arising, which have
some advantage over older forms; and those forms, which are already
dominant, or have some advantage over the other forms in their own
country, would naturally oftenest give rise to new varieties or
incipient species; for these latter must be victorious in a still
higher degree in order to be preserved and to survive. We have
distinct evidence on this head, in the plants which are dominant, that
is, which are commonest in their own homes, and are most widely
diffused, having produced the greatest number of new varieties. It is
also natural that the dominant, varying, and far-spreading species,
which already have invaded to a certain extent the territories of
other species, should be those which would have the best chance of
spreading still further, and of giving rise in new countries to new
varieties and species. The process of diffusion may often be very
slow, being dependent on climatal and geographical changes, or on
strange accidents, but in the long run the dominant forms will
generally succeed in spreading. The diffusion would, it is probable,
be slower with the terrestrial inhabitants of distinct continents than
with the marine inhabitants of the continuous sea. We might therefore
expect to find, as we apparently do find, a less strict degree of
parallel succession in the productions of the land than of the sea.

Dominant species spreading from any region might encounter still more
dominant species, and then their triumphant course, or even their
existence, would cease. We know not at all precisely what are all the
conditions most favourable for the multiplication of new and dominant
species; but we can, I think, clearly see that a number of
individuals, from giving a better chance of the appearance of
favourable variations, and that severe competition with many already
existing forms, would be highly favourable, as would be the power of
spreading into new territories. A certain amount of isolation,
recurring at long intervals of time, would probably be also
favourable, as before explained. One quarter of the world may have
been most favourable for the production of new and dominant species on
the land, and another for those in the waters of the sea. If two great
regions had been for a long period favourably circumstanced in an
equal degree, whenever their inhabitants met, the battle would be
prolonged and severe; and some from one birthplace and some from the
other might be victorious. But in the course of time, the forms
dominant in the highest degree, wherever produced, would tend
everywhere to prevail. As they prevailed, they would cause the
extinction of other and inferior forms; and as these inferior forms
would be allied in groups by inheritance, whole groups would tend
slowly to disappear; though here and there a single member might long
be enabled to survive.

Thus, as it seems to me, the parallel, and, taken in a large sense,
simultaneous, succession of the same forms of life throughout the
world, accords well with the principle of new species having been
formed by dominant species spreading widely and varying; the new
species thus produced being themselves dominant owing to inheritance,
and to having already had some advantage over their parents or over
other species; these again spreading, varying, and producing new
species. The forms which are beaten and which yield their places to
the new and victorious forms, will generally be allied in groups, from
inheriting some inferiority in common; and therefore as new and
improved groups spread throughout the world, old groups will disappear
from the world; and the succession of forms in both ways will
everywhere tend to correspond.

There is one other remark connected with this subject worth making. I
have given my reasons for believing that all our greater fossiliferous
formations were deposited during periods of subsidence; and that blank
intervals of vast duration occurred during the periods when the bed of
the sea was either stationary or rising, and likewise when sediment
was not thrown down quickly enough to embed and preserve organic
remains. During these long and blank intervals I suppose that the
inhabitants of each region underwent a considerable amount of
modification and extinction, and that there was much migration from
other parts of the world. As we have reason to believe that large
areas are affected by the same movement, it is probable that strictly
contemporaneous formations have often been accumulated over very wide
spaces in the same quarter of the world; but we are far from having
any right to conclude that this has invariably been the case, and that
large areas have invariably been affected by the same movements. When
two formations have been deposited in two regions during nearly, but
not exactly the same period, we should find in both, from the causes
explained in the foregoing paragraphs, the same general succession in
the forms of life; but the species would not exactly correspond; for
there will have been a little more time in the one region than in the
other for modification, extinction, and immigration.

I suspect that cases of this nature have occurred in Europe. Mr.
Prestwich, in his admirable Memoirs on the eocene deposits of England
and France, is able to draw a close general parallelism between the
successive stages in the two countries; but when he compares certain
stages in England with those in France, although he finds in both a
curious accordance in the numbers of the species belonging to the same
genera, yet the species themselves differ in a manner very difficult
to account for, considering the proximity of the two areas,--unless,
indeed, it be assumed that an isthmus separated two seas inhabited by
distinct, but contemporaneous, faunas. Lyell has made similar
observations on some of the later tertiary formations. Barrande, also,
shows that there is a striking general parallelism in the successive
Silurian deposits of Bohemia and Scandinavia; nevertheless he finds a
surprising amount of difference in the species. If the several
formations in these regions have not been deposited during the same
exact periods,--a formation in one region often corresponding with a
blank interval in the other,--and if in both regions the species have
gone on slowly changing during the accumulation of the several
formations and during the long intervals of time between them; in this
case, the several formations in the two regions could be arranged in
the same order, in accordance with the general succession of the form
of life, and the order would falsely appear to be strictly parallel;
nevertheless the species would not all be the same in the apparently
corresponding stages in the two regions.
$
ON THE AFFINITIES OF EXTINCT SPECIES TO EACH OTHER, AND TO LIVING
FORMS.

Let us now look to the mutual affinities of extinct and living
species. They all fall into one grand natural system; and this fact is
at once explained on the principle of descent. The more ancient any
form is, the more, as a general rule, it differs from living forms.
But, as Buckland long ago remarked, all fossils can be classed either
in still existing groups, or between them. That the extinct forms of
life help to fill up the wide intervals between existing genera,
families, and orders, cannot be disputed. For if we confine our
attention either to the living or to the extinct alone, the series is
far less perfect than if we combine both into one general system. With
respect to the Vertebrata, whole pages could be filled with striking
illustrations from our great palaeontologist, Owen, showing how
extinct animals fall in between existing groups. Cuvier ranked the
Ruminants and Pachyderms, as the two most distinct orders of mammals;
but Owen has discovered so many fossil links, that he has had to alter
the whole classification of these two orders; and has placed certain
pachyderms in the same sub-order with ruminants: for example, he
dissolves by fine gradations the apparently wide difference between
the pig and the camel. In regard to the Invertebrata, Barrande, and a
higher authority could not be named, asserts that he is every day
taught that palaeozoic animals, though belonging to the same orders,
families, or genera with those living at the present day, were not at
this early epoch limited in such distinct groups as they now are.

Some writers have objected to any extinct species or group of species
being considered as intermediate between living species or groups. If
by this term it is meant that an extinct form is directly intermediate
in all its characters between two living forms, the objection is
probably valid. But I apprehend that in a perfectly natural
classification many fossil species would have to stand between living
species, and some extinct genera between living genera, even between
genera belonging to distinct families. The most common case,
especially with respect to very distinct groups, such as fish and
reptiles, seems to be, that supposing them to be distinguished at the
present day from each other by a dozen characters, the ancient members
of the same two groups would be distinguished by a somewhat lesser
number of characters, so that the two groups, though formerly quite
distinct, at that period made some small approach to each other.

It is a common belief that the more ancient a form is, by so much the
more it tends to connect by some of its characters groups now widely
separated from each other. This remark no doubt must be restricted to
those groups which have undergone much change in the course of
geological ages; and it would be difficult to prove the truth of the
proposition, for every now and then even a living animal, as the
Lepidosiren, is discovered having affinities directed towards very
distinct groups. Yet if we compare the older Reptiles and Batrachians,
the older Fish, the older Cephalopods, and the eocene Mammals, with
the more recent members of the same classes, we must admit that there
is some truth in the remark.

Let us see how far these several facts and inferences accord with the
theory of descent with modification. As the subject is somewhat
complex, I must request the reader to turn to the diagram in the
fourth chapter. We may suppose that the numbered letters represent
genera, and the dotted lines diverging from them the species in each
genus. The diagram is much too simple, too few genera and too few
species being given, but this is unimportant for us. The horizontal
lines may represent successive geological formations, and all the
forms beneath the uppermost line may be considered as extinct. The
three existing genera, a14, q14, p14, will form a small family; b14
and f14 a closely allied family or sub-family; and o14, e14, m14, a
third family. These three families, together with the many extinct
genera on the several lines of descent diverging from the parent-form
A, will form an order; for all will have inherited something in common
from their ancient and common progenitor. On the principle of the
continued tendency to divergence of character, which was formerly
illustrated by this diagram, the more recent any form is, the more it
will generally differ from its ancient progenitor. Hence we can
understand the rule that the most ancient fossils differ most from
existing forms. We must not, however, assume that divergence of
character is a necessary contingency; it depends solely on the
descendants from a species being thus enabled to seize on many and
different places in the economy of nature. Therefore it is quite
possible, as we have seen in the case of some Silurian forms, that a
species might go on being slightly modified in relation to its
slightly altered conditions of life, and yet retain throughout a vast
period the same general characteristics. This is represented in the
diagram by the letter F14.

All the many forms, extinct and recent, descended from A, make, as
before remarked, one order; and this order, from the continued effects
of extinction and divergence of character, has become divided into
several sub-families and families, some of which are supposed to have
perished at different periods, and some to have endured to the present
day.

By looking at the diagram we can see that if many of the extinct
forms, supposed to be embedded in the successive formations, were
discovered at several points low down in the series, the three
existing families on the uppermost line would be rendered less
distinct from each other. If, for instance, the genera a1, a5, a10,
f8, m3, m6, m9 were disinterred, these three families would be so
closely linked together that they probably would have to be united
into one great family, in nearly the same manner as has occurred with
ruminants and pachyderms. Yet he who objected to call the extinct
genera, which thus linked the living genera of three families
together, intermediate in character, would be justified, as they are
intermediate, not directly, but only by a long and circuitous course
through many widely different forms. If many extinct forms were to be
discovered above one of the middle horizontal lines or geological
formations--for instance, above Number VI.--but none from beneath this
line, then only the two families on the left hand (namely, a14, etc.,
and b14, etc.) would have to be united into one family; and the two
other families (namely, a14 to f14 now including five genera, and o14
to m14) would yet remain distinct. These two families, however, would
be less distinct from each other than they were before the discovery
of the fossils. If, for instance, we suppose the existing genera of
the two families to differ from each other by a dozen characters, in
this case the genera, at the early period marked VI., would differ by
a lesser number of characters; for at this early stage of descent they
have not diverged in character from the common progenitor of the
order, nearly so much as they subsequently diverged. Thus it comes
that ancient and extinct genera are often in some slight degree
intermediate in character between their modified descendants, or
between their collateral relations.

In nature the case will be far more complicated than is represented in
the diagram; for the groups will have been more numerous, they will
have endured for extremely unequal lengths of time, and will have been
modified in various degrees. As we possess only the last volume of the
geological record, and that in a very broken condition, we have no
right to expect, except in very rare cases, to fill up wide intervals
in the natural system, and thus unite distinct families or orders. All
that we have a right to expect, is that those groups, which have
within known geological periods undergone much modification, should in
the older formations make some slight approach to each other; so that
the older members should differ less from each other in some of their
characters than do the existing members of the same groups; and this
by the concurrent evidence of our best palaeontologists seems
frequently to be the case.

Thus, on the theory of descent with modification, the main facts with
respect to the mutual affinities of the extinct forms of life to each
other and to living forms, seem to me explained in a satisfactory
manner. And they are wholly inexplicable on any other view.

On this same theory, it is evident that the fauna of any great period
in the earth's history will be intermediate in general character
between that which preceded and that which succeeded it. Thus, the
species which lived at the sixth great stage of descent in the diagram
are the modified offspring of those which lived at the fifth stage,
and are the parents of those which became still more modified at the
seventh stage; hence they could hardly fail to be nearly intermediate
in character between the forms of life above and below. We must,
however, allow for the entire extinction of some preceding forms, and
for the coming in of quite new forms by immigration, and for a large
amount of modification, during the long and blank intervals between
the successive formations. Subject to these allowances, the fauna of
each geological period undoubtedly is intermediate in character,
between the preceding and succeeding faunas. I need give only one
instance, namely, the manner in which the fossils of the Devonian
system, when this system was first discovered, were at once recognised
by palaeontologists as intermediate in character between those of the
overlying carboniferous, and underlying Silurian system. But each
fauna is not necessarily exactly intermediate, as unequal intervals of
time have elapsed between consecutive formations.

It is no real objection to the truth of the statement, that the fauna
of each period as a whole is nearly intermediate in character between
the preceding and succeeding faunas, that certain genera offer
exceptions to the rule. For instance, mastodons and elephants, when
arranged by Dr. Falconer in two series, first according to their
mutual affinities and then according to their periods of existence, do
not accord in arrangement. The species extreme in character are not
the oldest, or the most recent; nor are those which are intermediate
in character, intermediate in age. But supposing for an instant, in
this and other such cases, that the record of the first appearance and
disappearance of the species was perfect, we have no reason to believe
that forms successively produced necessarily endure for corresponding
lengths of time: a very ancient form might occasionally last much
longer than a form elsewhere subsequently produced, especially in the
case of terrestrial productions inhabiting separated districts. To
compare small things with great: if the principal living and extinct
races of the domestic pigeon were arranged as well as they could be in
serial affinity, this arrangement would not closely accord with the
order in time of their production, and still less with the order of
their disappearance; for the parent rock-pigeon now lives; and many
varieties between the rock-pigeon and the carrier have become extinct;
and carriers which are extreme in the important character of length of
beak originated earlier than short-beaked tumblers, which are at the
opposite end of the series in this same respect.

Closely connected with the statement, that the organic remains from an
intermediate formation are in some degree intermediate in character,
is the fact, insisted on by all palaeontologists, that fossils from
two consecutive formations are far more closely related to each other,
than are the fossils from two remote formations. Pictet gives as a
well-known instance, the general resemblance of the organic remains
from the several stages of the chalk formation, though the species are
distinct in each stage. This fact alone, from its generality, seems to
have shaken Professor Pictet in his firm belief in the immutability of
species. He who is acquainted with the distribution of existing
species over the globe, will not attempt to account for the close
resemblance of the distinct species in closely consecutive formations,
by the physical conditions of the ancient areas having remained nearly
the same. Let it be remembered that the forms of life, at least those
inhabiting the sea, have changed almost simultaneously throughout the
world, and therefore under the most different climates and conditions.
Consider the prodigious vicissitudes of climate during the pleistocene
period, which includes the whole glacial period, and note how little
the specific forms of the inhabitants of the sea have been affected.

On the theory of descent, the full meaning of the fact of fossil
remains from closely consecutive formations, though ranked as distinct
species, being closely related, is obvious. As the accumulation of
each formation has often been interrupted, and as long blank intervals
have intervened between successive formations, we ought not to expect
to find, as I attempted to show in the last chapter, in any one or two
formations all the intermediate varieties between the species which
appeared at the commencement and close of these periods; but we ought
to find after intervals, very long as measured by years, but only
moderately long as measured geologically, closely allied forms, or, as
they have been called by some authors, representative species; and
these we assuredly do find. We find, in short, such evidence of the
slow and scarcely sensible mutation of specific forms, as we have a
just right to expect to find.

ON THE STATE OF DEVELOPMENT OF ANCIENT FORMS.

There has been much discussion whether recent forms are more highly
developed than ancient. I will not here enter on this subject, for
naturalists have not as yet defined to each other's satisfaction what
is meant by high and low forms. But in one particular sense the more
recent forms must, on my theory, be higher than the more ancient; for
each new species is formed by having had some advantage in the
struggle for life over other and preceding forms. If under a nearly
similar climate, the eocene inhabitants of one quarter of the world
were put into competition with the existing inhabitants of the same or
some other quarter, the eocene fauna or flora would certainly be
beaten and exterminated; as would a secondary fauna by an eocene, and
a palaeozoic fauna by a secondary fauna. I do not doubt that this
process of improvement has affected in a marked and sensible manner
the organisation of the more recent and victorious forms of life, in
comparison with the ancient and beaten forms; but I can see no way of
testing this sort of progress. Crustaceans, for instance, not the
highest in their own class, may have beaten the highest molluscs. From
the extraordinary manner in which European productions have recently
spread over New Zealand, and have seized on places which must have
been previously occupied, we may believe, if all the animals and
plants of Great Britain were set free in New Zealand, that in the
course of time a multitude of British forms would become thoroughly
naturalized there, and would exterminate many of the natives. On the
other hand, from what we see now occurring in New Zealand, and from
hardly a single inhabitant of the southern hemisphere having become
wild in any part of Europe, we may doubt, if all the productions of
New Zealand were set free in Great Britain, whether any considerable
number would be enabled to seize on places now occupied by our native
plants and animals. Under this point of view, the productions of Great
Britain may be said to be higher than those of New Zealand. Yet the
most skilful naturalist from an examination of the species of the two
countries could not have foreseen this result.

Agassiz insists that ancient animals resemble to a certain extent the
embryos of recent animals of the same classes; or that the geological
succession of extinct forms is in some degree parallel to the
embryological development of recent forms. I must follow Pictet and
Huxley in thinking that the truth of this doctrine is very far from
proved. Yet I fully expect to see it hereafter confirmed, at least in
regard to subordinate groups, which have branched off from each other
within comparatively recent times. For this doctrine of Agassiz
accords well with the theory of natural selection. In a future chapter
I shall attempt to show that the adult differs from its embryo, owing
to variations supervening at a not early age, and being inherited at a
corresponding age. This process, whilst it leaves the embryo almost
unaltered, continually adds, in the course of successive generations,
more and more difference to the adult.

Thus the embryo comes to be left as a sort of picture, preserved by
nature, of the ancient and less modified condition of each animal.
This view may be true, and yet it may never be capable of full proof.
Seeing, for instance, that the oldest known mammals, reptiles, and
fish strictly belong to their own proper classes, though some of these
old forms are in a slight degree less distinct from each other than
are the typical members of the same groups at the present day, it
would be vain to look for animals having the common embryological
character of the Vertebrata, until beds far beneath the lowest
Silurian strata are discovered--a discovery of which the chance is
very small.

ON THE SUCCESSION OF THE SAME TYPES WITHIN THE SAME AREAS, DURING THE
LATER TERTIARY PERIODS.

Mr. Clift many years ago showed that the fossil mammals from the
Australian caves were closely allied to the living marsupials of that
continent. In South America, a similar relationship is manifest, even
to an uneducated eye, in the gigantic pieces of armour like those of
the armadillo, found in several parts of La Plata; and Professor Owen
has shown in the most striking manner that most of the fossil mammals,
buried there in such numbers, are related to South American types.
This relationship is even more clearly seen in the wonderful
collection of fossil bones made by MM. Lund and Clausen in the caves
of Brazil. I was so much impressed with these facts that I strongly
insisted, in 1839 and 1845, on this "law of the succession of
types,"--on "this wonderful relationship in the same continent between
the dead and the living." Professor Owen has subsequently extended the
same generalisation to the mammals of the Old World. We see the same
law in this author's restorations of the extinct and gigantic birds of
New Zealand. We see it also in the birds of the caves of Brazil. Mr.
Woodward has shown that the same law holds good with sea-shells, but
from the wide distribution of most genera of molluscs, it is not well
displayed by them. Other cases could be added, as the relation between
the extinct and living land-shells of Madeira; and between the extinct
and living brackish-water shells of the Aralo-Caspian Sea.

Now what does this remarkable law of the succession of the same types
within the same areas mean? He would be a bold man, who after
comparing the present climate of Australia and of parts of South
America under the same latitude, would attempt to account, on the one
hand, by dissimilar physical conditions for the dissimilarity of the
inhabitants of these two continents, and, on the other hand, by
similarity of conditions, for the uniformity of the same types in each
during the later tertiary periods. Nor can it be pretended that it is
an immutable law that marsupials should have been chiefly or solely
produced in Australia; or that Edentata and other American types
should have been solely produced in South America. For we know that
Europe in ancient times was peopled by numerous marsupials; and I have
shown in the publications above alluded to, that in America the law of
distribution of terrestrial mammals was formerly different from what
it now is. North America formerly partook strongly of the present
character of the southern half of the continent; and the southern half
was formerly more closely allied, than it is at present, to the
northern half. In a similar manner we know from Falconer and Cautley's
discoveries, that northern India was formerly more closely related in
its mammals to Africa than it is at the present time. Analogous facts
could be given in relation to the distribution of marine animals.

On the theory of descent with modification, the great law of the long
enduring, but not immutable, succession of the same types within the
same areas, is at once explained; for the inhabitants of each quarter
of the world will obviously tend to leave in that quarter, during the
next succeeding period of time, closely allied though in some degree
modified descendants. If the inhabitants of one continent formerly
differed greatly from those of another continent, so will their
modified descendants still differ in nearly the same manner and
degree. But after very long intervals of time and after great
geographical changes, permitting much inter-migration, the feebler
will yield to the more dominant forms, and there will be nothing
immutable in the laws of past and present distribution.

It may be asked in ridicule, whether I suppose that the megatherium
and other allied huge monsters have left behind them in South America
the sloth, armadillo, and anteater, as their degenerate descendants.
This cannot for an instant be admitted. These huge animals have become
wholly extinct, and have left no progeny. But in the caves of Brazil,
there are many extinct species which are closely allied in size and in
other characters to the species still living in South America; and
some of these fossils may be the actual progenitors of living species.
It must not be forgotten that, on my theory, all the species of the
same genus have descended from some one species; so that if six
genera, each having eight species, be found in one geological
formation, and in the next succeeding formation there be six other
allied or representative genera with the same number of species, then
we may conclude that only one species of each of the six older genera
has left modified descendants, constituting the six new genera. The
other seven species of the old genera have all died out and have left
no progeny. Or, which would probably be a far commoner case, two or
three species of two or three alone of the six older genera will have
been the parents of the six new genera; the other old species and the
other whole genera having become utterly extinct. In failing orders,
with the genera and species decreasing in numbers, as apparently is
the case of the Edentata of South America, still fewer genera and
species will have left modified blood-descendants.

SUMMARY OF THE PRECEDING AND PRESENT CHAPTERS.

I have attempted to show that the geological record is extremely
imperfect; that only a small portion of the globe has been
geologically explored with care; that only certain classes of organic
beings have been largely preserved in a fossil state; that the number
both of specimens and of species, preserved in our museums, is
absolutely as nothing compared with the incalculable number of
generations which must have passed away even during a single
formation; that, owing to subsidence being necessary for the
accumulation of fossiliferous deposits thick enough to resist future
degradation, enormous intervals of time have elapsed between the
successive formations; that there has probably been more extinction
during the periods of subsidence, and more variation during the
periods of elevation, and during the latter the record will have been
least perfectly kept; that each single formation has not been
continuously deposited; that the duration of each formation is,
perhaps, short compared with the average duration of specific forms;
that migration has played an important part in the first appearance of
new forms in any one area and formation; that widely ranging species
are those which have varied most, and have oftenest given rise to new
species; and that varieties have at first often been local. All these
causes taken conjointly, must have tended to make the geological
record extremely imperfect, and will to a large extent explain why we
do not find interminable varieties, connecting together all the
extinct and existing forms of life by the finest graduated steps.

He who rejects these views on the nature of the geological record,
will rightly reject my whole theory. For he may ask in vain where are
the numberless transitional links which must formerly have connected
the closely allied or representative species, found in the several
stages of the same great formation. He may disbelieve in the enormous
intervals of time which have elapsed between our consecutive
formations; he may overlook how important a part migration must have
played, when the formations of any one great region alone, as that of
Europe, are considered; he may urge the apparent, but often falsely
apparent, sudden coming in of whole groups of species. He may ask
where are the remains of those infinitely numerous organisms which
must have existed long before the first bed of the Silurian system was
deposited: I can answer this latter question only hypothetically, by
saying that as far as we can see, where our oceans now extend they
have for an enormous period extended, and where our oscillating
continents now stand they have stood ever since the Silurian epoch;
but that long before that period, the world may have presented a
wholly different aspect; and that the older continents, formed of
formations older than any known to us, may now all be in a
metamorphosed condition, or may lie buried under the ocean.

Passing from these difficulties, all the other great leading facts in
palaeontology seem to me simply to follow on the theory of descent
with modification through natural selection. We can thus understand
how it is that new species come in slowly and successively; how
species of different classes do not necessarily change together, or at
the same rate, or in the same degree; yet in the long run that all
undergo modification to some extent. The extinction of old forms is
the almost inevitable consequence of the production of new forms. We
can understand why when a species has once disappeared it never
reappears. Groups of species increase in numbers slowly, and endure
for unequal periods of time; for the process of modification is
necessarily slow, and depends on many complex contingencies. The
dominant species of the larger dominant groups tend to leave many
modified descendants, and thus new sub-groups and groups are formed.
As these are formed, the species of the less vigorous groups, from
their inferiority inherited from a common progenitor, tend to become
extinct together, and to leave no modified offspring on the face of
the earth. But the utter extinction of a whole group of species may
often be a very slow process, from the survival of a few descendants,
lingering in protected and isolated situations. When a group has once
wholly disappeared, it does not reappear; for the link of generation
has been broken.

We can understand how the spreading of the dominant forms of life,
which are those that oftenest vary, will in the long run tend to
people the world with allied, but modified, descendants; and these
will generally succeed in taking the places of those groups of species
which are their inferiors in the struggle for existence. Hence, after
long intervals of time, the productions of the world will appear to
have changed simultaneously.

We can understand how it is that all the forms of life, ancient and
recent, make together one grand system; for all are connected by
generation. We can understand, from the continued tendency to
divergence of character, why the more ancient a form is, the more it
generally differs from those now living. Why ancient and extinct forms
often tend to fill up gaps between existing forms, sometimes blending
two groups previously classed as distinct into one; but more commonly
only bringing them a little closer together. The more ancient a form
is, the more often, apparently, it displays characters in some degree
intermediate between groups now distinct; for the more ancient a form
is, the more nearly it will be related to, and consequently resemble,
the common progenitor of groups, since become widely divergent.
Extinct forms are seldom directly intermediate between existing forms;
but are intermediate only by a long and circuitous course through many
extinct and very different forms. We can clearly see why the organic
remains of closely consecutive formations are more closely allied to
each other, than are those of remote formations; for the forms are
more closely linked together by generation: we can clearly see why the
remains of an intermediate formation are intermediate in character.

The inhabitants of each successive period in the world's history have
beaten their predecessors in the race for life, and are, in so far,
higher in the scale of nature; and this may account for that vague yet
ill-defined sentiment, felt by many palaeontologists, that
organisation on the whole has progressed. If it should hereafter be
proved that ancient animals resemble to a certain extent the embryos
of more recent animals of the same class, the fact will be
intelligible. The succession of the same types of structure within the
same areas during the later geological periods ceases to be
mysterious, and is simply explained by inheritance.

If then the geological record be as imperfect as I believe it to be,
and it may at least be asserted that the record cannot be proved to be
much more perfect, the main objections to the theory of natural
selection are greatly diminished or disappear. On the other hand, all
the chief laws of palaeontology plainly proclaim, as it seems to me,
that species have been produced by ordinary generation: old forms
having been supplanted by new and improved forms of life, produced by
the laws of variation still acting round us, and preserved by Natural
Selection.


CHAPTER 11. GEOGRAPHICAL DISTRIBUTION.

Present distribution cannot be accounted for by differences in
physical conditions.
Importance of barriers.
Affinity of the productions of the same continent.
Centres of creation.
Means of dispersal, by changes of climate and of the level of the
land, and by occasional means.
Dispersal during the Glacial period co-extensive with the world.

In considering the distribution of organic beings over the face of the
globe, the first great fact which strikes us is, that neither the
similarity nor the dissimilarity of the inhabitants of various regions
can be accounted for by their climatal and other physical conditions.
Of late, almost every author who has studied the subject has come to
this conclusion. The case of America alone would almost suffice to
prove its truth: for if we exclude the northern parts where the
circumpolar land is almost continuous, all authors agree that one of
the most fundamental divisions in geographical distribution is that
between the New and Old Worlds; yet if we travel over the vast
American continent, from the central parts of the United States to its
extreme southern point, we meet with the most diversified conditions;
the most humid districts, arid deserts, lofty mountains, grassy
plains, forests, marshes, lakes, and great rivers, under almost every
temperature. There is hardly a climate or condition in the Old World
which cannot be paralleled in the New--at least as closely as the same
species generally require; for it is a most rare case to find a group
of organisms confined to any small spot, having conditions peculiar in
only a slight degree; for instance, small areas in the Old World could
be pointed out hotter than any in the New World, yet these are not
inhabited by a peculiar fauna or flora. Notwithstanding this
parallelism in the conditions of the Old and New Worlds, how widely
different are their living productions!

In the southern hemisphere, if we compare large tracts of land in
Australia, South Africa, and western South America, between latitudes
25 deg and 35 deg, we shall find parts extremely similar in all their
conditions, yet it would not be possible to point out three faunas and
floras more utterly dissimilar. Or again we may compare the
productions of South America south of lat. 35 deg with those north of
25 deg, which consequently inhabit a considerably different climate,
and they will be found incomparably more closely related to each
other, than they are to the productions of Australia or Africa under
nearly the same climate. Analogous facts could be given with respect
to the inhabitants of the sea.

A second great fact which strikes us in our general review is, that
barriers of any kind, or obstacles to free migration, are related in a
close and important manner to the differences between the productions
of various regions. We see this in the great difference of nearly all
the terrestrial productions of the New and Old Worlds, excepting in
the northern parts, where the land almost joins, and where, under a
slightly different climate, there might have been free migration for
the northern temperate forms, as there now is for the strictly arctic
productions. We see the same fact in the great difference between the
inhabitants of Australia, Africa, and South America under the same
latitude: for these countries are almost as much isolated from each
other as is possible. On each continent, also, we see the same fact;
for on the opposite sides of lofty and continuous mountain-ranges, and
of great deserts, and sometimes even of large rivers, we find
different productions; though as mountain chains, deserts, etc., are
not as impassable, or likely to have endured so long as the oceans
separating continents, the differences are very inferior in degree to
those characteristic of distinct continents.

Turning to the sea, we find the same law. No two marine faunas are
more distinct, with hardly a fish, shell, or crab in common, than
those of the eastern and western shores of South and Central America;
yet these great faunas are separated only by the narrow, but
impassable, isthmus of Panama. Westward of the shores of America, a
wide space of open ocean extends, with not an island as a
halting-place for emigrants; here we have a barrier of another kind,
and as soon as this is passed we meet in the eastern islands of the
Pacific, with another and totally distinct fauna. So that here three
marine faunas range far northward and southward, in parallel lines not
far from each other, under corresponding climates; but from being
separated from each other by impassable barriers, either of land or
open sea, they are wholly distinct. On the other hand, proceeding
still further westward from the eastern islands of the tropical parts
of the Pacific, we encounter no impassable barriers, and we have
innumerable islands as halting-places, until after travelling over a
hemisphere we come to the shores of Africa; and over this vast space
we meet with no well-defined and distinct marine faunas. Although
hardly one shell, crab or fish is common to the above-named three
approximate faunas of Eastern and Western America and the eastern
Pacific islands, yet many fish range from the Pacific into the Indian
Ocean, and many shells are common to the eastern islands of the
Pacific and the eastern shores of Africa, on almost exactly opposite
meridians of longitude.

A third great fact, partly included in the foregoing statements, is
the affinity of the productions of the same continent or sea, though
the species themselves are distinct at different points and stations.
It is a law of the widest generality, and every continent offers
innumerable instances. Nevertheless the naturalist in travelling, for
instance, from north to south never fails to be struck by the manner
in which successive groups of beings, specifically distinct, yet
clearly related, replace each other. He hears from closely allied, yet
distinct kinds of birds, notes nearly similar, and sees their nests
similarly constructed, but not quite alike, with eggs coloured in
nearly the same manner. The plains near the Straits of Magellan are
inhabited by one species of Rhea (American ostrich), and northward the
plains of La Plata by another species of the same genus; and not by a
true ostrich or emeu, like those found in Africa and Australia under
the same latitude. On these same plains of La Plata, we see the agouti
and bizcacha, animals having nearly the same habits as our hares and
rabbits and belonging to the same order of Rodents, but they plainly
display an American type of structure. We ascend the lofty peaks of
the Cordillera and we find an alpine species of bizcacha; we look to
the waters, and we do not find the beaver or musk-rat, but the coypu
and capybara, rodents of the American type. Innumerable other
instances could be given. If we look to the islands off the American
shore, however much they may differ in geological structure, the
inhabitants, though they may be all peculiar species, are essentially
American. We may look back to past ages, as shown in the last chapter,
and we find American types then prevalent on the American continent
and in the American seas. We see in these facts some deep organic
bond, prevailing throughout space and time, over the same areas of
land and water, and independent of their physical conditions. The
naturalist must feel little curiosity, who is not led to inquire what
this bond is.

This bond, on my theory, is simply inheritance, that cause which
alone, as far as we positively know, produces organisms quite like,
or, as we see in the case of varieties nearly like each other. The
dissimilarity of the inhabitants of different regions may be
attributed to modification through natural selection, and in a quite
subordinate degree to the direct influence of different physical
conditions. The degree of dissimilarity will depend on the migration
of the more dominant forms of life from one region into another having
been effected with more or less ease, at periods more or less
remote;--on the nature and number of the former immigrants;--and on
their action and reaction, in their mutual struggles for life;--the
relation of organism to organism being, as I have already often
remarked, the most important of all relations. Thus the high
importance of barriers comes into play by checking migration; as does
time for the slow process of modification through natural selection.
Widely-ranging species, abounding in individuals, which have already
triumphed over many competitors in their own widely-extended homes
will have the best chance of seizing on new places, when they spread
into new countries. In their new homes they will be exposed to new
conditions, and will frequently undergo further modification and
improvement; and thus they will become still further victorious, and
will produce groups of modified descendants. On this principle of
inheritance with modification, we can understand how it is that
sections of genera, whole genera, and even families are confined to
the same areas, as is so commonly and notoriously the case.

I believe, as was remarked in the last chapter, in no law of necessary
development. As the variability of each species is an independent
property, and will be taken advantage of by natural selection, only so
far as it profits the individual in its complex struggle for life, so
the degree of modification in different species will be no uniform
quantity. If, for instance, a number of species, which stand in direct
competition with each other, migrate in a body into a new and
afterwards isolated country, they will be little liable to
modification; for neither migration nor isolation in themselves can do
anything. These principles come into play only by bringing organisms
into new relations with each other, and in a lesser degree with the
surrounding physical conditions. As we have seen in the last chapter
that some forms have retained nearly the same character from an
enormously remote geological period, so certain species have migrated
over vast spaces, and have not become greatly modified.

On these views, it is obvious, that the several species of the same
genus, though inhabiting the most distant quarters of the world, must
originally have proceeded from the same source, as they have descended
from the same progenitor. In the case of those species, which have
undergone during whole geological periods but little modification,
there is not much difficulty in believing that they may have migrated
from the same region; for during the vast geographical and climatal
changes which will have supervened since ancient times, almost any
amount of migration is possible. But in many other cases, in which we
have reason to believe that the species of a genus have been produced
within comparatively recent times, there is great difficulty on this
head. It is also obvious that the individuals of the same species,
though now inhabiting distant and isolated regions, must have
proceeded from one spot, where their parents were first produced: for,
as explained in the last chapter, it is incredible that individuals
identically the same should ever have been produced through natural
selection from parents specifically distinct.

We are thus brought to the question which has been largely discussed
by naturalists, namely, whether species have been created at one or
more points of the earth's surface. Undoubtedly there are very many
cases of extreme difficulty, in understanding how the same species
could possibly have migrated from some one point to the several
distant and isolated points, where now found. Nevertheless the
simplicity of the view that each species was first produced within a
single region captivates the mind. He who rejects it, rejects the vera
causa of ordinary generation with subsequent migration, and calls in
the agency of a miracle. It is universally admitted, that in most
cases the area inhabited by a species is continuous; and when a plant
or animal inhabits two points so distant from each other, or with an
interval of such a nature, that the space could not be easily passed
over by migration, the fact is given as something remarkable and
exceptional. The capacity of migrating across the sea is more
distinctly limited in terrestrial mammals, than perhaps in any other
organic beings; and, accordingly, we find no inexplicable cases of the
same mammal inhabiting distant points of the world. No geologist will
feel any difficulty in such cases as Great Britain having been
formerly united to Europe, and consequently possessing the same
quadrupeds. But if the same species can be produced at two separate
points, why do we not find a single mammal common to Europe and
Australia or South America? The conditions of life are nearly the
same, so that a multitude of European animals and plants have become
naturalised in America and Australia; and some of the aboriginal
plants are identically the same at these distant points of the
northern and southern hemispheres? The answer, as I believe, is, that
mammals have not been able to migrate, whereas some plants, from their
varied means of dispersal, have migrated across the vast and broken
interspace. The great and striking influence which barriers of every
kind have had on distribution, is intelligible only on the view that
the great majority of species have been produced on one side alone,
and have not been able to migrate to the other side. Some few
families, many sub-families, very many genera, and a still greater
number of sections of genera are confined to a single region; and it
has been observed by several naturalists, that the most natural
genera, or those genera in which the species are most closely related
to each other, are generally local, or confined to one area. What a
strange anomaly it would be, if, when coming one step lower in the
series, to the individuals of the same species, a directly opposite
rule prevailed; and species were not local, but had been produced in
two or more distinct areas!

Hence it seems to me, as it has to many other naturalists, that the
view of each species having been produced in one area alone, and
having subsequently migrated from that area as far as its powers of
migration and subsistence under past and present conditions permitted,
is the most probable. Undoubtedly many cases occur, in which we cannot
explain how the same species could have passed from one point to the
other. But the geographical and climatal changes, which have certainly
occurred within recent geological times, must have interrupted or
rendered discontinuous the formerly continuous range of many species.
So that we are reduced to consider whether the exceptions to
continuity of range are so numerous and of so grave a nature, that we
ought to give up the belief, rendered probable by general
considerations, that each species has been produced within one area,
and has migrated thence as far as it could. It would be hopelessly
tedious to discuss all the exceptional cases of the same species, now
living at distant and separated points; nor do I for a moment pretend
that any explanation could be offered of many such cases. But after
some preliminary remarks, I will discuss a few of the most striking
classes of facts; namely, the existence of the same species on the
summits of distant mountain-ranges, and at distant points in the
arctic and antarctic regions; and secondly (in the following chapter),
the wide distribution of freshwater productions; and thirdly, the
occurrence of the same terrestrial species on islands and on the
mainland, though separated by hundreds of miles of open sea. If the
existence of the same species at distant and isolated points of the
earth's surface, can in many instances be explained on the view of
each species having migrated from a single birthplace; then,
considering our ignorance with respect to former climatal and
geographical changes and various occasional means of transport, the
belief that this has been the universal law, seems to me incomparably
the safest.

In discussing this subject, we shall be enabled at the same time to
consider a point equally important for us, namely, whether the several
distinct species of a genus, which on my theory have all descended
from a common progenitor, can have migrated (undergoing modification
during some part of their migration) from the area inhabited by their
progenitor. If it can be shown to be almost invariably the case, that
a region, of which most of its inhabitants are closely related to, or
belong to the same genera with the species of a second region, has
probably received at some former period immigrants from this other
region, my theory will be strengthened; for we can clearly understand,
on the principle of modification, why the inhabitants of a region
should be related to those of another region, whence it has been
stocked. A volcanic island, for instance, upheaved and formed at the
distance of a few hundreds of miles from a continent, would probably
receive from it in the course of time a few colonists, and their
descendants, though modified, would still be plainly related by
inheritance to the inhabitants of the continent. Cases of this nature
are common, and are, as we shall hereafter more fully see,
inexplicable on the theory of independent creation. This view of the
relation of species in one region to those in another, does not differ
much (by substituting the word variety for species) from that lately
advanced in an ingenious paper by Mr. Wallace, in which he concludes,
that "every species has come into existence coincident both in space
and time with a pre-existing closely allied species." And I now know
from correspondence, that this coincidence he attributes to generation
with modification.

The previous remarks on "single and multiple centres of creation" do
not directly bear on another allied question,--namely whether all the
individuals of the same species have descended from a single pair, or
single hermaphrodite, or whether, as some authors suppose, from many
individuals simultaneously created. With those organic beings which
never intercross (if such exist), the species, on my theory, must have
descended from a succession of improved varieties, which will never
have blended with other individuals or varieties, but will have
supplanted each other; so that, at each successive stage of
modification and improvement, all the individuals of each variety will
have descended from a single parent. But in the majority of cases,
namely, with all organisms which habitually unite for each birth, or
which often intercross, I believe that during the slow process of
modification the individuals of the species will have been kept nearly
uniform by intercrossing; so that many individuals will have gone on
simultaneously changing, and the whole amount of modification will not
have been due, at each stage, to descent from a single parent. To
illustrate what I mean: our English racehorses differ slightly from
the horses of every other breed; but they do not owe their difference
and superiority to descent from any single pair, but to continued care
in selecting and training many individuals during many generations.

Before discussing the three classes of facts, which I have selected as
presenting the greatest amount of difficulty on the theory of "single
centres of creation," I must say a few words on the means of
dispersal.

MEANS OF DISPERSAL.

Sir C. Lyell and other authors have ably treated this subject. I can
give here only the briefest abstract of the more important facts.
Change of climate must have had a powerful influence on migration: a
region when its climate was different may have been a high road for
migration, but now be impassable; I shall, however, presently have to
discuss this branch of the subject in some detail. Changes of level in
the land must also have been highly influential: a narrow isthmus now
separates two marine faunas; submerge it, or let it formerly have been
submerged, and the two faunas will now blend or may formerly have
blended: where the sea now extends, land may at a former period have
connected islands or possibly even continents together, and thus have
allowed terrestrial productions to pass from one to the other. No
geologist will dispute that great mutations of level have occurred
within the period of existing organisms. Edward Forbes insisted that
all the islands in the Atlantic must recently have been connected with
Europe or Africa, and Europe likewise with America. Other authors have
thus hypothetically bridged over every ocean, and have united almost
every island to some mainland. If indeed the arguments used by Forbes
are to be trusted, it must be admitted that scarcely a single island
exists which has not recently been united to some continent. This view
cuts the Gordian knot of the dispersal of the same species to the most
distant points, and removes many a difficulty: but to the best of my
judgment we are not authorized in admitting such enormous geographical
changes within the period of existing species. It seems to me that we
have abundant evidence of great oscillations of level in our
continents; but not of such vast changes in their position and
extension, as to have united them within the recent period to each
other and to the several intervening oceanic islands. I freely admit
the former existence of many islands, now buried beneath the sea,
which may have served as halting places for plants and for many
animals during their migration. In the coral-producing oceans such
sunken islands are now marked, as I believe, by rings of coral or
atolls standing over them. Whenever it is fully admitted, as I believe
it will some day be, that each species has proceeded from a single
birthplace, and when in the course of time we know something definite
about the means of distribution, we shall be enabled to speculate with
security on the former extension of the land. But I do not believe
that it will ever be proved that within the recent period continents
which are now quite separate, have been continuously, or almost
continuously, united with each other, and with the many existing
oceanic islands. Several facts in distribution,--such as the great
difference in the marine faunas on the opposite sides of almost every
continent,--the close relation of the tertiary inhabitants of several
lands and even seas to their present inhabitants,--a certain degree of
relation (as we shall hereafter see) between the distribution of
mammals and the depth of the sea,--these and other such facts seem to
me opposed to the admission of such prodigious geographical
revolutions within the recent period, as are necessitated on the view
advanced by Forbes and admitted by his many followers. The nature and
relative proportions of the inhabitants of oceanic islands likewise
seem to me opposed to the belief of their former continuity with
continents. Nor does their almost universally volcanic composition
favour the admission that they are the wrecks of sunken
continents;--if they had originally existed as mountain-ranges on the
land, some at least of the islands would have been formed, like other
mountain-summits, of granite, metamorphic schists, old fossiliferous
or other such rocks, instead of consisting of mere piles of volcanic
matter.

I must now say a few words on what are called accidental means, but
which more properly might be called occasional means of distribution.
I shall here confine myself to plants. In botanical works, this or
that plant is stated to be ill adapted for wide dissemination; but for
transport across the sea, the greater or less facilities may be said
to be almost wholly unknown. Until I tried, with Mr. Berkeley's aid, a
few experiments, it was not even known how far seeds could resist the
injurious action of sea-water. To my surprise I found that out of 87
kinds, 64 germinated after an immersion of 28 days, and a few survived
an immersion of 137 days. For convenience sake I chiefly tried small
seeds, without the capsule or fruit; and as all of these sank in a few
days, they could not be floated across wide spaces of the sea, whether
or not they were injured by the salt-water. Afterwards I tried some
larger fruits, capsules, etc., and some of these floated for a long
time. It is well known what a difference there is in the buoyancy of
green and seasoned timber; and it occurred to me that floods might
wash down plants or branches, and that these might be dried on the
banks, and then by a fresh rise in the stream be washed into the sea.
Hence I was led to dry stems and branches of 94 plants with ripe
fruit, and to place them on sea water. The majority sank quickly, but
some which whilst green floated for a very short time, when dried
floated much longer; for instance, ripe hazel-nuts sank immediately,
but when dried, they floated for 90 days and afterwards when planted
they germinated; an asparagus plant with ripe berries floated for 23
days, when dried it floated for 85 days, and the seeds afterwards
germinated: the ripe seeds of Helosciadium sank in two days, when
dried they floated for above 90 days, and afterwards germinated.
Altogether out of the 94 dried plants, 18 floated for above 28 days,
and some of the 18 floated for a very much longer period. So that as
64/87 seeds germinated after an immersion of 28 days; and as 18/94
plants with ripe fruit (but not all the same species as in the
foregoing experiment) floated, after being dried, for above 28 days,
as far as we may infer anything from these scanty facts, we may
conclude that the seeds of 14/100 plants of any country might be
floated by sea-currents during 28 days, and would retain their power
of germination. In Johnston's Physical Atlas, the average rate of the
several Atlantic currents is 33 miles per diem (some currents running
at the rate of 60 miles per diem); on this average, the seeds of
14/100 plants belonging to one country might be floated across 924
miles of sea to another country; and when stranded, if blown to a
favourable spot by an inland gale, they would germinate.

Subsequently to my experiments, M. Martens tried similar ones, but in
a much better manner, for he placed the seeds in a box in the actual
sea, so that they were alternately wet and exposed to the air like
really floating plants. He tried 98 seeds, mostly different from mine;
but he chose many large fruits and likewise seeds from plants which
live near the sea; and this would have favoured the average length of
their flotation and of their resistance to the injurious action of the
salt-water. On the other hand he did not previously dry the plants or
branches with the fruit; and this, as we have seen, would have caused
some of them to have floated much longer. The result was that 18/98 of
his seeds floated for 42 days, and were then capable of germination.
But I do not doubt that plants exposed to the waves would float for a
less time than those protected from violent movement as in our
experiments. Therefore it would perhaps be safer to assume that the
seeds of about 10/100 plants of a flora, after having been dried,
could be floated across a space of sea 900 miles in width, and would
then germinate. The fact of the larger fruits often floating longer
than the small, is interesting; as plants with large seeds or fruit
could hardly be transported by any other means; and Alph. de Candolle
has shown that such plants generally have restricted ranges.

But seeds may be occasionally transported in another manner. Drift
timber is thrown up on most islands, even on those in the midst of the
widest oceans; and the natives of the coral-islands in the Pacific,
procure stones for their tools, solely from the roots of drifted
trees, these stones being a valuable royal tax. I find on examination,
that when irregularly shaped stones are embedded in the roots of
trees, small parcels of earth are very frequently enclosed in their
interstices and behind them,--so perfectly that not a particle could
be washed away in the longest transport: out of one small portion of
earth thus COMPLETELY enclosed by wood in an oak about 50 years old,
three dicotyledonous plants germinated: I am certain of the accuracy
of this observation. Again, I can show that the carcasses of birds,
when floating on the sea, sometimes escape being immediately devoured;
and seeds of many kinds in the crops of floating birds long retain
their vitality: peas and vetches, for instance, are killed by even a
few days' immersion in sea-water; but some taken out of the crop of a
pigeon, which had floated on artificial salt-water for 30 days, to my
surprise nearly all germinated.

Living birds can hardly fail to be highly effective agents in the
transportation of seeds. I could give many facts showing how
frequently birds of many kinds are blown by gales to vast distances
across the ocean. We may I think safely assume that under such
circumstances their rate of flight would often be 35 miles an hour;
and some authors have given a far higher estimate. I have never seen
an instance of nutritious seeds passing through the intestines of a
bird; but hard seeds of fruit will pass uninjured through even the
digestive organs of a turkey. In the course of two months, I picked up
in my garden 12 kinds of seeds, out of the excrement of small birds,
and these seemed perfect, and some of them, which I tried, germinated.
But the following fact is more important: the crops of birds do not
secrete gastric juice, and do not in the least injure, as I know by
trial, the germination of seeds; now after a bird has found and
devoured a large supply of food, it is positively asserted that all
the grains do not pass into the gizzard for 12 or even 18 hours. A
bird in this interval might easily be blown to the distance of 500
miles, and hawks are known to look out for tired birds, and the
contents of their torn crops might thus readily get scattered. Mr.
Brent informs me that a friend of his had to give up flying
carrier-pigeons from France to England, as the hawks on the English
coast destroyed so many on their arrival. Some hawks and owls bolt
their prey whole, and after an interval of from twelve to twenty
hours, disgorge pellets, which, as I know from experiments made in the
Zoological Gardens, include seeds capable of germination. Some seeds
of the oat, wheat, millet, canary, hemp, clover, and beet germinated
after having been from twelve to twenty-one hours in the stomachs of
different birds of prey; and two seeds of beet grew after having been
thus retained for two days and fourteen hours. Freshwater fish, I
find, eat seeds of many land and water plants: fish are frequently
devoured by birds, and thus the seeds might be transported from place
to place. I forced many kinds of seeds into the stomachs of dead fish,
and then gave their bodies to fishing-eagles, storks, and pelicans;
these birds after an interval of many hours, either rejected the seeds
in pellets or passed them in their excrement; and several of these
seeds retained their power of germination. Certain seeds, however,
were always killed by this process.

Although the beaks and feet of birds are generally quite clean, I can
show that earth sometimes adheres to them: in one instance I removed
twenty-two grains of dry argillaceous earth from one foot of a
partridge, and in this earth there was a pebble quite as large as the
seed of a vetch. Thus seeds might occasionally be transported to great
distances; for many facts could be given showing that soil almost
everywhere is charged with seeds. Reflect for a moment on the millions
of quails which annually cross the Mediterranean; and can we doubt
that the earth adhering to their feet would sometimes include a few
minute seeds? But I shall presently have to recur to this subject.

As icebergs are known to be sometimes loaded with earth and stones,
and have even carried brushwood, bones, and the nest of a land-bird, I
can hardly doubt that they must occasionally have transported seeds
from one part to another of the arctic and antarctic regions, as
suggested by Lyell; and during the Glacial period from one part of the
now temperate regions to another. In the Azores, from the large number
of the species of plants common to Europe, in comparison with the
plants of other oceanic islands nearer to the mainland, and (as
remarked by Mr. H. C. Watson) from the somewhat northern character of
the flora in comparison with the latitude, I suspected that these
islands had been partly stocked by ice-borne seeds, during the Glacial
epoch. At my request Sir C. Lyell wrote to M. Hartung to inquire
whether he had observed erratic boulders on these islands, and he
answered that he had found large fragments of granite and other rocks,
which do not occur in the archipelago. Hence we may safely infer that
icebergs formerly landed their rocky burthens on the shores of these
mid-ocean islands, and it is at least possible that they may have
brought thither the seeds of northern plants.

Considering that the several above means of transport, and that
several other means, which without doubt remain to be discovered, have
been in action year after year, for centuries and tens of thousands of
years, it would I think be a marvellous fact if many plants had not
thus become widely transported. These means of transport are sometimes
called accidental, but this is not strictly correct: the currents of
the sea are not accidental, nor is the direction of prevalent gales of
wind. It should be observed that scarcely any means of transport would
carry seeds for very great distances; for seeds do not retain their
vitality when exposed for a great length of time to the action of
seawater; nor could they be long carried in the crops or intestines of
birds. These means, however, would suffice for occasional transport
across tracts of sea some hundred miles in breadth, or from island to
island, or from a continent to a neighbouring island, but not from one
distant continent to another. The floras of distant continents would
not by such means become mingled in any great degree; but would remain
as distinct as we now see them to be. The currents, from their course,
would never bring seeds from North America to Britain, though they
might and do bring seeds from the West Indies to our western shores,
where, if not killed by so long an immersion in salt-water, they could
not endure our climate. Almost every year, one or two land-birds are
blown across the whole Atlantic Ocean, from North America to the
western shores of Ireland and England; but seeds could be transported
by these wanderers only by one means, namely, in dirt sticking to
their feet, which is in itself a rare accident. Even in this case, how
small would the chance be of a seed falling on favourable soil, and
coming to maturity! But it would be a great error to argue that
because a well-stocked island, like Great Britain, has not, as far as
is known (and it would be very difficult to prove this), received
within the last few centuries, through occasional means of transport,
immigrants from Europe or any other continent, that a poorly-stocked
island, though standing more remote from the mainland, would not
receive colonists by similar means. I do not doubt that out of twenty
seeds or animals transported to an island, even if far less
well-stocked than Britain, scarcely more than one would be so well
fitted to its new home, as to become naturalised. But this, as it
seems to me, is no valid argument against what would be effected by
occasional means of transport, during the long lapse of geological
time, whilst an island was being upheaved and formed, and before it
had become fully stocked with inhabitants. On almost bare land, with
few or no destructive insects or birds living there, nearly every
seed, which chanced to arrive, would be sure to germinate and survive.

DISPERSAL DURING THE GLACIAL PERIOD.

The identity of many plants and animals, on mountain-summits,
separated from each other by hundreds of miles of lowlands, where the
Alpine species could not possibly exist, is one of the most striking
cases known of the same species living at distant points, without the
apparent possibility of their having migrated from one to the other.
It is indeed a remarkable fact to see so many of the same plants
living on the snowy regions of the Alps or Pyrenees, and in the
extreme northern parts of Europe; but it is far more remarkable, that
the plants on the White Mountains, in the United States of America,
are all the same with those of Labrador, and nearly all the same, as
we hear from Asa Gray, with those on the loftiest mountains of Europe.
Even as long ago as 1747, such facts led Gmelin to conclude that the
same species must have been independently created at several distinct
points; and we might have remained in this same belief, had not
Agassiz and others called vivid attention to the Glacial period,
which, as we shall immediately see, affords a simple explanation of
these facts. We have evidence of almost every conceivable kind,
organic and inorganic, that within a very recent geological period,
central Europe and North America suffered under an Arctic climate. The
ruins of a house burnt by fire do not tell their tale more plainly,
than do the mountains of Scotland and Wales, with their scored flanks,
polished surfaces, and perched boulders, of the icy streams with which
their valleys were lately filled. So greatly has the climate of Europe
changed, that in Northern Italy, gigantic moraines, left by old
glaciers, are now clothed by the vine and maize. Throughout a large
part of the United States, erratic boulders, and rocks scored by
drifted icebergs and coast-ice, plainly reveal a former cold period.

The former influence of the glacial climate on the distribution of the
inhabitants of Europe, as explained with remarkable clearness by
Edward Forbes, is substantially as follows. But we shall follow the
changes more readily, by supposing a new glacial period to come slowly
on, and then pass away, as formerly occurred. As the cold came on, and
as each more southern zone became fitted for arctic beings and
ill-fitted for their former more temperate inhabitants, the latter
would be supplanted and arctic productions would take their places.
The inhabitants of the more temperate regions would at the same time
travel southward, unless they were stopped by barriers, in which case
they would perish. The mountains would become covered with snow and
ice, and their former Alpine inhabitants would descend to the plains.
By the time that the cold had reached its maximum, we should have a
uniform arctic fauna and flora, covering the central parts of Europe,
as far south as the Alps and Pyrenees, and even stretching into Spain.
The now temperate regions of the United States would likewise be
covered by arctic plants and animals, and these would be nearly the
same with those of Europe; for the present circumpolar inhabitants,
which we suppose to have everywhere travelled southward, are
remarkably uniform round the world. We may suppose that the Glacial
period came on a little earlier or later in North America than in
Europe, so will the southern migration there have been a little
earlier or later; but this will make no difference in the final
result.

As the warmth returned, the arctic forms would retreat northward,
closely followed up in their retreat by the productions of the more
temperate regions. And as the snow melted from the bases of the
mountains, the arctic forms would seize on the cleared and thawed
ground, always ascending higher and higher, as the warmth increased,
whilst their brethren were pursuing their northern journey. Hence,
when the warmth had fully returned, the same arctic species, which had
lately lived in a body together on the lowlands of the Old and New
Worlds, would be left isolated on distant mountain-summits (having
been exterminated on all lesser heights) and in the arctic regions of
both hemispheres.

Thus we can understand the identity of many plants at points so
immensely remote as on the mountains of the United States and of
Europe. We can thus also understand the fact that the Alpine plants of
each mountain-range are more especially related to the arctic forms
living due north or nearly due north of them: for the migration as the
cold came on, and the re-migration on the returning warmth, will
generally have been due south and north. The Alpine plants, for
example, of Scotland, as remarked by Mr. H. C. Watson, and those of
the Pyrenees, as remarked by Ramond, are more especially allied to the
plants of northern Scandinavia; those of the United States to
Labrador; those of the mountains of Siberia to the arctic regions of
that country. These views, grounded as they are on the perfectly
well-ascertained occurrence of a former Glacial period, seem to me to
explain in so satisfactory a manner the present distribution of the
Alpine and Arctic productions of Europe and America, that when in
other regions we find the same species on distant mountain-summits, we
may almost conclude without other evidence, that a colder climate
permitted their former migration across the low intervening tracts,
since become too warm for their existence.

If the climate, since the Glacial period, has ever been in any degree
warmer than at present (as some geologists in the United States
believe to have been the case, chiefly from the distribution of the
fossil Gnathodon), then the arctic and temperate productions will at a
very late period have marched a little further north, and subsequently
have retreated to their present homes; but I have met with no
satisfactory evidence with respect to this intercalated slightly
warmer period, since the Glacial period.

The arctic forms, during their long southern migration and
re-migration northward, will have been exposed to nearly the same
climate, and, as is especially to be noticed, they will have kept in a
body together; consequently their mutual relations will not have been
much disturbed, and, in accordance with the principles inculcated in
this volume, they will not have been liable to much modification. But
with our Alpine productions, left isolated from the moment of the
returning warmth, first at the bases and ultimately on the summits of
the mountains, the case will have been somewhat different; for it is
not likely that all the same arctic species will have been left on
mountain ranges distant from each other, and have survived there ever
since; they will, also, in all probability have become mingled with
ancient Alpine species, which must have existed on the mountains
before the commencement of the Glacial epoch, and which during its
coldest period will have been temporarily driven down to the plains;
they will, also, have been exposed to somewhat different climatal
influences. Their mutual relations will thus have been in some degree
disturbed; consequently they will have been liable to modification;
and this we find has been the case; for if we compare the present
Alpine plants and animals of the several great European
mountain-ranges, though very many of the species are identically the
same, some present varieties, some are ranked as doubtful forms, and
some few are distinct yet closely allied or representative species.

In illustrating what, as I believe, actually took place during the
Glacial period, I assumed that at its commencement the arctic
productions were as uniform round the polar regions as they are at the
present day. But the foregoing remarks on distribution apply not only
to strictly arctic forms, but also to many sub-arctic and to some few
northern temperate forms, for some of these are the same on the lower
mountains and on the plains of North America and Europe; and it may be
reasonably asked how I account for the necessary degree of uniformity
of the sub-arctic and northern temperate forms round the world, at the
commencement of the Glacial period. At the present day, the sub-arctic
and northern temperate productions of the Old and New Worlds are
separated from each other by the Atlantic Ocean and by the extreme
northern part of the Pacific. During the Glacial period, when the
inhabitants of the Old and New Worlds lived further southwards than at
present, they must have been still more completely separated by wider
spaces of ocean. I believe the above difficulty may be surmounted by
looking to still earlier changes of climate of an opposite nature. We
have good reason to believe that during the newer Pliocene period,
before the Glacial epoch, and whilst the majority of the inhabitants
of the world were specifically the same as now, the climate was warmer
than at the present day. Hence we may suppose that the organisms now
living under the climate of latitude 60 deg, during the Pliocene
period lived further north under the Polar Circle, in latitude 66
deg-67 deg; and that the strictly arctic productions then lived on the
broken land still nearer to the pole. Now if we look at a globe, we
shall see that under the Polar Circle there is almost continuous land
from western Europe, through Siberia, to eastern America. And to this
continuity of the circumpolar land, and to the consequent freedom for
intermigration under a more favourable climate, I attribute the
necessary amount of uniformity in the sub-arctic and northern
temperate productions of the Old and New Worlds, at a period anterior
to the Glacial epoch.

Believing, from reasons before alluded to, that our continents have
long remained in nearly the same relative position, though subjected
to large, but partial oscillations of level, I am strongly inclined to
extend the above view, and to infer that during some earlier and still
warmer period, such as the older Pliocene period, a large number of
the same plants and animals inhabited the almost continuous
circumpolar land; and that these plants and animals, both in the Old
and New Worlds, began slowly to migrate southwards as the climate
became less warm, long before the commencement of the Glacial period.
We now see, as I believe, their descendants, mostly in a modified
condition, in the central parts of Europe and the United States. On
this view we can understand the relationship, with very little
identity, between the productions of North America and Europe,--a
relationship which is most remarkable, considering the distance of the
two areas, and their separation by the Atlantic Ocean. We can further
understand the singular fact remarked on by several observers, that
the productions of Europe and America during the later tertiary stages
were more closely related to each other than they are at the present
time; for during these warmer periods the northern parts of the Old
and New Worlds will have been almost continuously united by land,
serving as a bridge, since rendered impassable by cold, for the
inter-migration of their inhabitants.

During the slowly decreasing warmth of the Pliocene period, as soon as
the species in common, which inhabited the New and Old Worlds,
migrated south of the Polar Circle, they must have been completely cut
off from each other. This separation, as far as the more temperate
productions are concerned, took place long ages ago. And as the plants
and animals migrated southward, they will have become mingled in the
one great region with the native American productions, and have had to
compete with them; and in the other great region, with those of the
Old World. Consequently we have here everything favourable for much
modification,--for far more modification than with the Alpine
productions, left isolated, within a much more recent period, on the
several mountain-ranges and on the arctic lands of the two Worlds.
Hence it has come, that when we compare the now living productions of
the temperate regions of the New and Old Worlds, we find very few
identical species (though Asa Gray has lately shown that more plants
are identical than was formerly supposed), but we find in every great
class many forms, which some naturalists rank as geographical races,
and others as distinct species; and a host of closely allied or
representative forms which are ranked by all naturalists as
specifically distinct.

As on the land, so in the waters of the sea, a slow southern migration
of a marine fauna, which during the Pliocene or even a somewhat
earlier period, was nearly uniform along the continuous shores of the
Polar Circle, will account, on the theory of modification, for many
closely allied forms now living in areas completely sundered. Thus, I
think, we can understand the presence of many existing and tertiary
representative forms on the eastern and western shores of temperate
North America; and the still more striking case of many closely allied
crustaceans (as described in Dana's admirable work), of some fish and
other marine animals, in the Mediterranean and in the seas of
Japan,--areas now separated by a continent and by nearly a hemisphere
of equatorial ocean.

These cases of relationship, without identity, of the inhabitants of
seas now disjoined, and likewise of the past and present inhabitants
of the temperate lands of North America and Europe, are inexplicable
on the theory of creation. We cannot say that they have been created
alike, in correspondence with the nearly similar physical conditions
of the areas; for if we compare, for instance, certain parts of South
America with the southern continents of the Old World, we see
countries closely corresponding in all their physical conditions, but
with their inhabitants utterly dissimilar.

But we must return to our more immediate subject, the Glacial period.
I am convinced that Forbes's view may be largely extended. In Europe
we have the plainest evidence of the cold period, from the western
shores of Britain to the Oural range, and southward to the Pyrenees.
We may infer, from the frozen mammals and nature of the mountain
vegetation, that Siberia was similarly affected. Along the Himalaya,
at points 900 miles apart, glaciers have left the marks of their
former low descent; and in Sikkim, Dr. Hooker saw maize growing on
gigantic ancient moraines. South of the equator, we have some direct
evidence of former glacial action in New Zealand; and the same plants,
found on widely separated mountains in this island, tell the same
story. If one account which has been published can be trusted, we have
direct evidence of glacial action in the south-eastern corner of
Australia.

Looking to America; in the northern half, ice-borne fragments of rock
have been observed on the eastern side as far south as lat. 36 deg-37
deg, and on the shores of the Pacific, where the climate is now so
different, as far south as lat. 46 deg; erratic boulders have, also,
been noticed on the Rocky Mountains. In the Cordillera of Equatorial
South America, glaciers once extended far below their present level.
In central Chile I was astonished at the structure of a vast mound of
detritus, about 800 feet in height, crossing a valley of the Andes;
and this I now feel convinced was a gigantic moraine, left far below
any existing glacier. Further south on both sides of the continent,
from lat. 41 deg to the southernmost extremity, we have the clearest
evidence of former glacial action, in huge boulders transported far
from their parent source.

We do not know that the Glacial epoch was strictly simultaneous at
these several far distant points on opposite sides of the world. But
we have good evidence in almost every case, that the epoch was
included within the latest geological period. We have, also, excellent
evidence, that it endured for an enormous time, as measured by years,
at each point. The cold may have come on, or have ceased, earlier at
one point of the globe than at another, but seeing that it endured for
long at each, and that it was contemporaneous in a geological sense,
it seems to me probable that it was, during a part at least of the
period, actually simultaneous throughout the world. Without some
distinct evidence to the contrary, we may at least admit as probable
that the glacial action was simultaneous on the eastern and western
sides of North America, in the Cordillera under the equator and under
the warmer temperate zones, and on both sides of the southern
extremity of the continent. If this be admitted, it is difficult to
avoid believing that the temperature of the whole world was at this
period simultaneously cooler. But it would suffice for my purpose, if
the temperature was at the same time lower along certain broad belts
of longitude.

On this view of the whole world, or at least of broad longitudinal
belts, having been simultaneously colder from pole to pole, much light
can be thrown on the present distribution of identical and allied
species. In America, Dr. Hooker has shown that between forty and fifty
of the flowering plants of Tierra del Fuego, forming no inconsiderable
part of its scanty flora, are common to Europe, enormously remote as
these two points are; and there are many closely allied species. On
the lofty mountains of equatorial America a host of peculiar species
belonging to European genera occur. On the highest mountains of
Brazil, some few European genera were found by Gardner, which do not
exist in the wide intervening hot countries. So on the Silla of
Caraccas the illustrious Humboldt long ago found species belonging to
genera characteristic of the Cordillera. On the mountains of
Abyssinia, several European forms and some few representatives of the
peculiar flora of the Cape of Good Hope occur. At the Cape of Good
Hope a very few European species, believed not to have been introduced
by man, and on the mountains, some few representative European forms
are found, which have not been discovered in the intertropical parts
of Africa. On the Himalaya, and on the isolated mountain-ranges of the
peninsula of India, on the heights of Ceylon, and on the volcanic
cones of Java, many plants occur, either identically the same or
representing each other, and at the same time representing plants of
Europe, not found in the intervening hot lowlands. A list of the
genera collected on the loftier peaks of Java raises a picture of a
collection made on a hill in Europe! Still more striking is the fact
that southern Australian forms are clearly represented by plants
growing on the summits of the mountains of Borneo. Some of these
Australian forms, as I hear from Dr. Hooker, extend along the heights
of the peninsula of Malacca, and are thinly scattered, on the one hand
over India and on the other as far north as Japan.

On the southern mountains of Australia, Dr. F. Muller has discovered
several European species; other species, not introduced by man, occur
on the lowlands; and a long list can be given, as I am informed by Dr.
Hooker, of European genera, found in Australia, but not in the
intermediate torrid regions. In the admirable 'Introduction to the
Flora of New Zealand,' by Dr. Hooker, analogous and striking facts are
given in regard to the plants of that large island. Hence we see that
throughout the world, the plants growing on the more lofty mountains,
and on the temperate lowlands of the northern and southern
hemispheres, are sometimes identically the same; but they are much
oftener specifically distinct, though related to each other in a most
remarkable manner.

This brief abstract applies to plants alone: some strictly analogous
facts could be given on the distribution of terrestrial animals. In
marine productions, similar cases occur; as an example, I may quote a
remark by the highest authority, Professor Dana, that "it is certainly
a wonderful fact that New Zealand should have a closer resemblance in
its crustacea to Great Britain, its antipode, than to any other part
of the world." Sir J. Richardson, also, speaks of the reappearance on
the shores of New Zealand, Tasmania, etc., of northern forms of fish.
Dr. Hooker informs me that twenty-five species of Algae are common to
New Zealand and to Europe, but have not been found in the intermediate
tropical seas.

It should be observed that the northern species and forms found in the
southern parts of the southern hemisphere, and on the mountain-ranges
of the intertropical regions, are not arctic, but belong to the
northern temperate zones. As Mr. H. C. Watson has recently remarked,
"In receding from polar towards equatorial latitudes, the Alpine or
mountain floras really become less and less arctic." Many of the forms
living on the mountains of the warmer regions of the earth and in the
southern hemisphere are of doubtful value, being ranked by some
naturalists as specifically distinct, by others as varieties; but some
are certainly identical, and many, though closely related to northern
forms, must be ranked as distinct species.

Now let us see what light can be thrown on the foregoing facts, on the
belief, supported as it is by a large body of geological evidence,
that the whole world, or a large part of it, was during the Glacial
period simultaneously much colder than at present. The Glacial period,
as measured by years, must have been very long; and when we remember
over what vast spaces some naturalised plants and animals have spread
within a few centuries, this period will have been ample for any
amount of migration. As the cold came slowly on, all the tropical
plants and other productions will have retreated from both sides
towards the equator, followed in the rear by the temperate
productions, and these by the arctic; but with the latter we are not
now concerned. The tropical plants probably suffered much extinction;
how much no one can say; perhaps formerly the tropics supported as
many species as we see at the present day crowded together at the Cape
of Good Hope, and in parts of temperate Australia. As we know that
many tropical plants and animals can withstand a considerable amount
of cold, many might have escaped extermination during a moderate fall
of temperature, more especially by escaping into the warmest spots.
But the great fact to bear in mind is, that all tropical productions
will have suffered to a certain extent. On the other hand, the
temperate productions, after migrating nearer to the equator, though
they will have been placed under somewhat new conditions, will have
suffered less. And it is certain that many temperate plants, if
protected from the inroads of competitors, can withstand a much warmer
climate than their own. Hence, it seems to me possible, bearing in
mind that the tropical productions were in a suffering state and could
not have presented a firm front against intruders, that a certain
number of the more vigorous and dominant temperate forms might have
penetrated the native ranks and have reached or even crossed the
equator. The invasion would, of course, have been greatly favoured by
high land, and perhaps by a dry climate; for Dr. Falconer informs me
that it is the damp with the heat of the tropics which is so
destructive to perennial plants from a temperate climate. On the other
hand, the most humid and hottest districts will have afforded an
asylum to the tropical natives. The mountain-ranges north-west of the
Himalaya, and the long line of the Cordillera, seem to have afforded
two great lines of invasion: and it is a striking fact, lately
communicated to me by Dr. Hooker, that all the flowering plants, about
forty-six in number, common to Tierra del Fuego and to Europe still
exist in North America, which must have lain on the line of march. But
I do not doubt that some temperate productions entered and crossed
even the LOWLANDS of the tropics at the period when the cold was most
intense,--when arctic forms had migrated some twenty-five degrees of
latitude from their native country and covered the land at the foot of
the Pyrenees. At this period of extreme cold, I believe that the
climate under the equator at the level of the sea was about the same
with that now felt there at the height of six or seven thousand feet.
During this the coldest period, I suppose that large spaces of the
tropical lowlands were clothed with a mingled tropical and temperate
vegetation, like that now growing with strange luxuriance at the base
of the Himalaya, as graphically described by Hooker.

Thus, as I believe, a considerable number of plants, a few terrestrial
animals, and some marine productions, migrated during the Glacial
period from the northern and southern temperate zones into the
intertropical regions, and some even crossed the equator. As the
warmth returned, these temperate forms would naturally ascend the
higher mountains, being exterminated on the lowlands; those which had
not reached the equator, would re-migrate northward or southward
towards their former homes; but the forms, chiefly northern, which had
crossed the equator, would travel still further from their homes into
the more temperate latitudes of the opposite hemisphere. Although we
have reason to believe from geological evidence that the whole body of
arctic shells underwent scarcely any modification during their long
southern migration and re-migration northward, the case may have been
wholly different with those intruding forms which settled themselves
on the intertropical mountains, and in the southern hemisphere. These
being surrounded by strangers will have had to compete with many new
forms of life; and it is probable that selected modifications in their
structure, habits, and constitutions will have profited them. Thus
many of these wanderers, though still plainly related by inheritance
to their brethren of the northern or southern hemispheres, now exist
in their new homes as well-marked varieties or as distinct species.

It is a remarkable fact, strongly insisted on by Hooker in regard to
America, and by Alph. de Candolle in regard to Australia, that many
more identical plants and allied forms have apparently migrated from
the north to the south, than in a reversed direction. We see, however,
a few southern vegetable forms on the mountains of Borneo and
Abyssinia. I suspect that this preponderant migration from north to
south is due to the greater extent of land in the north, and to the
northern forms having existed in their own homes in greater numbers,
and having consequently been advanced through natural selection and
competition to a higher stage of perfection or dominating power, than
the southern forms. And thus, when they became commingled during the
Glacial period, the northern forms were enabled to beat the less
powerful southern forms. Just in the same manner as we see at the
present day, that very many European productions cover the ground in
La Plata, and in a lesser degree in Australia, and have to a certain
extent beaten the natives; whereas extremely few southern forms have
become naturalised in any part of Europe, though hides, wool, and
other objects likely to carry seeds have been largely imported into
Europe during the last two or three centuries from La Plata, and
during the last thirty or forty years from Australia. Something of the
same kind must have occurred on the intertropical mountains: no doubt
before the Glacial period they were stocked with endemic Alpine forms;
but these have almost everywhere largely yielded to the more dominant
forms, generated in the larger areas and more efficient workshops of
the north. In many islands the native productions are nearly equalled
or even outnumbered by the naturalised; and if the natives have not
been actually exterminated, their numbers have been greatly reduced,
and this is the first stage towards extinction. A mountain is an
island on the land; and the intertropical mountains before the Glacial
period must have been completely isolated; and I believe that the
productions of these islands on the land yielded to those produced
within the larger areas of the north, just in the same way as the
productions of real islands have everywhere lately yielded to
continental forms, naturalised by man's agency.

I am far from supposing that all difficulties are removed on the view
here given in regard to the range and affinities of the allied species
which live in the northern and southern temperate zones and on the
mountains of the intertropical regions. Very many difficulties remain
to be solved. I do not pretend to indicate the exact lines and means
of migration, or the reason why certain species and not others have
migrated; why certain species have been modified and have given rise
to new groups of forms, and others have remained unaltered. We cannot
hope to explain such facts, until we can say why one species and not
another becomes naturalised by man's agency in a foreign land; why one
ranges twice or thrice as far, and is twice or thrice as common, as
another species within their own homes.

I have said that many difficulties remain to be solved: some of the
most remarkable are stated with admirable clearness by Dr. Hooker in
his botanical works on the antarctic regions. These cannot be here
discussed. I will only say that as far as regards the occurrence of
identical species at points so enormously remote as Kerguelen Land,
New Zealand, and Fuegia, I believe that towards the close of the
Glacial period, icebergs, as suggested by Lyell, have been largely
concerned in their dispersal. But the existence of several quite
distinct species, belonging to genera exclusively confined to the
south, at these and other distant points of the southern hemisphere,
is, on my theory of descent with modification, a far more remarkable
case of difficulty. For some of these species are so distinct, that we
cannot suppose that there has been time since the commencement of the
Glacial period for their migration, and for their subsequent
modification to the necessary degree. The facts seem to me to indicate
that peculiar and very distinct species have migrated in radiating
lines from some common centre; and I am inclined to look in the
southern, as in the northern hemisphere, to a former and warmer
period, before the commencement of the Glacial period, when the
antarctic lands, now covered with ice, supported a highly peculiar and
isolated flora. I suspect that before this flora was exterminated by
the Glacial epoch, a few forms were widely dispersed to various points
of the southern hemisphere by occasional means of transport, and by
the aid, as halting-places, of existing and now sunken islands, and
perhaps at the commencement of the Glacial period, by icebergs. By
these means, as I believe, the southern shores of America, Australia,
New Zealand have become slightly tinted by the same peculiar forms of
vegetable life.

Sir C. Lyell in a striking passage has speculated, in language almost
identical with mine, on the effects of great alternations of climate
on geographical distribution. I believe that the world has recently
felt one of his great cycles of change; and that on this view,
combined with modification through natural selection, a multitude of
facts in the present distribution both of the same and of allied forms
of life can be explained. The living waters may be said to have flowed
during one short period from the north and from the south, and to have
crossed at the equator; but to have flowed with greater force from the
north so as to have freely inundated the south. As the tide leaves its
drift in horizontal lines, though rising higher on the shores where
the tide rises highest, so have the living waters left their living
drift on our mountain-summits, in a line gently rising from the arctic
lowlands to a great height under the equator. The various beings thus
left stranded may be compared with savage races of man, driven up and
surviving in the mountain-fastnesses of almost every land, which serve
as a record, full of interest to us, of the former inhabitants of the
surrounding lowlands.


CHAPTER 12. GEOGRAPHICAL DISTRIBUTION--continued.

Distribution of fresh-water productions.
On the inhabitants of oceanic islands.
Absence of Batrachians and of terrestrial Mammals.
On the relation of the inhabitants of islands to those of the nearest
mainland.
On colonisation from the nearest source with subsequent modification.
Summary of the last and present chapters.

As lakes and river-systems are separated from each other by barriers
of land, it might have been thought that fresh-water productions would
not have ranged widely within the same country, and as the sea is
apparently a still more impassable barrier, that they never would have
extended to distant countries. But the case is exactly the reverse.
Not only have many fresh-water species, belonging to quite different
classes, an enormous range, but allied species prevail in a remarkable
manner throughout the world. I well remember, when first collecting in
the fresh waters of Brazil, feeling much surprise at the similarity of
the fresh-water insects, shells, etc., and at the dissimilarity of the
surrounding terrestrial beings, compared with those of Britain.

But this power in fresh-water productions of ranging widely, though so
unexpected, can, I think, in most cases be explained by their having
become fitted, in a manner highly useful to them, for short and
frequent migrations from pond to pond, or from stream to stream; and
liability to wide dispersal would follow from this capacity as an
almost necessary consequence. We can here consider only a few cases.
In regard to fish, I believe that the same species never occur in the
fresh waters of distant continents. But on the same continent the
species often range widely and almost capriciously; for two
river-systems will have some fish in common and some different. A few
facts seem to favour the possibility of their occasional transport by
accidental means; like that of the live fish not rarely dropped by
whirlwinds in India, and the vitality of their ova when removed from
the water. But I am inclined to attribute the dispersal of fresh-water
fish mainly to slight changes within the recent period in the level of
the land, having caused rivers to flow into each other. Instances,
also, could be given of this having occurred during floods, without
any change of level. We have evidence in the loess of the Rhine of
considerable changes of level in the land within a very recent
geological period, and when the surface was peopled by existing land
and fresh-water shells. The wide difference of the fish on opposite
sides of continuous mountain-ranges, which from an early period must
have parted river-systems and completely prevented their inosculation,
seems to lead to this same conclusion. With respect to allied
fresh-water fish occurring at very distant points of the world, no
doubt there are many cases which cannot at present be explained: but
some fresh-water fish belong to very ancient forms, and in such cases
there will have been ample time for great geographical changes, and
consequently time and means for much migration. In the second place,
salt-water fish can with care be slowly accustomed to live in fresh
water; and, according to Valenciennes, there is hardly a single group
of fishes confined exclusively to fresh water, so that we may imagine
that a marine member of a fresh-water group might travel far along the
shores of the sea, and subsequently become modified and adapted to the
fresh waters of a distant land.

Some species of fresh-water shells have a very wide range, and allied
species, which, on my theory, are descended from a common parent and
must have proceeded from a single source, prevail throughout the
world. Their distribution at first perplexed me much, as their ova are
not likely to be transported by birds, and they are immediately killed
by sea water, as are the adults. I could not even understand how some
naturalised species have rapidly spread throughout the same country.
But two facts, which I have observed--and no doubt many others remain
to be observed--throw some light on this subject. When a duck suddenly
emerges from a pond covered with duck-weed, I have twice seen these
little plants adhering to its back; and it has happened to me, in
removing a little duck-weed from one aquarium to another, that I have
quite unintentionally stocked the one with fresh-water shells from the
other. But another agency is perhaps more effectual: I suspended a
duck's feet, which might represent those of a bird sleeping in a
natural pond, in an aquarium, where many ova of fresh-water shells
were hatching; and I found that numbers of the extremely minute and
just hatched shells crawled on the feet, and clung to them so firmly
that when taken out of the water they could not be jarred off, though
at a somewhat more advanced age they would voluntarily drop off. These
just hatched molluscs, though aquatic in their nature, survived on the
duck's feet, in damp air, from twelve to twenty hours; and in this
length of time a duck or heron might fly at least six or seven hundred
miles, and would be sure to alight on a pool or rivulet, if blown
across sea to an oceanic island or to any other distant point. Sir
Charles Lyell also informs me that a Dyticus has been caught with an
Ancylus (a fresh-water shell like a limpet) firmly adhering to it; and
a water-beetle of the same family, a Colymbetes, once flew on board
the 'Beagle,' when forty-five miles distant from the nearest land: how
much farther it might have flown with a favouring gale no one can
tell.

With respect to plants, it has long been known what enormous ranges
many fresh-water and even marsh-species have, both over continents and
to the most remote oceanic islands. This is strikingly shown, as
remarked by Alph. de Candolle, in large groups of terrestrial plants,
which have only a very few aquatic members; for these latter seem
immediately to acquire, as if in consequence, a very wide range. I
think favourable means of dispersal explain this fact. I have before
mentioned that earth occasionally, though rarely, adheres in some
quantity to the feet and beaks of birds. Wading birds, which frequent
the muddy edges of ponds, if suddenly flushed, would be the most
likely to have muddy feet. Birds of this order I can show are the
greatest wanderers, and are occasionally found on the most remote and
barren islands in the open ocean; they would not be likely to alight
on the surface of the sea, so that the dirt would not be washed off
their feet; when making land, they would be sure to fly to their
natural fresh-water haunts. I do not believe that botanists are aware
how charged the mud of ponds is with seeds: I have tried several
little experiments, but will here give only the most striking case: I
took in February three table-spoonfuls of mud from three different
points, beneath water, on the edge of a little pond; this mud when dry
weighed only 6 3/4 ounces; I kept it covered up in my study for six
months, pulling up and counting each plant as it grew; the plants were
of many kinds, and were altogether 537 in number; and yet the viscid
mud was all contained in a breakfast cup! Considering these facts, I
think it would be an inexplicable circumstance if water-birds did not
transport the seeds of fresh-water plants to vast distances, and if
consequently the range of these plants was not very great. The same
agency may have come into play with the eggs of some of the smaller
fresh-water animals.

Other and unknown agencies probably have also played a part. I have
stated that fresh-water fish eat some kinds of seeds, though they
reject many other kinds after having swallowed them; even small fish
swallow seeds of moderate size, as of the yellow water-lily and
Potamogeton. Herons and other birds, century after century, have gone
on daily devouring fish; they then take flight and go to other waters,
or are blown across the sea; and we have seen that seeds retain their
power of germination, when rejected in pellets or in excrement, many
hours afterwards. When I saw the great size of the seeds of that fine
water-lily, the Nelumbium, and remembered Alph. de Candolle's remarks
on this plant, I thought that its distribution must remain quite
inexplicable; but Audubon states that he found the seeds of the great
southern water-lily (probably, according to Dr. Hooker, the Nelumbium
luteum) in a heron's stomach; although I do not know the fact, yet
analogy makes me believe that a heron flying to another pond and
getting a hearty meal of fish, would probably reject from its stomach
a pellet containing the seeds of the Nelumbium undigested; or the
seeds might be dropped by the bird whilst feeding its young, in the
same way as fish are known sometimes to be dropped.

In considering these several means of distribution, it should be
remembered that when a pond or stream is first formed, for instance,
on a rising islet, it will be unoccupied; and a single seed or egg
will have a good chance of succeeding. Although there will always be a
struggle for life between the individuals of the species, however few,
already occupying any pond, yet as the number of kinds is small,
compared with those on the land, the competition will probably be less
severe between aquatic than between terrestrial species; consequently
an intruder from the waters of a foreign country, would have a better
chance of seizing on a place, than in the case of terrestrial
colonists. We should, also, remember that some, perhaps many,
fresh-water productions are low in the scale of nature, and that we
have reason to believe that such low beings change or become modified
less quickly than the high; and this will give longer time than the
average for the migration of the same aquatic species. We should not
forget the probability of many species having formerly ranged as
continuously as fresh-water productions ever can range, over immense
areas, and having subsequently become extinct in intermediate regions.
But the wide distribution of fresh-water plants and of the lower
animals, whether retaining the same identical form or in some degree
modified, I believe mainly depends on the wide dispersal of their
seeds and eggs by animals, more especially by fresh-water birds, which
have large powers of flight, and naturally travel from one to another
and often distant piece of water. Nature, like a careful gardener,
thus takes her seeds from a bed of a particular nature, and drops them
in another equally well fitted for them.

ON THE INHABITANTS OF OCEANIC ISLANDS.

We now come to the last of the three classes of facts, which I have
selected as presenting the greatest amount of difficulty, on the view
that all the individuals both of the same and of allied species have
descended from a single parent; and therefore have all proceeded from
a common birthplace, notwithstanding that in the course of time they
have come to inhabit distant points of the globe. I have already
stated that I cannot honestly admit Forbes's view on continental
extensions, which, if legitimately followed out, would lead to the
belief that within the recent period all existing islands have been
nearly or quite joined to some continent. This view would remove many
difficulties, but it would not, I think, explain all the facts in
regard to insular productions. In the following remarks I shall not
confine myself to the mere question of dispersal; but shall consider
some other facts, which bear on the truth of the two theories of
independent creation and of descent with modification.

The species of all kinds which inhabit oceanic islands are few in
number compared with those on equal continental areas: Alph. de
Candolle admits this for plants, and Wollaston for insects. If we look
to the large size and varied stations of New Zealand, extending over
780 miles of latitude, and compare its flowering plants, only 750 in
number, with those on an equal area at the Cape of Good Hope or in
Australia, we must, I think, admit that something quite independently
of any difference in physical conditions has caused so great a
difference in number. Even the uniform county of Cambridge has 847
plants, and the little island of Anglesea 764, but a few ferns and a
few introduced plants are included in these numbers, and the
comparison in some other respects is not quite fair. We have evidence
that the barren island of Ascension aboriginally possessed under
half-a-dozen flowering plants; yet many have become naturalised on it,
as they have on New Zealand and on every other oceanic island which
can be named. In St. Helena there is reason to believe that the
naturalised plants and animals have nearly or quite exterminated many
native productions. He who admits the doctrine of the creation of each
separate species, will have to admit, that a sufficient number of the
best adapted plants and animals have not been created on oceanic
islands; for man has unintentionally stocked them from various sources
far more fully and perfectly than has nature.

Although in oceanic islands the number of kinds of inhabitants is
scanty, the proportion of endemic species (i.e. those found nowhere
else in the world) is often extremely large. If we compare, for
instance, the number of the endemic land-shells in Madeira, or of the
endemic birds in the Galapagos Archipelago, with the number found on
any continent, and then compare the area of the islands with that of
the continent, we shall see that this is true. This fact might have
been expected on my theory, for, as already explained, species
occasionally arriving after long intervals in a new and isolated
district, and having to compete with new associates, will be eminently
liable to modification, and will often produce groups of modified
descendants. But it by no means follows, that, because in an island
nearly all the species of one class are peculiar, those of another
class, or of another section of the same class, are peculiar; and this
difference seems to depend on the species which do not become modified
having immigrated with facility and in a body, so that their mutual
relations have not been much disturbed. Thus in the Galapagos Islands
nearly every land-bird, but only two out of the eleven marine birds,
are peculiar; and it is obvious that marine birds could arrive at
these islands more easily than land-birds. Bermuda, on the other hand,
which lies at about the same distance from North America as the
Galapagos Islands do from South America, and which has a very peculiar
soil, does not possess one endemic land bird; and we know from Mr. J.
M. Jones's admirable account of Bermuda, that very many North American
birds, during their great annual migrations, visit either periodically
or occasionally this island. Madeira does not possess one peculiar
bird, and many European and African birds are almost every year blown
there, as I am informed by Mr. E. V. Harcourt. So that these two
islands of Bermuda and Madeira have been stocked by birds, which for
long ages have struggled together in their former homes, and have
become mutually adapted to each other; and when settled in their new
homes, each kind will have been kept by the others to their proper
places and habits, and will consequently have been little liable to
modification. Madeira, again, is inhabited by a wonderful number of
peculiar land-shells, whereas not one species of sea-shell is confined
to its shores: now, though we do not know how seashells are dispersed,
yet we can see that their eggs or larvae, perhaps attached to seaweed
or floating timber, or to the feet of wading-birds, might be
transported far more easily than land-shells, across three or four
hundred miles of open sea. The different orders of insects in Madeira
apparently present analogous facts.

Oceanic islands are sometimes deficient in certain classes, and their
places are apparently occupied by the other inhabitants; in the
Galapagos Islands reptiles, and in New Zealand gigantic wingless
birds, take the place of mammals. In the plants of the Galapagos
Islands, Dr. Hooker has shown that the proportional numbers of the
different orders are very different from what they are elsewhere. Such
cases are generally accounted for by the physical conditions of the
islands; but this explanation seems to me not a little doubtful.
Facility of immigration, I believe, has been at least as important as
the nature of the conditions.

Many remarkable little facts could be given with respect to the
inhabitants of remote islands. For instance, in certain islands not
tenanted by mammals, some of the endemic plants have beautifully
hooked seeds; yet few relations are more striking than the adaptation
of hooked seeds for transportal by the wool and fur of quadrupeds.
This case presents no difficulty on my view, for a hooked seed might
be transported to an island by some other means; and the plant then
becoming slightly modified, but still retaining its hooked seeds,
would form an endemic species, having as useless an appendage as any
rudimentary organ,--for instance, as the shrivelled wings under the
soldered elytra of many insular beetles. Again, islands often possess
trees or bushes belonging to orders which elsewhere include only
herbaceous species; now trees, as Alph. de Candolle has shown,
generally have, whatever the cause may be, confined ranges. Hence
trees would be little likely to reach distant oceanic islands; and an
herbaceous plant, though it would have no chance of successfully
competing in stature with a fully developed tree, when established on
an island and having to compete with herbaceous plants alone, might
readily gain an advantage by growing taller and taller and overtopping
the other plants. If so, natural selection would often tend to add to
the stature of herbaceous plants when growing on an island, to
whatever order they belonged, and thus convert them first into bushes
and ultimately into trees.

With respect to the absence of whole orders on oceanic islands, Bory
St. Vincent long ago remarked that Batrachians (frogs, toads, newts)
have never been found on any of the many islands with which the great
oceans are studded. I have taken pains to verify this assertion, and I
have found it strictly true. I have, however, been assured that a frog
exists on the mountains of the great island of New Zealand; but I
suspect that this exception (if the information be correct) may be
explained through glacial agency. This general absence of frogs,
toads, and newts on so many oceanic islands cannot be accounted for by
their physical conditions; indeed it seems that islands are peculiarly
well fitted for these animals; for frogs have been introduced into
Madeira, the Azores, and Mauritius, and have multiplied so as to
become a nuisance. But as these animals and their spawn are known to
be immediately killed by sea-water, on my view we can see that there
would be great difficulty in their transportal across the sea, and
therefore why they do not exist on any oceanic island. But why, on the
theory of creation, they should not have been created there, it would
be very difficult to explain.

Mammals offer another and similar case. I have carefully searched the
oldest voyages, but have not finished my search; as yet I have not
found a single instance, free from doubt, of a terrestrial mammal
(excluding domesticated animals kept by the natives) inhabiting an
island situated above 300 miles from a continent or great continental
island; and many islands situated at a much less distance are equally
barren. The Falkland Islands, which are inhabited by a wolf-like fox,
come nearest to an exception; but this group cannot be considered as
oceanic, as it lies on a bank connected with the mainland; moreover,
icebergs formerly brought boulders to its western shores, and they may
have formerly transported foxes, as so frequently now happens in the
arctic regions. Yet it cannot be said that small islands will not
support small mammals, for they occur in many parts of the world on
very small islands, if close to a continent; and hardly an island can
be named on which our smaller quadrupeds have not become naturalised
and greatly multiplied. It cannot be said, on the ordinary view of
creation, that there has not been time for the creation of mammals;
many volcanic islands are sufficiently ancient, as shown by the
stupendous degradation which they have suffered and by their tertiary
strata: there has also been time for the production of endemic species
belonging to other classes; and on continents it is thought that
mammals appear and disappear at a quicker rate than other and lower
animals. Though terrestrial mammals do not occur on oceanic islands,
aerial mammals do occur on almost every island. New Zealand possesses
two bats found nowhere else in the world: Norfolk Island, the Viti
Archipelago, the Bonin Islands, the Caroline and Marianne
Archipelagoes, and Mauritius, all possess their peculiar bats. Why, it
may be asked, has the supposed creative force produced bats and no
other mammals on remote islands? On my view this question can easily
be answered; for no terrestrial mammal can be transported across a
wide space of sea, but bats can fly across. Bats have been seen
wandering by day far over the Atlantic Ocean; and two North American
species either regularly or occasionally visit Bermuda, at the
distance of 600 miles from the mainland. I hear from Mr. Tomes, who
has specially studied this family, that many of the same species have
enormous ranges, and are found on continents and on far distant
islands. Hence we have only to suppose that such wandering species
have been modified through natural selection in their new homes in
relation to their new position, and we can understand the presence of
endemic bats on islands, with the absence of all terrestrial mammals.

Besides the absence of terrestrial mammals in relation to the
remoteness of islands from continents, there is also a relation, to a
certain extent independent of distance, between the depth of the sea
separating an island from the neighbouring mainland, and the presence
in both of the same mammiferous species or of allied species in a more
or less modified condition. Mr. Windsor Earl has made some striking
observations on this head in regard to the great Malay Archipelago,
which is traversed near Celebes by a space of deep ocean; and this
space separates two widely distinct mammalian faunas. On either side
the islands are situated on moderately deep submarine banks, and they
are inhabited by closely allied or identical quadrupeds. No doubt some
few anomalies occur in this great archipelago, and there is much
difficulty in forming a judgment in some cases owing to the probable
naturalisation of certain mammals through man's agency; but we shall
soon have much light thrown on the natural history of this archipelago
by the admirable zeal and researches of Mr. Wallace. I have not as yet
had time to follow up this subject in all other quarters of the world;
but as far as I have gone, the relation generally holds good. We see
Britain separated by a shallow channel from Europe, and the mammals
are the same on both sides; we meet with analogous facts on many
islands separated by similar channels from Australia. The West Indian
Islands stand on a deeply submerged bank, nearly 1000 fathoms in
depth, and here we find American forms, but the species and even the
genera are distinct. As the amount of modification in all cases
depends to a certain degree on the lapse of time, and as during
changes of level it is obvious that islands separated by shallow
channels are more likely to have been continuously united within a
recent period to the mainland than islands separated by deeper
channels, we can understand the frequent relation between the depth of
the sea and the degree of affinity of the mammalian inhabitants of
islands with those of a neighbouring continent,--an inexplicable
relation on the view of independent acts of creation.

All the foregoing remarks on the inhabitants of oceanic
islands,--namely, the scarcity of kinds--the richness in endemic forms
in particular classes or sections of classes,--the absence of whole
groups, as of batrachians, and of terrestrial mammals notwithstanding
the presence of aerial bats,--the singular proportions of certain
orders of plants,--herbaceous forms having been developed into trees,
etc.,--seem to me to accord better with the view of occasional means
of transport having been largely efficient in the long course of time,
than with the view of all our oceanic islands having been formerly
connected by continuous land with the nearest continent; for on this
latter view the migration would probably have been more complete; and
if modification be admitted, all the forms of life would have been
more equally modified, in accordance with the paramount importance of
the relation of organism to organism.

I do not deny that there are many and grave difficulties in
understanding how several of the inhabitants of the more remote
islands, whether still retaining the same specific form or modified
since their arrival, could have reached their present homes. But the
probability of many islands having existed as halting-places, of which
not a wreck now remains, must not be overlooked. I will here give a
single instance of one of the cases of difficulty. Almost all oceanic
islands, even the most isolated and smallest, are inhabited by
land-shells, generally by endemic species, but sometimes by species
found elsewhere. Dr. Aug. A. Gould has given several interesting cases
in regard to the land-shells of the islands of the Pacific. Now it is
notorious that land-shells are very easily killed by salt; their eggs,
at least such as I have tried, sink in sea-water and are killed by it.
Yet there must be, on my view, some unknown, but highly efficient
means for their transportal. Would the just-hatched young occasionally
crawl on and adhere to the feet of birds roosting on the ground, and
thus get transported? It occurred to me that land-shells, when
hybernating and having a membranous diaphragm over the mouth of the
shell, might be floated in chinks of drifted timber across moderately
wide arms of the sea. And I found that several species did in this
state withstand uninjured an immersion in sea-water during seven days:
one of these shells was the Helix pomatia, and after it had again
hybernated I put it in sea-water for twenty days, and it perfectly
recovered. As this species has a thick calcareous operculum, I removed
it, and when it had formed a new membranous one, I immersed it for
fourteen days in sea-water, and it recovered and crawled away: but
more experiments are wanted on this head. The most striking and
important fact for us in regard to the inhabitants of islands, is
their affinity to those of the nearest mainland, without being
actually the same species. Numerous instances could be given of this
fact. I will give only one, that of the Galapagos Archipelago,
situated under the equator, between 500 and 600 miles from the shores
of South America. Here almost every product of the land and water
bears the unmistakeable stamp of the American continent. There are
twenty-six land birds, and twenty-five of these are ranked by Mr.
Gould as distinct species, supposed to have been created here; yet the
close affinity of most of these birds to American species in every
character, in their habits, gestures, and tones of voice, was
manifest. So it is with the other animals, and with nearly all the
plants, as shown by Dr. Hooker in his admirable memoir on the Flora of
this archipelago. The naturalist, looking at the inhabitants of these
volcanic islands in the Pacific, distant several hundred miles from
the continent, yet feels that he is standing on American land. Why
should this be so? why should the species which are supposed to have
been created in the Galapagos Archipelago, and nowhere else, bear so
plain a stamp of affinity to those created in America? There is
nothing in the conditions of life, in the geological nature of the
islands, in their height or climate, or in the proportions in which
the several classes are associated together, which resembles closely
the conditions of the South American coast: in fact there is a
considerable dissimilarity in all these respects. On the other hand,
there is a considerable degree of resemblance in the volcanic nature
of the soil, in climate, height, and size of the islands, between the
Galapagos and Cape de Verde Archipelagos: but what an entire and
absolute difference in their inhabitants! The inhabitants of the Cape
de Verde Islands are related to those of Africa, like those of the
Galapagos to America. I believe this grand fact can receive no sort of
explanation on the ordinary view of independent creation; whereas on
the view here maintained, it is obvious that the Galapagos Islands
would be likely to receive colonists, whether by occasional means of
transport or by formerly continuous land, from America; and the Cape
de Verde Islands from Africa; and that such colonists would be liable
to modification;--the principle of inheritance still betraying their
original birthplace.

Many analogous facts could be given: indeed it is an almost universal
rule that the endemic productions of islands are related to those of
the nearest continent, or of other near islands. The exceptions are
few, and most of them can be explained. Thus the plants of Kerguelen
Land, though standing nearer to Africa than to America, are related,
and that very closely, as we know from Dr. Hooker's account, to those
of America: but on the view that this island has been mainly stocked
by seeds brought with earth and stones on icebergs, drifted by the
prevailing currents, this anomaly disappears. New Zealand in its
endemic plants is much more closely related to Australia, the nearest
mainland, than to any other region: and this is what might have been
expected; but it is also plainly related to South America, which,
although the next nearest continent, is so enormously remote, that the
fact becomes an anomaly. But this difficulty almost disappears on the
view that both New Zealand, South America, and other southern lands
were long ago partially stocked from a nearly intermediate though
distant point, namely from the antarctic islands, when they were
clothed with vegetation, before the commencement of the Glacial
period. The affinity, which, though feeble, I am assured by Dr. Hooker
is real, between the flora of the south-western corner of Australia
and of the Cape of Good Hope, is a far more remarkable case, and is at
present inexplicable: but this affinity is confined to the plants, and
will, I do not doubt, be some day explained.

The law which causes the inhabitants of an archipelago, though
specifically distinct, to be closely allied to those of the nearest
continent, we sometimes see displayed on a small scale, yet in a most
interesting manner, within the limits of the same archipelago. Thus
the several islands of the Galapagos Archipelago are tenanted, as I
have elsewhere shown, in a quite marvellous manner, by very closely
related species; so that the inhabitants of each separate island,
though mostly distinct, are related in an incomparably closer degree
to each other than to the inhabitants of any other part of the world.
And this is just what might have been expected on my view, for the
islands are situated so near each other that they would almost
certainly receive immigrants from the same original source, or from
each other. But this dissimilarity between the endemic inhabitants of
the islands may be used as an argument against my views; for it may be
asked, how has it happened in the several islands situated within
sight of each other, having the same geological nature, the same
height, climate, etc., that many of the immigrants should have been
differently modified, though only in a small degree. This long
appeared to me a great difficulty: but it arises in chief part from
the deeply-seated error of considering the physical conditions of a
country as the most important for its inhabitants; whereas it cannot,
I think, be disputed that the nature of the other inhabitants, with
which each has to compete, is at least as important, and generally a
far more important element of success. Now if we look to those
inhabitants of the Galapagos Archipelago which are found in other
parts of the world (laying on one side for the moment the endemic
species, which cannot be here fairly included, as we are considering
how they have come to be modified since their arrival), we find a
considerable amount of difference in the several islands. This
difference might indeed have been expected on the view of the islands
having been stocked by occasional means of transport--a seed, for
instance, of one plant having been brought to one island, and that of
another plant to another island. Hence when in former times an
immigrant settled on any one or more of the islands, or when it
subsequently spread from one island to another, it would undoubtedly
be exposed to different conditions of life in the different islands,
for it would have to compete with different sets of organisms: a
plant, for instance, would find the best-fitted ground more perfectly
occupied by distinct plants in one island than in another, and it
would be exposed to the attacks of somewhat different enemies. If then
it varied, natural selection would probably favour different varieties
in the different islands. Some species, however, might spread and yet
retain the same character throughout the group, just as we see on
continents some species spreading widely and remaining the same.

The really surprising fact in this case of the Galapagos Archipelago,
and in a lesser degree in some analogous instances, is that the new
species formed in the separate islands have not quickly spread to the
other islands. But the islands, though in sight of each other, are
separated by deep arms of the sea, in most cases wider than the
British Channel, and there is no reason to suppose that they have at
any former period been continuously united. The currents of the sea
are rapid and sweep across the archipelago, and gales of wind are
extraordinarily rare; so that the islands are far more effectually
separated from each other than they appear to be on a map.
Nevertheless a good many species, both those found in other parts of
the world and those confined to the archipelago, are common to the
several islands, and we may infer from certain facts that these have
probably spread from some one island to the others. But we often take,
I think, an erroneous view of the probability of closely allied
species invading each other's territory, when put into free
intercommunication. Undoubtedly if one species has any advantage
whatever over another, it will in a very brief time wholly or in part
supplant it; but if both are equally well fitted for their own places
in nature, both probably will hold their own places and keep separate
for almost any length of time. Being familiar with the fact that many
species, naturalised through man's agency, have spread with
astonishing rapidity over new countries, we are apt to infer that most
species would thus spread; but we should remember that the forms which
become naturalised in new countries are not generally closely allied
to the aboriginal inhabitants, but are very distinct species,
belonging in a large proportion of cases, as shown by Alph. de
Candolle, to distinct genera. In the Galapagos Archipelago, many even
of the birds, though so well adapted for flying from island to island,
are distinct on each; thus there are three closely-allied species of
mocking-thrush, each confined to its own island. Now let us suppose
the mocking-thrush of Chatham Island to be blown to Charles Island,
which has its own mocking-thrush: why should it succeed in
establishing itself there? We may safely infer that Charles Island is
well stocked with its own species, for annually more eggs are laid
there than can possibly be reared; and we may infer that the
mocking-thrush peculiar to Charles Island is at least as well fitted
for its home as is the species peculiar to Chatham Island. Sir C.
Lyell and Mr. Wollaston have communicated to me a remarkable fact
bearing on this subject; namely, that Madeira and the adjoining islet
of Porto Santo possess many distinct but representative land-shells,
some of which live in crevices of stone; and although large quantities
of stone are annually transported from Porto Santo to Madeira, yet
this latter island has not become colonised by the Porto Santo
species: nevertheless both islands have been colonised by some
European land-shells, which no doubt had some advantage over the
indigenous species. From these considerations I think we need not
greatly marvel at the endemic and representative species, which
inhabit the several islands of the Galapagos Archipelago, not having
universally spread from island to island. In many other instances, as
in the several districts of the same continent, pre-occupation has
probably played an important part in checking the commingling of
species under the same conditions of life. Thus, the south-east and
south-west corners of Australia have nearly the same physical
conditions, and are united by continuous land, yet they are inhabited
by a vast number of distinct mammals, birds, and plants.

The principle which determines the general character of the fauna and
flora of oceanic islands, namely, that the inhabitants, when not
identically the same, yet are plainly related to the inhabitants of
that region whence colonists could most readily have been
derived,--the colonists having been subsequently modified and better
fitted to their new homes,--is of the widest application throughout
nature. We see this on every mountain, in every lake and marsh. For
Alpine species, excepting in so far as the same forms, chiefly of
plants, have spread widely throughout the world during the recent
Glacial epoch, are related to those of the surrounding lowlands;--thus
we have in South America, Alpine humming-birds, Alpine rodents, Alpine
plants, etc., all of strictly American forms, and it is obvious that a
mountain, as it became slowly upheaved, would naturally be colonised
from the surrounding lowlands. So it is with the inhabitants of lakes
and marshes, excepting in so far as great facility of transport has
given the same general forms to the whole world. We see this same
principle in the blind animals inhabiting the caves of America and of
Europe. Other analogous facts could be given. And it will, I believe,
be universally found to be true, that wherever in two regions, let
them be ever so distant, many closely allied or representative species
occur, there will likewise be found some identical species, showing,
in accordance with the foregoing view, that at some former period
there has been intercommunication or migration between the two
regions. And wherever many closely-allied species occur, there will be
found many forms which some naturalists rank as distinct species, and
some as varieties; these doubtful forms showing us the steps in the
process of modification.

This relation between the power and extent of migration of a species,
either at the present time or at some former period under different
physical conditions, and the existence at remote points of the world
of other species allied to it, is shown in another and more general
way. Mr. Gould remarked to me long ago, that in those genera of birds
which range over the world, many of the species have very wide ranges.
I can hardly doubt that this rule is generally true, though it would
be difficult to prove it. Amongst mammals, we see it strikingly
displayed in Bats, and in a lesser degree in the Felidae and Canidae.
We see it, if we compare the distribution of butterflies and beetles.
So it is with most fresh-water productions, in which so many genera
range over the world, and many individual species have enormous
ranges. It is not meant that in world-ranging genera all the species
have a wide range, or even that they have on an AVERAGE a wide range;
but only that some of the species range very widely; for the facility
with which widely-ranging species vary and give rise to new forms will
largely determine their average range. For instance, two varieties of
the same species inhabit America and Europe, and the species thus has
an immense range; but, if the variation had been a little greater, the
two varieties would have been ranked as distinct species, and the
common range would have been greatly reduced. Still less is it meant,
that a species which apparently has the capacity of crossing barriers
and ranging widely, as in the case of certain powerfully-winged birds,
will necessarily range widely; for we should never forget that to
range widely implies not only the power of crossing barriers, but the
more important power of being victorious in distant lands in the
struggle for life with foreign associates. But on the view of all the
species of a genus having descended from a single parent, though now
distributed to the most remote points of the world, we ought to find,
and I believe as a general rule we do find, that some at least of the
species range very widely; for it is necessary that the unmodified
parent should range widely, undergoing modification during its
diffusion, and should place itself under diverse conditions favourable
for the conversion of its offspring, firstly into new varieties and
ultimately into new species.

In considering the wide distribution of certain genera, we should bear
in mind that some are extremely ancient, and must have branched off
from a common parent at a remote epoch; so that in such cases there
will have been ample time for great climatal and geographical changes
and for accidents of transport; and consequently for the migration of
some of the species into all quarters of the world, where they may
have become slightly modified in relation to their new conditions.
There is, also, some reason to believe from geological evidence that
organisms low in the scale within each great class, generally change
at a slower rate than the higher forms; and consequently the lower
forms will have had a better chance of ranging widely and of still
retaining the same specific character. This fact, together with the
seeds and eggs of many low forms being very minute and better fitted
for distant transportation, probably accounts for a law which has long
been observed, and which has lately been admirably discussed by Alph.
de Candolle in regard to plants, namely, that the lower any group of
organisms is, the more widely it is apt to range.

The relations just discussed,--namely, low and slowly-changing
organisms ranging more widely than the high,--some of the species of
widely-ranging genera themselves ranging widely,--such facts, as
alpine, lacustrine, and marsh productions being related (with the
exceptions before specified) to those on the surrounding low lands and
dry lands, though these stations are so different--the very close
relation of the distinct species which inhabit the islets of the same
archipelago,--and especially the striking relation of the inhabitants
of each whole archipelago or island to those of the nearest
mainland,--are, I think, utterly inexplicable on the ordinary view of
the independent creation of each species, but are explicable on the
view of colonisation from the nearest and readiest source, together
with the subsequent modification and better adaptation of the
colonists to their new homes.

SUMMARY OF LAST AND PRESENT CHAPTERS.

In these chapters I have endeavoured to show, that if we make due
allowance for our ignorance of the full effects of all the changes of
climate and of the level of the land, which have certainly occurred
within the recent period, and of other similar changes which may have
occurred within the same period; if we remember how profoundly
ignorant we are with respect to the many and curious means of
occasional transport,--a subject which has hardly ever been properly
experimentised on; if we bear in mind how often a species may have
ranged continuously over a wide area, and then have become extinct in
the intermediate tracts, I think the difficulties in believing that
all the individuals of the same species, wherever located, have
descended from the same parents, are not insuperable. And we are led
to this conclusion, which has been arrived at by many naturalists
under the designation of single centres of creation, by some general
considerations, more especially from the importance of barriers and
from the analogical distribution of sub-genera, genera, and families.

With respect to the distinct species of the same genus, which on my
theory must have spread from one parent-source; if we make the same
allowances as before for our ignorance, and remember that some forms
of life change most slowly, enormous periods of time being thus
granted for their migration, I do not think that the difficulties are
insuperable; though they often are in this case, and in that of the
individuals of the same species, extremely grave.

As exemplifying the effects of climatal changes on distribution, I
have attempted to show how important has been the influence of the
modern Glacial period, which I am fully convinced simultaneously
affected the whole world, or at least great meridional belts. As
showing how diversified are the means of occasional transport, I have
discussed at some little length the means of dispersal of fresh-water
productions.

If the difficulties be not insuperable in admitting that in the long
course of time the individuals of the same species, and likewise of
allied species, have proceeded from some one source; then I think all
the grand leading facts of geographical distribution are explicable on
the theory of migration (generally of the more dominant forms of
life), together with subsequent modification and the multiplication of
new forms. We can thus understand the high importance of barriers,
whether of land or water, which separate our several zoological and
botanical provinces. We can thus understand the localisation of
sub-genera, genera, and families; and how it is that under different
latitudes, for instance in South America, the inhabitants of the
plains and mountains, of the forests, marshes, and deserts, are in so
mysterious a manner linked together by affinity, and are likewise
linked to the extinct beings which formerly inhabited the same
continent. Bearing in mind that the mutual relations of organism to
organism are of the highest importance, we can see why two areas
having nearly the same physical conditions should often be inhabited
by very different forms of life; for according to the length of time
which has elapsed since new inhabitants entered one region; according
to the nature of the communication which allowed certain forms and not
others to enter, either in greater or lesser numbers; according or
not, as those which entered happened to come in more or less direct
competition with each other and with the aborigines; and according as
the immigrants were capable of varying more or less rapidly, there
would ensue in different regions, independently of their physical
conditions, infinitely diversified conditions of life,--there would be
an almost endless amount of organic action and reaction,--and we
should find, as we do find, some groups of beings greatly, and some
only slightly modified,--some developed in great force, some existing
in scanty numbers--in the different great geographical provinces of
the world.

On these same principles, we can understand, as I have endeavoured to
show, why oceanic islands should have few inhabitants, but of these a
great number should be endemic or peculiar; and why, in relation to
the means of migration, one group of beings, even within the same
class, should have all its species endemic, and another group should
have all its species common to other quarters of the world. We can see
why whole groups of organisms, as batrachians and terrestrial mammals,
should be absent from oceanic islands, whilst the most isolated
islands possess their own peculiar species of aerial mammals or bats.
We can see why there should be some relation between the presence of
mammals, in a more or less modified condition, and the depth of the
sea between an island and the mainland. We can clearly see why all the
inhabitants of an archipelago, though specifically distinct on the
several islets, should be closely related to each other, and likewise
be related, but less closely, to those of the nearest continent or
other source whence immigrants were probably derived. We can see why
in two areas, however distant from each other, there should be a
correlation, in the presence of identical species, of varieties, of
doubtful species, and of distinct but representative species.

As the late Edward Forbes often insisted, there is a striking
parallelism in the laws of life throughout time and space: the laws
governing the succession of forms in past times being nearly the same
with those governing at the present time the differences in different
areas. We see this in many facts. The endurance of each species and
group of species is continuous in time; for the exceptions to the rule
are so few, that they may fairly be attributed to our not having as
yet discovered in an intermediate deposit the forms which are therein
absent, but which occur above and below: so in space, it certainly is
the general rule that the area inhabited by a single species, or by a
group of species, is continuous; and the exceptions, which are not
rare, may, as I have attempted to show, be accounted for by migration
at some former period under different conditions or by occasional
means of transport, and by the species having become extinct in the
intermediate tracts. Both in time and space, species and groups of
species have their points of maximum development. Groups of species,
belonging either to a certain period of time, or to a certain area,
are often characterised by trifling characters in common, as of
sculpture or colour. In looking to the long succession of ages, as in
now looking to distant provinces throughout the world, we find that
some organisms differ little, whilst others belonging to a different
class, or to a different order, or even only to a different family of
the same order, differ greatly. In both time and space the lower
members of each class generally change less than the higher; but there
are in both cases marked exceptions to the rule. On my theory these
several relations throughout time and space are intelligible; for
whether we look to the forms of life which have changed during
successive ages within the same quarter of the world, or to those
which have changed after having migrated into distant quarters, in
both cases the forms within each class have been connected by the same
bond of ordinary generation; and the more nearly any two forms are
related in blood, the nearer they will generally stand to each other
in time and space; in both cases the laws of variation have been the
same, and modifications have been accumulated by the same power of
natural selection.


CHAPTER 13. MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY:
EMBRYOLOGY: RUDIMENTARY ORGANS.

CLASSIFICATION, groups subordinate to groups.
Natural system.
Rules and difficulties in classification, explained on the theory of
descent with modification.
Classification of varieties.
Descent always used in classification.
Analogical or adaptive characters.
Affinities, general, complex and radiating.
Extinction separates and defines groups.
MORPHOLOGY, between members of the same class, between parts of the
same individual.
EMBRYOLOGY, laws of, explained by variations not supervening at an
early age, and being inherited at a corresponding age.
RUDIMENTARY ORGANS; their origin explained.
Summary.

From the first dawn of life, all organic beings are found to resemble
each other in descending degrees, so that they can be classed in
groups under groups. This classification is evidently not arbitrary
like the grouping of the stars in constellations. The existence of
groups would have been of simple signification, if one group had been
exclusively fitted to inhabit the land, and another the water; one to
feed on flesh, another on vegetable matter, and so on; but the case is
widely different in nature; for it is notorious how commonly members
of even the same subgroup have different habits. In our second and
fourth chapters, on Variation and on Natural Selection, I have
attempted to show that it is the widely ranging, the much diffused and
common, that is the dominant species belonging to the larger genera,
which vary most. The varieties, or incipient species, thus produced
ultimately become converted, as I believe, into new and distinct
species; and these, on the principle of inheritance, tend to produce
other new and dominant species. Consequently the groups which are now
large, and which generally include many dominant species, tend to go
on increasing indefinitely in size. I further attempted to show that
from the varying descendants of each species trying to occupy as many
and as different places as possible in the economy of nature, there is
a constant tendency in their characters to diverge. This conclusion
was supported by looking at the great diversity of the forms of life
which, in any small area, come into the closest competition, and by
looking to certain facts in naturalisation.

I attempted also to show that there is a constant tendency in the
forms which are increasing in number and diverging in character, to
supplant and exterminate the less divergent, the less improved, and
preceding forms. I request the reader to turn to the diagram
illustrating the action, as formerly explained, of these several
principles; and he will see that the inevitable result is that the
modified descendants proceeding from one progenitor become broken up
into groups subordinate to groups. In the diagram each letter on the
uppermost line may represent a genus including several species; and
all the genera on this line form together one class, for all have
descended from one ancient but unseen parent, and, consequently, have
inherited something in common. But the three genera on the left hand
have, on this same principle, much in common, and form a sub-family,
distinct from that including the next two genera on the right hand,
which diverged from a common parent at the fifth stage of descent.
These five genera have also much, though less, in common; and they
form a family distinct from that including the three genera still
further to the right hand, which diverged at a still earlier period.
And all these genera, descended from (A), form an order distinct from
the genera descended from (I). So that we here have many species
descended from a single progenitor grouped into genera; and the genera
are included in, or subordinate to, sub-families, families, and
orders, all united into one class. Thus, the grand fact in natural
history of the subordination of group under group, which, from its
familiarity, does not always sufficiently strike us, is in my judgment
fully explained.

Naturalists try to arrange the species, genera, and families in each
class, on what is called the Natural System. But what is meant by this
system? Some authors look at it merely as a scheme for arranging
together those living objects which are most alike, and for separating
those which are most unlike; or as an artificial means for
enunciating, as briefly as possible, general propositions,--that is,
by one sentence to give the characters common, for instance, to all
mammals, by another those common to all carnivora, by another those
common to the dog-genus, and then by adding a single sentence, a full
description is given of each kind of dog. The ingenuity and utility of
this system are indisputable. But many naturalists think that
something more is meant by the Natural System; they believe that it
reveals the plan of the Creator; but unless it be specified whether
order in time or space, or what else is meant by the plan of the
Creator, it seems to me that nothing is thus added to our knowledge.
Such expressions as that famous one of Linnaeus, and which we often
meet with in a more or less concealed form, that the characters do not
make the genus, but that the genus gives the characters, seem to imply
that something more is included in our classification, than mere
resemblance. I believe that something more is included; and that
propinquity of descent,--the only known cause of the similarity of
organic beings,--is the bond, hidden as it is by various degrees of
modification, which is partially revealed to us by our
classifications.

Let us now consider the rules followed in classification, and the
difficulties which are encountered on the view that classification
either gives some unknown plan of creation, or is simply a scheme for
enunciating general propositions and of placing together the forms
most like each other. It might have been thought (and was in ancient
times thought) that those parts of the structure which determined the
habits of life, and the general place of each being in the economy of
nature, would be of very high importance in classification. Nothing
can be more false. No one regards the external similarity of a mouse
to a shrew, of a dugong to a whale, of a whale to a fish, as of any
importance. These resemblances, though so intimately connected with
the whole life of the being, are ranked as merely "adaptive or
analogical characters;" but to the consideration of these resemblances
we shall have to recur. It may even be given as a general rule, that
the less any part of the organisation is concerned with special
habits, the more important it becomes for classification. As an
instance: Owen, in speaking of the dugong, says, "The generative
organs being those which are most remotely related to the habits and
food of an animal, I have always regarded as affording very clear
indications of its true affinities. We are least likely in the
modifications of these organs to mistake a merely adaptive for an
essential character." So with plants, how remarkable it is that the
organs of vegetation, on which their whole life depends, are of little
signification, excepting in the first main divisions; whereas the
organs of reproduction, with their product the seed, are of paramount
importance!

We must not, therefore, in classifying, trust to resemblances in parts
of the organisation, however important they may be for the welfare of
the being in relation to the outer world. Perhaps from this cause it
has partly arisen, that almost all naturalists lay the greatest stress
on resemblances in organs of high vital or physiological importance.
No doubt this view of the classificatory importance of organs which
are important is generally, but by no means always, true. But their
importance for classification, I believe, depends on their greater
constancy throughout large groups of species; and this constancy
depends on such organs having generally been subjected to less change
in the adaptation of the species to their conditions of life. That the
mere physiological importance of an organ does not determine its
classificatory value, is almost shown by the one fact, that in allied
groups, in which the same organ, as we have every reason to suppose,
has nearly the same physiological value, its classificatory value is
widely different. No naturalist can have worked at any group without
being struck with this fact; and it has been most fully acknowledged
in the writings of almost every author. It will suffice to quote the
highest authority, Robert Brown, who in speaking of certain organs in
the Proteaceae, says their generic importance, "like that of all their
parts, not only in this but, as I apprehend, in every natural family,
is very unequal, and in some cases seems to be entirely lost." Again
in another work he says, the genera of the Connaraceae "differ in
having one or more ovaria, in the existence or absence of albumen, in
the imbricate or valvular aestivation. Any one of these characters
singly is frequently of more than generic importance, though here even
when all taken together they appear insufficient to separate Cnestis
from Connarus." To give an example amongst insects, in one great
division of the Hymenoptera, the antennae, as Westwood has remarked,
are most constant in structure; in another division they differ much,
and the differences are of quite subordinate value in classification;
yet no one probably will say that the antennae in these two divisions
of the same order are of unequal physiological importance. Any number
of instances could be given of the varying importance for
classification of the same important organ within the same group of
beings.

Again, no one will say that rudimentary or atrophied organs are of
high physiological or vital importance; yet, undoubtedly, organs in
this condition are often of high value in classification. No one will
dispute that the rudimentary teeth in the upper jaws of young
ruminants, and certain rudimentary bones of the leg, are highly
serviceable in exhibiting the close affinity between Ruminants and
Pachyderms. Robert Brown has strongly insisted on the fact that the
rudimentary florets are of the highest importance in the
classification of the Grasses.

Numerous instances could be given of characters derived from parts
which must be considered of very trifling physiological importance,
but which are universally admitted as highly serviceable in the
definition of whole groups. For instance, whether or not there is an
open passage from the nostrils to the mouth, the only character,
according to Owen, which absolutely distinguishes fishes and
reptiles--the inflection of the angle of the jaws in Marsupials--the
manner in which the wings of insects are folded--mere colour in
certain Algae--mere pubescence on parts of the flower in grasses--the
nature of the dermal covering, as hair or feathers, in the Vertebrata.
If the Ornithorhynchus had been covered with feathers instead of hair,
this external and trifling character would, I think, have been
considered by naturalists as important an aid in determining the
degree of affinity of this strange creature to birds and reptiles, as
an approach in structure in any one internal and important organ.

The importance, for classification, of trifling characters, mainly
depends on their being correlated with several other characters of
more or less importance. The value indeed of an aggregate of
characters is very evident in natural history. Hence, as has often
been remarked, a species may depart from its allies in several
characters, both of high physiological importance and of almost
universal prevalence, and yet leave us in no doubt where it should be
ranked. Hence, also, it has been found, that a classification founded
on any single character, however important that may be, has always
failed; for no part of the organisation is universally constant. The
importance of an aggregate of characters, even when none are
important, alone explains, I think, that saying of Linnaeus, that the
characters do not give the genus, but the genus gives the characters;
for this saying seems founded on an appreciation of many trifling
points of resemblance, too slight to be defined. Certain plants,
belonging to the Malpighiaceae, bear perfect and degraded flowers; in
the latter, as A. de Jussieu has remarked, "the greater number of the
characters proper to the species, to the genus, to the family, to the
class, disappear, and thus laugh at our classification." But when
Aspicarpa produced in France, during several years, only degraded
flowers, departing so wonderfully in a number of the most important
points of structure from the proper type of the order, yet M. Richard
sagaciously saw, as Jussieu observes, that this genus should still be
retained amongst the Malpighiaceae. This case seems to me well to
illustrate the spirit with which our classifications are sometimes
necessarily founded.

Practically when naturalists are at work, they do not trouble
themselves about the physiological value of the characters which they
use in defining a group, or in allocating any particular species. If
they find a character nearly uniform, and common to a great number of
forms, and not common to others, they use it as one of high value; if
common to some lesser number, they use it as of subordinate value.
This principle has been broadly confessed by some naturalists to be
the true one; and by none more clearly than by that excellent
botanist, Aug. St. Hilaire. If certain characters are always found
correlated with others, though no apparent bond of connexion can be
discovered between them, especial value is set on them. As in most
groups of animals, important organs, such as those for propelling the
blood, or for aerating it, or those for propagating the race, are
found nearly uniform, they are considered as highly serviceable in
classification; but in some groups of animals all these, the most
important vital organs, are found to offer characters of quite
subordinate value.

We can see why characters derived from the embryo should be of equal
importance with those derived from the adult, for our classifications
of course include all ages of each species. But it is by no means
obvious, on the ordinary view, why the structure of the embryo should
be more important for this purpose than that of the adult, which alone
plays its full part in the economy of nature. Yet it has been strongly
urged by those great naturalists, Milne Edwards and Agassiz, that
embryonic characters are the most important of any in the
classification of animals; and this doctrine has very generally been
admitted as true. The same fact holds good with flowering plants, of
which the two main divisions have been founded on characters derived
from the embryo,--on the number and position of the embryonic leaves
or cotyledons, and on the mode of development of the plumule and
radicle. In our discussion on embryology, we shall see why such
characters are so valuable, on the view of classification tacitly
including the idea of descent.

Our classifications are often plainly influenced by chains of
affinities. Nothing can be easier than to define a number of
characters common to all birds; but in the case of crustaceans, such
definition has hitherto been found impossible. There are crustaceans
at the opposite ends of the series, which have hardly a character in
common; yet the species at both ends, from being plainly allied to
others, and these to others, and so onwards, can be recognised as
unequivocally belonging to this, and to no other class of the
Articulata.

Geographical distribution has often been used, though perhaps not
quite logically, in classification, more especially in very large
groups of closely allied forms. Temminck insists on the utility or
even necessity of this practice in certain groups of birds; and it has
been followed by several entomologists and botanists.

Finally, with respect to the comparative value of the various groups
of species, such as orders, sub-orders, families, sub-families, and
genera, they seem to be, at least at present, almost arbitrary.
Several of the best botanists, such as Mr. Bentham and others, have
strongly insisted on their arbitrary value. Instances could be given
amongst plants and insects, of a group of forms, first ranked by
practised naturalists as only a genus, and then raised to the rank of
a sub-family or family; and this has been done, not because further
research has detected important structural differences, at first
overlooked, but because numerous allied species, with slightly
different grades of difference, have been subsequently discovered.

All the foregoing rules and aids and difficulties in classification
are explained, if I do not greatly deceive myself, on the view that
the natural system is founded on descent with modification; that the
characters which naturalists consider as showing true affinity between
any two or more species, are those which have been inherited from a
common parent, and, in so far, all true classification is
genealogical; that community of descent is the hidden bond which
naturalists have been unconsciously seeking, and not some unknown plan
of creation, or the enunciation of general propositions, and the mere
putting together and separating objects more or less alike.

But I must explain my meaning more fully. I believe that the
ARRANGEMENT of the groups within each class, in due subordination and
relation to the other groups, must be strictly genealogical in order
to be natural; but that the AMOUNT of difference in the several
branches or groups, though allied in the same degree in blood to their
common progenitor, may differ greatly, being due to the different
degrees of modification which they have undergone; and this is
expressed by the forms being ranked under different genera, families,
sections, or orders. The reader will best understand what is meant, if
he will take the trouble of referring to the diagram in the fourth
chapter. We will suppose the letters A to L to represent allied
genera, which lived during the Silurian epoch, and these have
descended from a species which existed at an unknown anterior period.
Species of three of these genera (A, F, and I) have transmitted
modified descendants to the present day, represented by the fifteen
genera (a14 to z14) on the uppermost horizontal line. Now all these
modified descendants from a single species, are represented as related
in blood or descent to the same degree; they may metaphorically be
called cousins to the same millionth degree; yet they differ widely
and in different degrees from each other. The forms descended from A,
now broken up into two or three families, constitute a distinct order
from those descended from I, also broken up into two families. Nor can
the existing species, descended from A, be ranked in the same genus
with the parent A; or those from I, with the parent I. But the
existing genus F14 may be supposed to have been but slightly modified;
and it will then rank with the parent-genus F; just as some few still
living organic beings belong to Silurian genera. So that the amount or
value of the differences between organic beings all related to each
other in the same degree in blood, has come to be widely different.
Nevertheless their genealogical ARRANGEMENT remains strictly true, not
only at the present time, but at each successive period of descent.
All the modified descendants from A will have inherited something in
common from their common parent, as will all the descendants from I;
so will it be with each subordinate branch of descendants, at each
successive period. If, however, we choose to suppose that any of the
descendants of A or of I have been so much modified as to have more or
less completely lost traces of their parentage, in this case, their
places in a natural classification will have been more or less
completely lost,--as sometimes seems to have occurred with existing
organisms. All the descendants of the genus F, along its whole line of
descent, are supposed to have been but little modified, and they yet
form a single genus. But this genus, though much isolated, will still
occupy its proper intermediate position; for F originally was
intermediate in character between A and I, and the several genera
descended from these two genera will have inherited to a certain
extent their characters. This natural arrangement is shown, as far as
is possible on paper, in the diagram, but in much too simple a manner.
If a branching diagram had not been used, and only the names of the
groups had been written in a linear series, it would have been still
less possible to have given a natural arrangement; and it is
notoriously not possible to represent in a series, on a flat surface,
the affinities which we discover in nature amongst the beings of the
same group. Thus, on the view which I hold, the natural system is
genealogical in its arrangement, like a pedigree; but the degrees of
modification which the different groups have undergone, have to be
expressed by ranking them under different so-called genera,
sub-families, families, sections, orders, and classes.

It may be worth while to illustrate this view of classification, by
taking the case of languages. If we possessed a perfect pedigree of
mankind, a genealogical arrangement of the races of man would afford
the best classification of the various languages now spoken throughout
the world; and if all extinct languages, and all intermediate and
slowly changing dialects, had to be included, such an arrangement
would, I think, be the only possible one. Yet it might be that some
very ancient language had altered little, and had given rise to few
new languages, whilst others (owing to the spreading and subsequent
isolation and states of civilisation of the several races, descended
from a common race) had altered much, and had given rise to many new
languages and dialects. The various degrees of difference in the
languages from the same stock, would have to be expressed by groups
subordinate to groups; but the proper or even only possible
arrangement would still be genealogical; and this would be strictly
natural, as it would connect together all languages, extinct and
modern, by the closest affinities, and would give the filiation and
origin of each tongue.

In confirmation of this view, let us glance at the classification of
varieties, which are believed or known to have descended from one
species. These are grouped under species, with sub-varieties under
varieties; and with our domestic productions, several other grades of
difference are requisite, as we have seen with pigeons. The origin of
the existence of groups subordinate to groups, is the same with
varieties as with species, namely, closeness of descent with various
degrees of modification. Nearly the same rules are followed in
classifying varieties, as with species. Authors have insisted on the
necessity of classing varieties on a natural instead of an artificial
system; we are cautioned, for instance, not to class two varieties of
the pine-apple together, merely because their fruit, though the most
important part, happens to be nearly identical; no one puts the
swedish and common turnips together, though the esculent and thickened
stems are so similar. Whatever part is found to be most constant, is
used in classing varieties: thus the great agriculturist Marshall says
the horns are very useful for this purpose with cattle, because they
are less variable than the shape or colour of the body, etc.; whereas
with sheep the horns are much less serviceable, because less constant.
In classing varieties, I apprehend if we had a real pedigree, a
genealogical classification would be universally preferred; and it has
been attempted by some authors. For we might feel sure, whether there
had been more or less modification, the principle of inheritance would
keep the forms together which were allied in the greatest number of
points. In tumbler pigeons, though some sub-varieties differ from the
others in the important character of having a longer beak, yet all are
kept together from having the common habit of tumbling; but the
short-faced breed has nearly or quite lost this habit; nevertheless,
without any reasoning or thinking on the subject, these tumblers are
kept in the same group, because allied in blood and alike in some
other respects. If it could be proved that the Hottentot had descended
from the Negro, I think he would be classed under the Negro group,
however much he might differ in colour and other important characters
from negroes.

With species in a state of nature, every naturalist has in fact
brought descent into his classification; for he includes in his lowest
grade, or that of a species, the two sexes; and how enormously these
sometimes differ in the most important characters, is known to every
naturalist: scarcely a single fact can be predicated in common of the
males and hermaphrodites of certain cirripedes, when adult, and yet no
one dreams of separating them. The naturalist includes as one species
the several larval stages of the same individual, however much they
may differ from each other and from the adult; as he likewise includes
the so-called alternate generations of Steenstrup, which can only in a
technical sense be considered as the same individual. He includes
monsters; he includes varieties, not solely because they closely
resemble the parent-form, but because they are descended from it. He
who believes that the cowslip is descended from the primrose, or
conversely, ranks them together as a single species, and gives a
single definition. As soon as three Orchidean forms (Monochanthus,
Myanthus, and Catasetum), which had previously been ranked as three
distinct genera, were known to be sometimes produced on the same
spike, they were immediately included as a single species. But it may
be asked, what ought we to do, if it could be proved that one species
of kangaroo had been produced, by a long course of modification, from
a bear? Ought we to rank this one species with bears, and what should
we do with the other species? The supposition is of course
preposterous; and I might answer by the argumentum ad hominem, and ask
what should be done if a perfect kangaroo were seen to come out of the
womb of a bear? According to all analogy, it would be ranked with
bears; but then assuredly all the other species of the kangaroo family
would have to be classed under the bear genus. The whole case is
preposterous; for where there has been close descent in common, there
will certainly be close resemblance or affinity.

As descent has universally been used in classing together the
individuals of the same species, though the males and females and
larvae are sometimes extremely different; and as it has been used in
classing varieties which have undergone a certain, and sometimes a
considerable amount of modification, may not this same element of
descent have been unconsciously used in grouping species under genera,
and genera under higher groups, though in these cases the modification
has been greater in degree, and has taken a longer time to complete? I
believe it has thus been unconsciously used; and only thus can I
understand the several rules and guides which have been followed by
our best systematists. We have no written pedigrees; we have to make
out community of descent by resemblances of any kind. Therefore we
choose those characters which, as far as we can judge, are the least
likely to have been modified in relation to the conditions of life to
which each species has been recently exposed. Rudimentary structures
on this view are as good as, or even sometimes better than, other
parts of the organisation. We care not how trifling a character may
be--let it be the mere inflection of the angle of the jaw, the manner
in which an insect's wing is folded, whether the skin be covered by
hair or feathers--if it prevail throughout many and different species,
especially those having very different habits of life, it assumes high
value; for we can account for its presence in so many forms with such
different habits, only by its inheritance from a common parent. We may
err in this respect in regard to single points of structure, but when
several characters, let them be ever so trifling, occur together
throughout a large group of beings having different habits, we may
feel almost sure, on the theory of descent, that these characters have
been inherited from a common ancestor. And we know that such
correlated or aggregated characters have especial value in
classification.

We can understand why a species or a group of species may depart, in
several of its most important characteristics, from its allies, and
yet be safely classed with them. This may be safely done, and is often
done, as long as a sufficient number of characters, let them be ever
so unimportant, betrays the hidden bond of community of descent. Let
two forms have not a single character in common, yet if these extreme
forms are connected together by a chain of intermediate groups, we may
at once infer their community of descent, and we put them all into the
same class. As we find organs of high physiological importance--those
which serve to preserve life under the most diverse conditions of
existence--are generally the most constant, we attach especial value
to them; but if these same organs, in another group or section of a
group, are found to differ much, we at once value them less in our
classification. We shall hereafter, I think, clearly see why
embryological characters are of such high classificatory importance.
Geographical distribution may sometimes be brought usefully into play
in classing large and widely-distributed genera, because all the
species of the same genus, inhabiting any distinct and isolated
region, have in all probability descended from the same parents.

We can understand, on these views, the very important distinction
between real affinities and analogical or adaptive resemblances.
Lamarck first called attention to this distinction, and he has been
ably followed by Macleay and others. The resemblance, in the shape of
the body and in the fin-like anterior limbs, between the dugong, which
is a pachydermatous animal, and the whale, and between both these
mammals and fishes, is analogical. Amongst insects there are
innumerable instances: thus Linnaeus, misled by external appearances,
actually classed an homopterous insect as a moth. We see something of
the same kind even in our domestic varieties, as in the thickened
stems of the common and swedish turnip. The resemblance of the
greyhound and racehorse is hardly more fanciful than the analogies
which have been drawn by some authors between very distinct animals.
On my view of characters being of real importance for classification,
only in so far as they reveal descent, we can clearly understand why
analogical or adaptive character, although of the utmost importance to
the welfare of the being, are almost valueless to the systematist. For
animals, belonging to two most distinct lines of descent, may readily
become adapted to similar conditions, and thus assume a close external
resemblance; but such resemblances will not reveal--will rather tend
to conceal their blood-relationship to their proper lines of descent.
We can also understand the apparent paradox, that the very same
characters are analogical when one class or order is compared with
another, but give true affinities when the members of the same class
or order are compared one with another: thus the shape of the body and
fin-like limbs are only analogical when whales are compared with
fishes, being adaptations in both classes for swimming through the
water; but the shape of the body and fin-like limbs serve as
characters exhibiting true affinity between the several members of the
whale family; for these cetaceans agree in so many characters, great
and small, that we cannot doubt that they have inherited their general
shape of body and structure of limbs from a common ancestor. So it is
with fishes.

As members of distinct classes have often been adapted by successive
slight modifications to live under nearly similar circumstances,--to
inhabit for instance the three elements of land, air, and water,--we
can perhaps understand how it is that a numerical parallelism has
sometimes been observed between the sub-groups in distinct classes. A
naturalist, struck by a parallelism of this nature in any one class,
by arbitrarily raising or sinking the value of the groups in other
classes (and all our experience shows that this valuation has hitherto
been arbitrary), could easily extend the parallelism over a wide
range; and thus the septenary, quinary, quaternary, and ternary
classifications have probably arisen.

As the modified descendants of dominant species, belonging to the
larger genera, tend to inherit the advantages, which made the groups
to which they belong large and their parents dominant, they are almost
sure to spread widely, and to seize on more and more places in the
economy of nature. The larger and more dominant groups thus tend to go
on increasing in size; and they consequently supplant many smaller and
feebler groups. Thus we can account for the fact that all organisms,
recent and extinct, are included under a few great orders, under still
fewer classes, and all in one great natural system. As showing how few
the higher groups are in number, and how widely spread they are
throughout the world, the fact is striking, that the discovery of
Australia has not added a single insect belonging to a new order; and
that in the vegetable kingdom, as I learn from Dr. Hooker, it has
added only two or three orders of small size.

In the chapter on geological succession I attempted to show, on the
principle of each group having generally diverged much in character
during the long-continued process of modification, how it is that the
more ancient forms of life often present characters in some slight
degree intermediate between existing groups. A few old and
intermediate parent-forms having occasionally transmitted to the
present day descendants but little modified, will give to us our
so-called osculant or aberrant groups. The more aberrant any form is,
the greater must be the number of connecting forms which on my theory
have been exterminated and utterly lost. And we have some evidence of
aberrant forms having suffered severely from extinction, for they are
generally represented by extremely few species; and such species as do
occur are generally very distinct from each other, which again implies
extinction. The genera Ornithorhynchus and Lepidosiren, for example,
would not have been less aberrant had each been represented by a dozen
species instead of by a single one; but such richness in species, as I
find after some investigation, does not commonly fall to the lot of
aberrant genera. We can, I think, account for this fact only by
looking at aberrant forms as failing groups conquered by more
successful competitors, with a few members preserved by some unusual
coincidence of favourable circumstances.

Mr. Waterhouse has remarked that, when a member belonging to one group
of animals exhibits an affinity to a quite distinct group, this
affinity in most cases is general and not special: thus, according to
Mr. Waterhouse, of all Rodents, the bizcacha is most nearly related to
Marsupials; but in the points in which it approaches this order, its
relations are general, and not to any one marsupial species more than
to another. As the points of affinity of the bizcacha to Marsupials
are believed to be real and not merely adaptive, they are due on my
theory to inheritance in common. Therefore we must suppose either that
all Rodents, including the bizcacha, branched off from some very
ancient Marsupial, which will have had a character in some degree
intermediate with respect to all existing Marsupials; or that both
Rodents and Marsupials branched off from a common progenitor, and that
both groups have since undergone much modification in divergent
directions. On either view we may suppose that the bizcacha has
retained, by inheritance, more of the character of its ancient
progenitor than have other Rodents; and therefore it will not be
specially related to any one existing Marsupial, but indirectly to all
or nearly all Marsupials, from having partially retained the character
of their common progenitor, or of an early member of the group. On the
other hand, of all Marsupials, as Mr. Waterhouse has remarked, the
phascolomys resembles most nearly, not any one species, but the
general order of Rodents. In this case, however, it may be strongly
suspected that the resemblance is only analogical, owing to the
phascolomys having become adapted to habits like those of a Rodent.
The elder De Candolle has made nearly similar observations on the
general nature of the affinities of distinct orders of plants.

On the principle of the multiplication and gradual divergence in
character of the species descended from a common parent, together with
their retention by inheritance of some characters in common, we can
understand the excessively complex and radiating affinities by which
all the members of the same family or higher group are connected
together. For the common parent of a whole family of species, now
broken up by extinction into distinct groups and sub-groups, will have
transmitted some of its characters, modified in various ways and
degrees, to all; and the several species will consequently be related
to each other by circuitous lines of affinity of various lengths (as
may be seen in the diagram so often referred to), mounting up through
many predecessors. As it is difficult to show the blood-relationship
between the numerous kindred of any ancient and noble family, even by
the aid of a genealogical tree, and almost impossible to do this
without this aid, we can understand the extraordinary difficulty which
naturalists have experienced in describing, without the aid of a
diagram, the various affinities which they perceive between the many
living and extinct members of the same great natural class.

Extinction, as we have seen in the fourth chapter, has played an
important part in defining and widening the intervals between the
several groups in each class. We may thus account even for the
distinctness of whole classes from each other--for instance, of birds
from all other vertebrate animals--by the belief that many ancient
forms of life have been utterly lost, through which the early
progenitors of birds were formerly connected with the early
progenitors of the other vertebrate classes. There has been less
entire extinction of the forms of life which once connected fishes
with batrachians. There has been still less in some other classes, as
in that of the Crustacea, for here the most wonderfully diverse forms
are still tied together by a long, but broken, chain of affinities.
Extinction has only separated groups: it has by no means made them;
for if every form which has ever lived on this earth were suddenly to
reappear, though it would be quite impossible to give definitions by
which each group could be distinguished from other groups, as all
would blend together by steps as fine as those between the finest
existing varieties, nevertheless a natural classification, or at least
a natural arrangement, would be possible. We shall see this by turning
to the diagram: the letters, A to L, may represent eleven Silurian
genera, some of which have produced large groups of modified
descendants. Every intermediate link between these eleven genera and
their primordial parent, and every intermediate link in each branch
and sub-branch of their descendants, may be supposed to be still
alive; and the links to be as fine as those between the finest
varieties. In this case it would be quite impossible to give any
definition by which the several members of the several groups could be
distinguished from their more immediate parents; or these parents from
their ancient and unknown progenitor. Yet the natural arrangement in
the diagram would still hold good; and, on the principle of
inheritance, all the forms descended from A, or from I, would have
something in common. In a tree we can specify this or that branch,
though at the actual fork the two unite and blend together. We could
not, as I have said, define the several groups; but we could pick out
types, or forms, representing most of the characters of each group,
whether large or small, and thus give a general idea of the value of
the differences between them. This is what we should be driven to, if
we were ever to succeed in collecting all the forms in any class which
have lived throughout all time and space. We shall certainly never
succeed in making so perfect a collection: nevertheless, in certain
classes, we are tending in this direction; and Milne Edwards has
lately insisted, in an able paper, on the high importance of looking
to types, whether or not we can separate and define the groups to
which such types belong.

Finally, we have seen that natural selection, which results from the
struggle for existence, and which almost inevitably induces extinction
and divergence of character in the many descendants from one dominant
parent-species, explains that great and universal feature in the
affinities of all organic beings, namely, their subordination in group
under group. We use the element of descent in classing the individuals
of both sexes and of all ages, although having few characters in
common, under one species; we use descent in classing acknowledged
varieties, however different they may be from their parent; and I
believe this element of descent is the hidden bond of connexion which
naturalists have sought under the term of the Natural System. On this
idea of the natural system being, in so far as it has been perfected,
genealogical in its arrangement, with the grades of difference between
the descendants from a common parent, expressed by the terms genera,
families, orders, etc., we can understand the rules which we are
compelled to follow in our classification. We can understand why we
value certain resemblances far more than others; why we are permitted
to use rudimentary and useless organs, or others of trifling
physiological importance; why, in comparing one group with a distinct
group, we summarily reject analogical or adaptive characters, and yet
use these same characters within the limits of the same group. We can
clearly see how it is that all living and extinct forms can be grouped
together in one great system; and how the several members of each
class are connected together by the most complex and radiating lines
of affinities. We shall never, probably, disentangle the inextricable
web of affinities between the members of any one class; but when we
have a distinct object in view, and do not look to some unknown plan
of creation, we may hope to make sure but slow progress.

MORPHOLOGY.

We have seen that the members of the same class, independently of
their habits of life, resemble each other in the general plan of their
organisation. This resemblance is often expressed by the term "unity
of type;" or by saying that the several parts and organs in the
different species of the class are homologous. The whole subject is
included under the general name of Morphology. This is the most
interesting department of natural history, and may be said to be its
very soul. What can be more curious than that the hand of a man,
formed for grasping, that of a mole for digging, the leg of the horse,
the paddle of the porpoise, and the wing of the bat, should all be
constructed on the same pattern, and should include the same bones, in
the same relative positions? Geoffroy St. Hilaire has insisted
strongly on the high importance of relative connexion in homologous
organs: the parts may change to almost any extent in form and size,
and yet they always remain connected together in the same order. We
never find, for instance, the bones of the arm and forearm, or of the
thigh and leg, transposed. Hence the same names can be given to the
homologous bones in widely different animals. We see the same great
law in the construction of the mouths of insects: what can be more
different than the immensely long spiral proboscis of a sphinx-moth,
the curious folded one of a bee or bug, and the great jaws of a
beetle?--yet all these organs, serving for such different purposes,
are formed by infinitely numerous modifications of an upper lip,
mandibles, and two pairs of maxillae. Analogous laws govern the
construction of the mouths and limbs of crustaceans. So it is with the
flowers of plants.

Nothing can be more hopeless than to attempt to explain this
similarity of pattern in members of the same class, by utility or by
the doctrine of final causes. The hopelessness of the attempt has been
expressly admitted by Owen in his most interesting work on the 'Nature
of Limbs.' On the ordinary view of the independent creation of each
being, we can only say that so it is;--that it has so pleased the
Creator to construct each animal and plant.

The explanation is manifest on the theory of the natural selection of
successive slight modifications,--each modification being profitable
in some way to the modified form, but often affecting by correlation
of growth other parts of the organisation. In changes of this nature,
there will be little or no tendency to modify the original pattern, or
to transpose parts. The bones of a limb might be shortened and widened
to any extent, and become gradually enveloped in thick membrane, so as
to serve as a fin; or a webbed foot might have all its bones, or
certain bones, lengthened to any extent, and the membrane connecting
them increased to any extent, so as to serve as a wing: yet in all
this great amount of modification there will be no tendency to alter
the framework of bones or the relative connexion of the several parts.
If we suppose that the ancient progenitor, the archetype as it may be
called, of all mammals, had its limbs constructed on the existing
general pattern, for whatever purpose they served, we can at once
perceive the plain signification of the homologous construction of the
limbs throughout the whole class. So with the mouths of insects, we
have only to suppose that their common progenitor had an upper lip,
mandibles, and two pair of maxillae, these parts being perhaps very
simple in form; and then natural selection will account for the
infinite diversity in structure and function of the mouths of insects.
Nevertheless, it is conceivable that the general pattern of an organ
might become so much obscured as to be finally lost, by the atrophy
and ultimately by the complete abortion of certain parts, by the
soldering together of other parts, and by the doubling or
multiplication of others,--variations which we know to be within the
limits of possibility. In the paddles of the extinct gigantic
sea-lizards, and in the mouths of certain suctorial crustaceans, the
general pattern seems to have been thus to a certain extent obscured.

There is another and equally curious branch of the present subject;
namely, the comparison not of the same part in different members of a
class, but of the different parts or organs in the same individual.
Most physiologists believe that the bones of the skull are homologous
with--that is correspond in number and in relative connexion with--the
elemental parts of a certain number of vertebrae. The anterior and
posterior limbs in each member of the vertebrate and articulate
classes are plainly homologous. We see the same law in comparing the
wonderfully complex jaws and legs in crustaceans. It is familiar to
almost every one, that in a flower the relative position of the
sepals, petals, stamens, and pistils, as well as their intimate
structure, are intelligible on the view that they consist of
metamorphosed leaves, arranged in a spire. In monstrous plants, we
often get direct evidence of the possibility of one organ being
transformed into another; and we can actually see in embryonic
crustaceans and in many other animals, and in flowers, that organs,
which when mature become extremely different, are at an early stage of
growth exactly alike.

How inexplicable are these facts on the ordinary view of creation! Why
should the brain be enclosed in a box composed of such numerous and
such extraordinarily shaped pieces of bone? As Owen has remarked, the
benefit derived from the yielding of the separate pieces in the act of
parturition of mammals, will by no means explain the same construction
in the skulls of birds. Why should similar bones have been created in
the formation of the wing and leg of a bat, used as they are for such
totally different purposes? Why should one crustacean, which has an
extremely complex mouth formed of many parts, consequently always have
fewer legs; or conversely, those with many legs have simpler mouths?
Why should the sepals, petals, stamens, and pistils in any individual
flower, though fitted for such widely different purposes, be all
constructed on the same pattern?

On the theory of natural selection, we can satisfactorily answer these
questions. In the vertebrata, we see a series of internal vertebrae
bearing certain processes and appendages; in the articulata, we see
the body divided into a series of segments, bearing external
appendages; and in flowering plants, we see a series of successive
spiral whorls of leaves. An indefinite repetition of the same part or
organ is the common characteristic (as Owen has observed) of all low
or little-modified forms; therefore we may readily believe that the
unknown progenitor of the vertebrata possessed many vertebrae; the
unknown progenitor of the articulata, many segments; and the unknown
progenitor of flowering plants, many spiral whorls of leaves. We have
formerly seen that parts many times repeated are eminently liable to
vary in number and structure; consequently it is quite probable that
natural selection, during a long-continued course of modification,
should have seized on a certain number of the primordially similar
elements, many times repeated, and have adapted them to the most
diverse purposes. And as the whole amount of modification will have
been effected by slight successive steps, we need not wonder at
discovering in such parts or organs, a certain degree of fundamental
resemblance, retained by the strong principle of inheritance.

In the great class of molluscs, though we can homologise the parts of
one species with those of another and distinct species, we can
indicate but few serial homologies; that is, we are seldom enabled to
say that one part or organ is homologous with another in the same
individual. And we can understand this fact; for in molluscs, even in
the lowest members of the class, we do not find nearly so much
indefinite repetition of any one part, as we find in the other great
classes of the animal and vegetable kingdoms.

Naturalists frequently speak of the skull as formed of metamorphosed
vertebrae: the jaws of crabs as metamorphosed legs; the stamens and
pistils of flowers as metamorphosed leaves; but it would in these
cases probably be more correct, as Professor Huxley has remarked, to
speak of both skull and vertebrae, both jaws and legs, etc.,--as
having been metamorphosed, not one from the other, but from some
common element. Naturalists, however, use such language only in a
metaphorical sense: they are far from meaning that during a long
course of descent, primordial organs of any kind--vertebrae in the one
case and legs in the other--have actually been modified into skulls or
jaws. Yet so strong is the appearance of a modification of this nature
having occurred, that naturalists can hardly avoid employing language
having this plain signification. On my view these terms may be used
literally; and the wonderful fact of the jaws, for instance, of a crab
retaining numerous characters, which they would probably have retained
through inheritance, if they had really been metamorphosed during a
long course of descent from true legs, or from some simple appendage,
is explained.

EMBRYOLOGY.

It has already been casually remarked that certain organs in the
individual, which when mature become widely different and serve for
different purposes, are in the embryo exactly alike. The embryos,
also, of distinct animals within the same class are often strikingly
similar: a better proof of this cannot be given, than a circumstance
mentioned by Agassiz, namely, that having forgotten to ticket the
embryo of some vertebrate animal, he cannot now tell whether it be
that of a mammal, bird, or reptile. The vermiform larvae of moths,
flies, beetles, etc., resemble each other much more closely than do
the mature insects; but in the case of larvae, the embryos are active,
and have been adapted for special lines of life. A trace of the law of
embryonic resemblance, sometimes lasts till a rather late age: thus
birds of the same genus, and of closely allied genera, often resemble
each other in their first and second plumage; as we see in the spotted
feathers in the thrush group. In the cat tribe, most of the species
are striped or spotted in lines; and stripes can be plainly
distinguished in the whelp of the lion. We occasionally though rarely
see something of this kind in plants: thus the embryonic leaves of the
ulex or furze, and the first leaves of the phyllodineous acaceas, are
pinnate or divided like the ordinary leaves of the leguminosae.

The points of structure, in which the embryos of widely different
animals of the same class resemble each other, often have no direct
relation to their conditions of existence. We cannot, for instance,
suppose that in the embryos of the vertebrata the peculiar loop-like
course of the arteries near the branchial slits are related to similar
conditions,--in the young mammal which is nourished in the womb of its
mother, in the egg of the bird which is hatched in a nest, and in the
spawn of a frog under water. We have no more reason to believe in such
a relation, than we have to believe that the same bones in the hand of
a man, wing of a bat, and fin of a porpoise, are related to similar
conditions of life. No one will suppose that the stripes on the whelp
of a lion, or the spots on the young blackbird, are of any use to
these animals, or are related to the conditions to which they are
exposed.

The case, however, is different when an animal during any part of its
embryonic career is active, and has to provide for itself. The period
of activity may come on earlier or later in life; but whenever it
comes on, the adaptation of the larva to its conditions of life is
just as perfect and as beautiful as in the adult animal. From such
special adaptations, the similarity of the larvae or active embryos of
allied animals is sometimes much obscured; and cases could be given of
the larvae of two species, or of two groups of species, differing
quite as much, or even more, from each other than do their adult
parents. In most cases, however, the larvae, though active, still obey
more or less closely the law of common embryonic resemblance.
Cirripedes afford a good instance of this: even the illustrious Cuvier
did not perceive that a barnacle was, as it certainly is, a
crustacean; but a glance at the larva shows this to be the case in an
unmistakeable manner. So again the two main divisions of cirripedes,
the pedunculated and sessile, which differ widely in external
appearance, have larvae in all their several stages barely
distinguishable.

The embryo in the course of development generally rises in
organisation: I use this expression, though I am aware that it is
hardly possible to define clearly what is meant by the organisation
being higher or lower. But no one probably will dispute that the
butterfly is higher than the caterpillar. In some cases, however, the
mature animal is generally considered as lower in the scale than the
larva, as with certain parasitic crustaceans. To refer once again to
cirripedes: the larvae in the first stage have three pairs of legs, a
very simple single eye, and a probosciformed mouth, with which they
feed largely, for they increase much in size. In the second stage,
answering to the chrysalis stage of butterflies, they have six pairs
of beautifully constructed natatory legs, a pair of magnificent
compound eyes, and extremely complex antennae; but they have a closed
and imperfect mouth, and cannot feed: their function at this stage is,
to search by their well-developed organs of sense, and to reach by
their active powers of swimming, a proper place on which to become
attached and to undergo their final metamorphosis. When this is
completed they are fixed for life: their legs are now converted into
prehensile organs; they again obtain a well-constructed mouth; but
they have no antennae, and their two eyes are now reconverted into a
minute, single, and very simple eye-spot. In this last and complete
state, cirripedes may be considered as either more highly or more
lowly organised than they were in the larval condition. But in some
genera the larvae become developed either into hermaphrodites having
the ordinary structure, or into what I have called complemental males:
and in the latter, the development has assuredly been retrograde; for
the male is a mere sack, which lives for a short time, and is
destitute of mouth, stomach, or other organ of importance, excepting
for reproduction.

We are so much accustomed to see differences in structure between the
embryo and the adult, and likewise a close similarity in the embryos
of widely different animals within the same class, that we might be
led to look at these facts as necessarily contingent in some manner on
growth. But there is no obvious reason why, for instance, the wing of
a bat, or the fin of a porpoise, should not have been sketched out
with all the parts in proper proportion, as soon as any structure
became visible in the embryo. And in some whole groups of animals and
in certain members of other groups, the embryo does not at any period
differ widely from the adult: thus Owen has remarked in regard to
cuttle-fish, "there is no metamorphosis; the cephalopodic character is
manifested long before the parts of the embryo are completed;" and
again in spiders, "there is nothing worthy to be called a
metamorphosis." The larvae of insects, whether adapted to the most
diverse and active habits, or quite inactive, being fed by their
parents or placed in the midst of proper nutriment, yet nearly all
pass through a similar worm-like stage of development; but in some few
cases, as in that of Aphis, if we look to the admirable drawings by
Professor Huxley of the development of this insect, we see no trace of
the vermiform stage.

How, then, can we explain these several facts in embryology,--namely
the very general, but not universal difference in structure between
the embryo and the adult;--of parts in the same individual embryo,
which ultimately become very unlike and serve for diverse purposes,
being at this early period of growth alike;--of embryos of different
species within the same class, generally, but not universally,
resembling each other;--of the structure of the embryo not being
closely related to its conditions of existence, except when the embryo
becomes at any period of life active and has to provide for
itself;--of the embryo apparently having sometimes a higher
organisation than the mature animal, into which it is developed. I
believe that all these facts can be explained, as follows, on the view
of descent with modification.

It is commonly assumed, perhaps from monstrosities often affecting the
embryo at a very early period, that slight variations necessarily
appear at an equally early period. But we have little evidence on this
head--indeed the evidence rather points the other way; for it is
notorious that breeders of cattle, horses, and various fancy animals,
cannot positively tell, until some time after the animal has been
born, what its merits or form will ultimately turn out. We see this
plainly in our own children; we cannot always tell whether the child
will be tall or short, or what its precise features will be. The
question is not, at what period of life any variation has been caused,
but at what period it is fully displayed. The cause may have acted,
and I believe generally has acted, even before the embryo is formed;
and the variation may be due to the male and female sexual elements
having been affected by the conditions to which either parent, or
their ancestors, have been exposed. Nevertheless an effect thus caused
at a very early period, even before the formation of the embryo, may
appear late in life; as when an hereditary disease, which appears in
old age alone, has been communicated to the offspring from the
reproductive element of one parent. Or again, as when the horns of
cross-bred cattle have been affected by the shape of the horns of
either parent. For the welfare of a very young animal, as long as it
remains in its mother's womb, or in the egg, or as long as it is
nourished and protected by its parent, it must be quite unimportant
whether most of its characters are fully acquired a little earlier or
later in life. It would not signify, for instance, to a bird which
obtained its food best by having a long beak, whether or not it
assumed a beak of this particular length, as long as it was fed by its
parents. Hence, I conclude, that it is quite possible, that each of
the many successive modifications, by which each species has acquired
its present structure, may have supervened at a not very early period
of life; and some direct evidence from our domestic animals supports
this view. But in other cases it is quite possible that each
successive modification, or most of them, may have appeared at an
extremely early period.

I have stated in the first chapter, that there is some evidence to
render it probable, that at whatever age any variation first appears
in the parent, it tends to reappear at a corresponding age in the
offspring. Certain variations can only appear at corresponding ages,
for instance, peculiarities in the caterpillar, cocoon, or imago
states of the silk-moth; or, again, in the horns of almost full-grown
cattle. But further than this, variations which, for all that we can
see, might have appeared earlier or later in life, tend to appear at a
corresponding age in the offspring and parent. I am far from meaning
that this is invariably the case; and I could give a good many cases
of variations (taking the word in the largest sense) which have
supervened at an earlier age in the child than in the parent.

These two principles, if their truth be admitted, will, I believe,
explain all the above specified leading facts in embryology. But first
let us look at a few analogous cases in domestic varieties. Some
authors who have written on Dogs, maintain that the greyhound and
bulldog, though appearing so different, are really varieties most
closely allied, and have probably descended from the same wild stock;
hence I was curious to see how far their puppies differed from each
other: I was told by breeders that they differed just as much as their
parents, and this, judging by the eye, seemed almost to be the case;
but on actually measuring the old dogs and their six-days old puppies,
I found that the puppies had not nearly acquired their full amount of
proportional difference. So, again, I was told that the foals of cart
and race-horses differed as much as the full-grown animals; and this
surprised me greatly, as I think it probable that the difference
between these two breeds has been wholly caused by selection under
domestication; but having had careful measurements made of the dam and
of a three-days old colt of a race and heavy cart-horse, I find that
the colts have by no means acquired their full amount of proportional
difference.

As the evidence appears to me conclusive, that the several domestic
breeds of Pigeon have descended from one wild species, I compared
young pigeons of various breeds, within twelve hours after being
hatched; I carefully measured the proportions (but will not here give
details) of the beak, width of mouth, length of nostril and of eyelid,
size of feet and length of leg, in the wild stock, in pouters,
fantails, runts, barbs, dragons, carriers, and tumblers. Now some of
these birds, when mature, differ so extraordinarily in length and form
of beak, that they would, I cannot doubt, be ranked in distinct
genera, had they been natural productions. But when the nestling birds
of these several breeds were placed in a row, though most of them
could be distinguished from each other, yet their proportional
differences in the above specified several points were incomparably
less than in the full-grown birds. Some characteristic points of
difference--for instance, that of the width of mouth--could hardly be
detected in the young. But there was one remarkable exception to this
rule, for the young of the short-faced tumbler differed from the young
of the wild rock-pigeon and of the other breeds, in all its
proportions, almost exactly as much as in the adult state.

The two principles above given seem to me to explain these facts in
regard to the later embryonic stages of our domestic varieties.
Fanciers select their horses, dogs, and pigeons, for breeding, when
they are nearly grown up: they are indifferent whether the desired
qualities and structures have been acquired earlier or later in life,
if the full-grown animal possesses them. And the cases just given,
more especially that of pigeons, seem to show that the characteristic
differences which give value to each breed, and which have been
accumulated by man's selection, have not generally first appeared at
an early period of life, and have been inherited by the offspring at a
corresponding not early period. But the case of the short-faced
tumbler, which when twelve hours old had acquired its proper
proportions, proves that this is not the universal rule; for here the
characteristic differences must either have appeared at an earlier
period than usual, or, if not so, the differences must have been
inherited, not at the corresponding, but at an earlier age.

Now let us apply these facts and the above two principles--which
latter, though not proved true, can be shown to be in some degree
probable--to species in a state of nature. Let us take a genus of
birds, descended on my theory from some one parent-species, and of
which the several new species have become modified through natural
selection in accordance with their diverse habits. Then, from the many
slight successive steps of variation having supervened at a rather
late age, and having been inherited at a corresponding age, the young
of the new species of our supposed genus will manifestly tend to
resemble each other much more closely than do the adults, just as we
have seen in the case of pigeons. We may extend this view to whole
families or even classes. The fore-limbs, for instance, which served
as legs in the parent-species, may become, by a long course of
modification, adapted in one descendant to act as hands, in another as
paddles, in another as wings; and on the above two principles--namely
of each successive modification supervening at a rather late age, and
being inherited at a corresponding late age--the fore-limbs in the
embryos of the several descendants of the parent-species will still
resemble each other closely, for they will not have been modified. But
in each individual new species, the embryonic fore-limbs will differ
greatly from the fore-limbs in the mature animal; the limbs in the
latter having undergone much modification at a rather late period of
life, and having thus been converted into hands, or paddles, or wings.
Whatever influence long-continued exercise or use on the one hand, and
disuse on the other, may have in modifying an organ, such influence
will mainly affect the mature animal, which has come to its full
powers of activity and has to gain its own living; and the effects
thus produced will be inherited at a corresponding mature age. Whereas
the young will remain unmodified, or be modified in a lesser degree,
by the effects of use and disuse.

In certain cases the successive steps of variation might supervene,
from causes of which we are wholly ignorant, at a very early period of
life, or each step might be inherited at an earlier period than that
at which it first appeared. In either case (as with the short-faced
tumbler) the young or embryo would closely resemble the mature
parent-form. We have seen that this is the rule of development in
certain whole groups of animals, as with cuttle-fish and spiders, and
with a few members of the great class of insects, as with Aphis. With
respect to the final cause of the young in these cases not undergoing
any metamorphosis, or closely resembling their parents from their
earliest age, we can see that this would result from the two following
contingencies; firstly, from the young, during a course of
modification carried on for many generations, having to provide for
their own wants at a very early stage of development, and secondly,
from their following exactly the same habits of life with their
parents; for in this case, it would be indispensable for the existence
of the species, that the child should be modified at a very early age
in the same manner with its parents, in accordance with their similar
habits. Some further explanation, however, of the embryo not
undergoing any metamorphosis is perhaps requisite. If, on the other
hand, it profited the young to follow habits of life in any degree
different from those of their parent, and consequently to be
constructed in a slightly different manner, then, on the principle of
inheritance at corresponding ages, the active young or larvae might
easily be rendered by natural selection different to any conceivable
extent from their parents. Such differences might, also, become
correlated with successive stages of development; so that the larvae,
in the first stage, might differ greatly from the larvae in the second
stage, as we have seen to be the case with cirripedes. The adult might
become fitted for sites or habits, in which organs of locomotion or of
the senses, etc., would be useless; and in this case the final
metamorphosis would be said to be retrograde.

As all the organic beings, extinct and recent, which have ever lived
on this earth have to be classed together, and as all have been
connected by the finest gradations, the best, or indeed, if our
collections were nearly perfect, the only possible arrangement, would
be genealogical. Descent being on my view the hidden bond of connexion
which naturalists have been seeking under the term of the natural
system. On this view we can understand how it is that, in the eyes of
most naturalists, the structure of the embryo is even more important
for classification than that of the adult. For the embryo is the
animal in its less modified state; and in so far it reveals the
structure of its progenitor. In two groups of animal, however much
they may at present differ from each other in structure and habits, if
they pass through the same or similar embryonic stages, we may feel
assured that they have both descended from the same or nearly similar
parents, and are therefore in that degree closely related. Thus,
community in embryonic structure reveals community of descent. It will
reveal this community of descent, however much the structure of the
adult may have been modified and obscured; we have seen, for instance,
that cirripedes can at once be recognised by their larvae as belonging
to the great class of crustaceans. As the embryonic state of each
species and group of species partially shows us the structure of their
less modified ancient progenitors, we can clearly see why ancient and
extinct forms of life should resemble the embryos of their
descendants,--our existing species. Agassiz believes this to be a law
of nature; but I am bound to confess that I only hope to see the law
hereafter proved true. It can be proved true in those cases alone in
which the ancient state, now supposed to be represented in many
embryos, has not been obliterated, either by the successive variations
in a long course of modification having supervened at a very early
age, or by the variations having been inherited at an earlier period
than that at which they first appeared. It should also be borne in
mind, that the supposed law of resemblance of ancient forms of life to
the embryonic stages of recent forms, may be true, but yet, owing to
the geological record not extending far enough back in time, may
remain for a long period, or for ever, incapable of demonstration.

Thus, as it seems to me, the leading facts in embryology, which are
second in importance to none in natural history, are explained on the
principle of slight modifications not appearing, in the many
descendants from some one ancient progenitor, at a very early period
in the life of each, though perhaps caused at the earliest, and being
inherited at a corresponding not early period. Embryology rises
greatly in interest, when we thus look at the embryo as a picture,
more or less obscured, of the common parent-form of each great class
of animals.

RUDIMENTARY, ATROPHIED, OR ABORTED ORGANS.

Organs or parts in this strange condition, bearing the stamp of
inutility, are extremely common throughout nature. For instance,
rudimentary mammae are very general in the males of mammals: I presume
that the "bastard-wing" in birds may be safely considered as a digit
in a rudimentary state: in very many snakes one lobe of the lungs is
rudimentary; in other snakes there are rudiments of the pelvis and
hind limbs. Some of the cases of rudimentary organs are extremely
curious; for instance, the presence of teeth in foetal whales, which
when grown up have not a tooth in their heads; and the presence of
teeth, which never cut through the gums, in the upper jaws of our
unborn calves. It has even been stated on good authority that
rudiments of teeth can be detected in the beaks of certain embryonic
birds. Nothing can be plainer than that wings are formed for flight,
yet in how many insects do we see wings so reduced in size as to be
utterly incapable of flight, and not rarely lying under wing-cases,
firmly soldered together!

The meaning of rudimentary organs is often quite unmistakeable: for
instance there are beetles of the same genus (and even of the same
species) resembling each other most closely in all respects, one of
which will have full-sized wings, and another mere rudiments of
membrane; and here it is impossible to doubt, that the rudiments
represent wings. Rudimentary organs sometimes retain their
potentiality, and are merely not developed: this seems to be the case
with the mammae of male mammals, for many instances are on record of
these organs having become well developed in full-grown males, and
having secreted milk. So again there are normally four developed and
two rudimentary teats in the udders of the genus Bos, but in our
domestic cows the two sometimes become developed and give milk. In
individual plants of the same species the petals sometimes occur as
mere rudiments, and sometimes in a well-developed state. In plants
with separated sexes, the male flowers often have a rudiment of a
pistil; and Kolreuter found that by crossing such male plants with an
hermaphrodite species, the rudiment of the pistil in the hybrid
offspring was much increased in size; and this shows that the rudiment
and the perfect pistil are essentially alike in nature.

An organ serving for two purposes, may become rudimentary or utterly
aborted for one, even the more important purpose; and remain perfectly
efficient for the other. Thus in plants, the office of the pistil is
to allow the pollen-tubes to reach the ovules protected in the ovarium
at its base. The pistil consists of a stigma supported on the style;
but in some Compositae, the male florets, which of course cannot be
fecundated, have a pistil, which is in a rudimentary state, for it is
not crowned with a stigma; but the style remains well developed, and
is clothed with hairs as in other compositae, for the purpose of
brushing the pollen out of the surrounding anthers. Again, an organ
may become rudimentary for its proper purpose, and be used for a
distinct object: in certain fish the swim-bladder seems to be
rudimentary for its proper function of giving buoyancy, but has become
converted into a nascent breathing organ or lung. Other similar
instances could be given.

Rudimentary organs in the individuals of the same species are very
liable to vary in degree of development and in other respects.
Moreover, in closely allied species, the degree to which the same
organ has been rendered rudimentary occasionally differs much. This
latter fact is well exemplified in the state of the wings of the
female moths in certain groups. Rudimentary organs may be utterly
aborted; and this implies, that we find in an animal or plant no trace
of an organ, which analogy would lead us to expect to find, and which
is occasionally found in monstrous individuals of the species. Thus in
the snapdragon (antirrhinum) we generally do not find a rudiment of a
fifth stamen; but this may sometimes be seen. In tracing the
homologies of the same part in different members of a class, nothing
is more common, or more necessary, than the use and discovery of
rudiments. This is well shown in the drawings given by Owen of the
bones of the leg of the horse, ox, and rhinoceros.

It is an important fact that rudimentary organs, such as teeth in the
upper jaws of whales and ruminants, can often be detected in the
embryo, but afterwards wholly disappear. It is also, I believe, a
universal rule, that a rudimentary part or organ is of greater size
relatively to the adjoining parts in the embryo, than in the adult; so
that the organ at this early age is less rudimentary, or even cannot
be said to be in any degree rudimentary. Hence, also, a rudimentary
organ in the adult, is often said to have retained its embryonic
condition.

I have now given the leading facts with respect to rudimentary organs.
In reflecting on them, every one must be struck with astonishment: for
the same reasoning power which tells us plainly that most parts and
organs are exquisitely adapted for certain purposes, tells us with
equal plainness that these rudimentary or atrophied organs, are
imperfect and useless. In works on natural history rudimentary organs
are generally said to have been created "for the sake of symmetry," or
in order "to complete the scheme of nature;" but this seems to me no
explanation, merely a restatement of the fact. Would it be thought
sufficient to say that because planets revolve in elliptic courses
round the sun, satellites follow the same course round the planets,
for the sake of symmetry, and to complete the scheme of nature? An
eminent physiologist accounts for the presence of rudimentary organs,
by supposing that they serve to excrete matter in excess, or injurious
to the system; but can we suppose that the minute papilla, which often
represents the pistil in male flowers, and which is formed merely of
cellular tissue, can thus act? Can we suppose that the formation of
rudimentary teeth which are subsequently absorbed, can be of any
service to the rapidly growing embryonic calf by the excretion of
precious phosphate of lime? When a man's fingers have been amputated,
imperfect nails sometimes appear on the stumps: I could as soon
believe that these vestiges of nails have appeared, not from unknown
laws of growth, but in order to excrete horny matter, as that the
rudimentary nails on the fin of the manatee were formed for this
purpose.

On my view of descent with modification, the origin of rudimentary
organs is simple. We have plenty of cases of rudimentary organs in our
domestic productions,--as the stump of a tail in tailless breeds,--the
vestige of an ear in earless breeds,--the reappearance of minute
dangling horns in hornless breeds of cattle, more especially,
according to Youatt, in young animals,--and the state of the whole
flower in the cauliflower. We often see rudiments of various parts in
monsters. But I doubt whether any of these cases throw light on the
origin of rudimentary organs in a state of nature, further than by
showing that rudiments can be produced; for I doubt whether species
under nature ever undergo abrupt changes. I believe that disuse has
been the main agency; that it has led in successive generations to the
gradual reduction of various organs, until they have become
rudimentary,--as in the case of the eyes of animals inhabiting dark
caverns, and of the wings of birds inhabiting oceanic islands, which
have seldom been forced to take flight, and have ultimately lost the
power of flying. Again, an organ useful under certain conditions,
might become injurious under others, as with the wings of beetles
living on small and exposed islands; and in this case natural
selection would continue slowly to reduce the organ, until it was
rendered harmless and rudimentary.

Any change in function, which can be effected by insensibly small
steps, is within the power of natural selection; so that an organ
rendered, during changed habits of life, useless or injurious for one
purpose, might easily be modified and used for another purpose. Or an
organ might be retained for one alone of its former functions. An
organ, when rendered useless, may well be variable, for its variations
cannot be checked by natural selection. At whatever period of life
disuse or selection reduces an organ, and this will generally be when
the being has come to maturity and to its full powers of action, the
principle of inheritance at corresponding ages will reproduce the
organ in its reduced state at the same age, and consequently will
seldom affect or reduce it in the embryo. Thus we can understand the
greater relative size of rudimentary organs in the embryo, and their
lesser relative size in the adult. But if each step of the process of
reduction were to be inherited, not at the corresponding age, but at
an extremely early period of life (as we have good reason to believe
to be possible) the rudimentary part would tend to be wholly lost, and
we should have a case of complete abortion. The principle, also, of
economy, explained in a former chapter, by which the materials forming
any part or structure, if not useful to the possessor, will be saved
as far as is possible, will probably often come into play; and this
will tend to cause the entire obliteration of a rudimentary organ.

As the presence of rudimentary organs is thus due to the tendency in
every part of the organisation, which has long existed, to be
inherited--we can understand, on the genealogical view of
classification, how it is that systematists have found rudimentary
parts as useful as, or even sometimes more useful than, parts of high
physiological importance. Rudimentary organs may be compared with the
letters in a word, still retained in the spelling, but become useless
in the pronunciation, but which serve as a clue in seeking for its
derivation. On the view of descent with modification, we may conclude
that the existence of organs in a rudimentary, imperfect, and useless
condition, or quite aborted, far from presenting a strange difficulty,
as they assuredly do on the ordinary doctrine of creation, might even
have been anticipated, and can be accounted for by the laws of
inheritance.

SUMMARY.

In this chapter I have attempted to show, that the subordination of
group to group in all organisms throughout all time; that the nature
of the relationship, by which all living and extinct beings are united
by complex, radiating, and circuitous lines of affinities into one
grand system; the rules followed and the difficulties encountered by
naturalists in their classifications; the value set upon characters,
if constant and prevalent, whether of high vital importance, or of the
most trifling importance, or, as in rudimentary organs, of no
importance; the wide opposition in value between analogical or
adaptive characters, and characters of true affinity; and other such
rules;--all naturally follow on the view of the common parentage of
those forms which are considered by naturalists as allied, together
with their modification through natural selection, with its
contingencies of extinction and divergence of character. In
considering this view of classification, it should be borne in mind
that the element of descent has been universally used in ranking
together the sexes, ages, and acknowledged varieties of the same
species, however different they may be in structure. If we extend the
use of this element of descent,--the only certainly known cause of
similarity in organic beings,--we shall understand what is meant by
the natural system: it is genealogical in its attempted arrangement,
with the grades of acquired difference marked by the terms varieties,
species, genera, families, orders, and classes.

On this same view of descent with modification, all the great facts in
Morphology become intelligible,--whether we look to the same pattern
displayed in the homologous organs, to whatever purpose applied, of
the different species of a class; or to the homologous parts
constructed on the same pattern in each individual animal and plant.

On the principle of successive slight variations, not necessarily or
generally supervening at a very early period of life, and being
inherited at a corresponding period, we can understand the great
leading facts in Embryology; namely, the resemblance in an individual
embryo of the homologous parts, which when matured will become widely
different from each other in structure and function; and the
resemblance in different species of a class of the homologous parts or
organs, though fitted in the adult members for purposes as different
as possible. Larvae are active embryos, which have become specially
modified in relation to their habits of life, through the principle of
modifications being inherited at corresponding ages. On this same
principle--and bearing in mind, that when organs are reduced in size,
either from disuse or selection, it will generally be at that period
of life when the being has to provide for its own wants, and bearing
in mind how strong is the principle of inheritance--the occurrence of
rudimentary organs and their final abortion, present to us no
inexplicable difficulties; on the contrary, their presence might have
been even anticipated. The importance of embryological characters and
of rudimentary organs in classification is intelligible, on the view
that an arrangement is only so far natural as it is genealogical.

Finally, the several classes of facts which have been considered in
this chapter, seem to me to proclaim so plainly, that the innumerable
species, genera, and families of organic beings, with which this world
is peopled, have all descended, each within its own class or group,
from common parents, and have all been modified in the course of
descent, that I should without hesitation adopt this view, even if it
were unsupported by other facts or arguments.


CHAPTER 14. RECAPITULATION AND CONCLUSION.

Recapitulation of the difficulties on the theory of Natural Selection.
Recapitulation of the general and special circumstances in its favour.
Causes of the general belief in the immutability of species.
How far the theory of natural selection may be extended.
Effects of its adoption on the study of Natural history.
Concluding remarks.

As this whole volume is one long argument, it may be convenient to the
reader to have the leading facts and inferences briefly recapitulated.

That many and grave objections may be advanced against the theory of
descent with modification through natural selection, I do not deny. I
have endeavoured to give to them their full force. Nothing at first
can appear more difficult to believe than that the more complex organs
and instincts should have been perfected, not by means superior to,
though analogous with, human reason, but by the accumulation of
innumerable slight variations, each good for the individual possessor.
Nevertheless, this difficulty, though appearing to our imagination
insuperably great, cannot be considered real if we admit the following
propositions, namely,--that gradations in the perfection of any organ
or instinct, which we may consider, either do now exist or could have
existed, each good of its kind,--that all organs and instincts are, in
ever so slight a degree, variable,--and, lastly, that there is a
struggle for existence leading to the preservation of each profitable
deviation of structure or instinct. The truth of these propositions
cannot, I think, be disputed.

It is, no doubt, extremely difficult even to conjecture by what
gradations many structures have been perfected, more especially
amongst broken and failing groups of organic beings; but we see so
many strange gradations in nature, as is proclaimed by the canon,
"Natura non facit saltum," that we ought to be extremely cautious in
saying that any organ or instinct, or any whole being, could not have
arrived at its present state by many graduated steps. There are, it
must be admitted, cases of special difficulty on the theory of natural
selection; and one of the most curious of these is the existence of
two or three defined castes of workers or sterile females in the same
community of ants; but I have attempted to show how this difficulty
can be mastered.

With respect to the almost universal sterility of species when first
crossed, which forms so remarkable a contrast with the almost
universal fertility of varieties when crossed, I must refer the reader
to the recapitulation of the facts given at the end of the eighth
chapter, which seem to me conclusively to show that this sterility is
no more a special endowment than is the incapacity of two trees to be
grafted together, but that it is incidental on constitutional
differences in the reproductive systems of the intercrossed species.
We see the truth of this conclusion in the vast difference in the
result, when the same two species are crossed reciprocally; that is,
when one species is first used as the father and then as the mother.

The fertility of varieties when intercrossed and of their mongrel
offspring cannot be considered as universal; nor is their very general
fertility surprising when we remember that it is not likely that
either their constitutions or their reproductive systems should have
been profoundly modified. Moreover, most of the varieties which have
been experimentised on have been produced under domestication; and as
domestication apparently tends to eliminate sterility, we ought not to
expect it also to produce sterility.

The sterility of hybrids is a very different case from that of first
crosses, for their reproductive organs are more or less functionally
impotent; whereas in first crosses the organs on both sides are in a
perfect condition. As we continually see that organisms of all kinds
are rendered in some degree sterile from their constitutions having
been disturbed by slightly different and new conditions of life, we
need not feel surprise at hybrids being in some degree sterile, for
their constitutions can hardly fail to have been disturbed from being
compounded of two distinct organisations. This parallelism is
supported by another parallel, but directly opposite, class of facts;
namely, that the vigour and fertility of all organic beings are
increased by slight changes in their conditions of life, and that the
offspring of slightly modified forms or varieties acquire from being
crossed increased vigour and fertility. So that, on the one hand,
considerable changes in the conditions of life and crosses between
greatly modified forms, lessen fertility; and on the other hand,
lesser changes in the conditions of life and crosses between less
modified forms, increase fertility.

Turning to geographical distribution, the difficulties encountered on
the theory of descent with modification are grave enough. All the
individuals of the same species, and all the species of the same
genus, or even higher group, must have descended from common parents;
and therefore, in however distant and isolated parts of the world they
are now found, they must in the course of successive generations have
passed from some one part to the others. We are often wholly unable
even to conjecture how this could have been effected. Yet, as we have
reason to believe that some species have retained the same specific
form for very long periods, enormously long as measured by years, too
much stress ought not to be laid on the occasional wide diffusion of
the same species; for during very long periods of time there will
always be a good chance for wide migration by many means. A broken or
interrupted range may often be accounted for by the extinction of the
species in the intermediate regions. It cannot be denied that we are
as yet very ignorant of the full extent of the various climatal and
geographical changes which have affected the earth during modern
periods; and such changes will obviously have greatly facilitated
migration. As an example, I have attempted to show how potent has been
the influence of the Glacial period on the distribution both of the
same and of representative species throughout the world. We are as yet
profoundly ignorant of the many occasional means of transport. With
respect to distinct species of the same genus inhabiting very distant
and isolated regions, as the process of modification has necessarily
been slow, all the means of migration will have been possible during a
very long period; and consequently the difficulty of the wide
diffusion of species of the same genus is in some degree lessened.

As on the theory of natural selection an interminable number of
intermediate forms must have existed, linking together all the species
in each group by gradations as fine as our present varieties, it may
be asked, Why do we not see these linking forms all around us? Why are
not all organic beings blended together in an inextricable chaos? With
respect to existing forms, we should remember that we have no right to
expect (excepting in rare cases) to discover DIRECTLY connecting links
between them, but only between each and some extinct and supplanted
form. Even on a wide area, which has during a long period remained
continuous, and of which the climate and other conditions of life
change insensibly in going from a district occupied by one species
into another district occupied by a closely allied species, we have no
just right to expect often to find intermediate varieties in the
intermediate zone. For we have reason to believe that only a few
species are undergoing change at any one period; and all changes are
slowly effected. I have also shown that the intermediate varieties
which will at first probably exist in the intermediate zones, will be
liable to be supplanted by the allied forms on either hand; and the
latter, from existing in greater numbers, will generally be modified
and improved at a quicker rate than the intermediate varieties, which
exist in lesser numbers; so that the intermediate varieties will, in
the long run, be supplanted and exterminated.

On this doctrine of the extermination of an infinitude of connecting
links, between the living and extinct inhabitants of the world, and at
each successive period between the extinct and still older species,
why is not every geological formation charged with such links? Why
does not every collection of fossil remains afford plain evidence of
the gradation and mutation of the forms of life? We meet with no such
evidence, and this is the most obvious and forcible of the many
objections which may be urged against my theory. Why, again, do whole
groups of allied species appear, though certainly they often falsely
appear, to have come in suddenly on the several geological stages? Why
do we not find great piles of strata beneath the Silurian system,
stored with the remains of the progenitors of the Silurian groups of
fossils? For certainly on my theory such strata must somewhere have
been deposited at these ancient and utterly unknown epochs in the
world's history.

I can answer these questions and grave objections only on the
supposition that the geological record is far more imperfect than most
geologists believe. It cannot be objected that there has not been time
sufficient for any amount of organic change; for the lapse of time has
been so great as to be utterly inappreciable by the human intellect.
The number of specimens in all our museums is absolutely as nothing
compared with the countless generations of countless species which
certainly have existed. We should not be able to recognise a species
as the parent of any one or more species if we were to examine them
ever so closely, unless we likewise possessed many of the intermediate
links between their past or parent and present states; and these many
links we could hardly ever expect to discover, owing to the
imperfection of the geological record. Numerous existing doubtful
forms could be named which are probably varieties; but who will
pretend that in future ages so many fossil links will be discovered,
that naturalists will be able to decide, on the common view, whether
or not these doubtful forms are varieties? As long as most of the
links between any two species are unknown, if any one link or
intermediate variety be discovered, it will simply be classed as
another and distinct species. Only a small portion of the world has
been geologically explored. Only organic beings of certain classes can
be preserved in a fossil condition, at least in any great number.
Widely ranging species vary most, and varieties are often at first
local,--both causes rendering the discovery of intermediate links less
likely. Local varieties will not spread into other and distant regions
until they are considerably modified and improved; and when they do
spread, if discovered in a geological formation, they will appear as
if suddenly created there, and will be simply classed as new species.
Most formations have been intermittent in their accumulation; and
their duration, I am inclined to believe, has been shorter than the
average duration of specific forms. Successive formations are
separated from each other by enormous blank intervals of time; for
fossiliferous formations, thick enough to resist future degradation,
can be accumulated only where much sediment is deposited on the
subsiding bed of the sea. During the alternate periods of elevation
and of stationary level the record will be blank. During these latter
periods there will probably be more variability in the forms of life;
during periods of subsidence, more extinction.

With respect to the absence of fossiliferous formations beneath the
lowest Silurian strata, I can only recur to the hypothesis given in
the ninth chapter. That the geological record is imperfect all will
admit; but that it is imperfect to the degree which I require, few
will be inclined to admit. If we look to long enough intervals of
time, geology plainly declares that all species have changed; and they
have changed in the manner which my theory requires, for they have
changed slowly and in a graduated manner. We clearly see this in the
fossil remains from consecutive formations invariably being much more
closely related to each other, than are the fossils from formations
distant from each other in time.

Such is the sum of the several chief objections and difficulties which
may justly be urged against my theory; and I have now briefly
recapitulated the answers and explanations which can be given to them.
I have felt these difficulties far too heavily during many years to
doubt their weight. But it deserves especial notice that the more
important objections relate to questions on which we are confessedly
ignorant; nor do we know how ignorant we are. We do not know all the
possible transitional gradations between the simplest and the most
perfect organs; it cannot be pretended that we know all the varied
means of Distribution during the long lapse of years, or that we know
how imperfect the Geological Record is. Grave as these several
difficulties are, in my judgment they do not overthrow the theory of
descent with modification.

Now let us turn to the other side of the argument. Under domestication
we see much variability. This seems to be mainly due to the
reproductive system being eminently susceptible to changes in the
conditions of life; so that this system, when not rendered impotent,
fails to reproduce offspring exactly like the parent-form. Variability
is governed by many complex laws,--by correlation of growth, by use
and disuse, and by the direct action of the physical conditions of
life. There is much difficulty in ascertaining how much modification
our domestic productions have undergone; but we may safely infer that
the amount has been large, and that modifications can be inherited for
long periods. As long as the conditions of life remain the same, we
have reason to believe that a modification, which has already been
inherited for many generations, may continue to be inherited for an
almost infinite number of generations. On the other hand we have
evidence that variability, when it has once come into play, does not
wholly cease; for new varieties are still occasionally produced by our
most anciently domesticated productions.

Man does not actually produce variability; he only unintentionally
exposes organic beings to new conditions of life, and then nature acts
on the organisation, and causes variability. But man can and does
select the variations given to him by nature, and thus accumulate them
in any desired manner. He thus adapts animals and plants for his own
benefit or pleasure. He may do this methodically, or he may do it
unconsciously by preserving the individuals most useful to him at the
time, without any thought of altering the breed. It is certain that he
can largely influence the character of a breed by selecting, in each
successive generation, individual differences so slight as to be quite
inappreciable by an uneducated eye. This process of selection has been
the great agency in the production of the most distinct and useful
domestic breeds. That many of the breeds produced by man have to a
large extent the character of natural species, is shown by the
inextricable doubts whether very many of them are varieties or
aboriginal species.

There is no obvious reason why the principles which have acted so
efficiently under domestication should not have acted under nature. In
the preservation of favoured individuals and races, during the
constantly-recurrent Struggle for Existence, we see the most powerful
and ever-acting means of selection. The struggle for existence
inevitably follows from the high geometrical ratio of increase which
is common to all organic beings. This high rate of increase is proved
by calculation, by the effects of a succession of peculiar seasons,
and by the results of naturalisation, as explained in the third
chapter. More individuals are born than can possibly survive. A grain
in the balance will determine which individual shall live and which
shall die,--which variety or species shall increase in number, and
which shall decrease, or finally become extinct. As the individuals of
the same species come in all respects into the closest competition
with each other, the struggle will generally be most severe between
them; it will be almost equally severe between the varieties of the
same species, and next in severity between the species of the same
genus. But the struggle will often be very severe between beings most
remote in the scale of nature. The slightest advantage in one being,
at any age or during any season, over those with which it comes into
competition, or better adaptation in however slight a degree to the
surrounding physical conditions, will turn the balance.

With animals having separated sexes there will in most cases be a
struggle between the males for possession of the females. The most
vigorous individuals, or those which have most successfully struggled
with their conditions of life, will generally leave most progeny. But
success will often depend on having special weapons or means of
defence, or on the charms of the males; and the slightest advantage
will lead to victory.

As geology plainly proclaims that each land has undergone great
physical changes, we might have expected that organic beings would
have varied under nature, in the same way as they generally have
varied under the changed conditions of domestication. And if there be
any variability under nature, it would be an unaccountable fact if
natural selection had not come into play. It has often been asserted,
but the assertion is quite incapable of proof, that the amount of
variation under nature is a strictly limited quantity. Man, though
acting on external characters alone and often capriciously, can
produce within a short period a great result by adding up mere
individual differences in his domestic productions; and every one
admits that there are at least individual differences in species under
nature. But, besides such differences, all naturalists have admitted
the existence of varieties, which they think sufficiently distinct to
be worthy of record in systematic works. No one can draw any clear
distinction between individual differences and slight varieties; or
between more plainly marked varieties and sub-species, and species.
Let it be observed how naturalists differ in the rank which they
assign to the many representative forms in Europe and North America.

If then we have under nature variability and a powerful agent always
ready to act and select, why should we doubt that variations in any
way useful to beings, under their excessively complex relations of
life, would be preserved, accumulated, and inherited? Why, if man can
by patience select variations most useful to himself, should nature
fail in selecting variations useful, under changing conditions of
life, to her living products? What limit can be put to this power,
acting during long ages and rigidly scrutinising the whole
constitution, structure, and habits of each creature,--favouring the
good and rejecting the bad? I can see no limit to this power, in
slowly and beautifully adapting each form to the most complex
relations of life. The theory of natural selection, even if we looked
no further than this, seems to me to be in itself probable. I have
already recapitulated, as fairly as I could, the opposed difficulties
and objections: now let us turn to the special facts and arguments in
favour of the theory.

On the view that species are only strongly marked and permanent
varieties, and that each species first existed as a variety, we can
see why it is that no line of demarcation can be drawn between
species, commonly supposed to have been produced by special acts of
creation, and varieties which are acknowledged to have been produced
by secondary laws. On this same view we can understand how it is that
in each region where many species of a genus have been produced, and
where they now flourish, these same species should present many
varieties; for where the manufactory of species has been active, we
might expect, as a general rule, to find it still in action; and this
is the case if varieties be incipient species. Moreover, the species
of the larger genera, which afford the greater number of varieties or
incipient species, retain to a certain degree the character of
varieties; for they differ from each other by a less amount of
difference than do the species of smaller genera. The closely allied
species also of the larger genera apparently have restricted ranges,
and they are clustered in little groups round other species--in which
respects they resemble varieties. These are strange relations on the
view of each species having been independently created, but are
intelligible if all species first existed as varieties.

As each species tends by its geometrical ratio of reproduction to
increase inordinately in number; and as the modified descendants of
each species will be enabled to increase by so much the more as they
become more diversified in habits and structure, so as to be enabled
to seize on many and widely different places in the economy of nature,
there will be a constant tendency in natural selection to preserve the
most divergent offspring of any one species. Hence during a
long-continued course of modification, the slight differences,
characteristic of varieties of the same species, tend to be augmented
into the greater differences characteristic of species of the same
genus. New and improved varieties will inevitably supplant and
exterminate the older, less improved and intermediate varieties; and
thus species are rendered to a large extent defined and distinct
objects. Dominant species belonging to the larger groups tend to give
birth to new and dominant forms; so that each large group tends to
become still larger, and at the same time more divergent in character.
But as all groups cannot thus succeed in increasing in size, for the
world would not hold them, the more dominant groups beat the less
dominant. This tendency in the large groups to go on increasing in
size and diverging in character, together with the almost inevitable
contingency of much extinction, explains the arrangement of all the
forms of life, in groups subordinate to groups, all within a few great
classes, which we now see everywhere around us, and which has
prevailed throughout all time. This grand fact of the grouping of all
organic beings seems to me utterly inexplicable on the theory of
creation.

As natural selection acts solely by accumulating slight, successive,
favourable variations, it can produce no great or sudden modification;
it can act only by very short and slow steps. Hence the canon of
"Natura non facit saltum," which every fresh addition to our knowledge
tends to make more strictly correct, is on this theory simply
intelligible. We can plainly see why nature is prodigal in variety,
though niggard in innovation. But why this should be a law of nature
if each species has been independently created, no man can explain.

Many other facts are, as it seems to me, explicable on this theory.
How strange it is that a bird, under the form of woodpecker, should
have been created to prey on insects on the ground; that upland geese,
which never or rarely swim, should have been created with webbed feet;
that a thrush should have been created to dive and feed on sub-aquatic
insects; and that a petrel should have been created with habits and
structure fitting it for the life of an auk or grebe! and so on in
endless other cases. But on the view of each species constantly trying
to increase in number, with natural selection always ready to adapt
the slowly varying descendants of each to any unoccupied or
ill-occupied place in nature, these facts cease to be strange, or
perhaps might even have been anticipated.

As natural selection acts by competition, it adapts the inhabitants of
each country only in relation to the degree of perfection of their
associates; so that we need feel no surprise at the inhabitants of any
one country, although on the ordinary view supposed to have been
specially created and adapted for that country, being beaten and
supplanted by the naturalised productions from another land. Nor ought
we to marvel if all the contrivances in nature be not, as far as we
can judge, absolutely perfect; and if some of them be abhorrent to our
ideas of fitness. We need not marvel at the sting of the bee causing
the bee's own death; at drones being produced in such vast numbers for
one single act, and being then slaughtered by their sterile sisters;
at the astonishing waste of pollen by our fir-trees; at the
instinctive hatred of the queen bee for her own fertile daughters; at
ichneumonidae feeding within the live bodies of caterpillars; and at
other such cases. The wonder indeed is, on the theory of natural
selection, that more cases of the want of absolute perfection have not
been observed.

The complex and little known laws governing variation are the same, as
far as we can see, with the laws which have governed the production of
so-called specific forms. In both cases physical conditions seem to
have produced but little direct effect; yet when varieties enter any
zone, they occasionally assume some of the characters of the species
proper to that zone. In both varieties and species, use and disuse
seem to have produced some effect; for it is difficult to resist this
conclusion when we look, for instance, at the logger-headed duck,
which has wings incapable of flight, in nearly the same condition as
in the domestic duck; or when we look at the burrowing tucutucu, which
is occasionally blind, and then at certain moles, which are habitually
blind and have their eyes covered with skin; or when we look at the
blind animals inhabiting the dark caves of America and Europe. In both
varieties and species correlation of growth seems to have played a
most important part, so that when one part has been modified other
parts are necessarily modified. In both varieties and species
reversions to long-lost characters occur. How inexplicable on the
theory of creation is the occasional appearance of stripes on the
shoulder and legs of the several species of the horse-genus and in
their hybrids! How simply is this fact explained if we believe that
these species have descended from a striped progenitor, in the same
manner as the several domestic breeds of pigeon have descended from
the blue and barred rock-pigeon!

On the ordinary view of each species having been independently
created, why should the specific characters, or those by which the
species of the same genus differ from each other, be more variable
than the generic characters in which they all agree? Why, for
instance, should the colour of a flower be more likely to vary in any
one species of a genus, if the other species, supposed to have been
created independently, have differently coloured flowers, than if all
the species of the genus have the same coloured flowers? If species
are only well-marked varieties, of which the characters have become in
a high degree permanent, we can understand this fact; for they have
already varied since they branched off from a common progenitor in
certain characters, by which they have come to be specifically
distinct from each other; and therefore these same characters would be
more likely still to be variable than the generic characters which
have been inherited without change for an enormous period. It is
inexplicable on the theory of creation why a part developed in a very
unusual manner in any one species of a genus, and therefore, as we may
naturally infer, of great importance to the species, should be
eminently liable to variation; but, on my view, this part has
undergone, since the several species branched off from a common
progenitor, an unusual amount of variability and modification, and
therefore we might expect this part generally to be still variable.
But a part may be developed in the most unusual manner, like the wing
of a bat, and yet not be more variable than any other structure, if
the part be common to many subordinate forms, that is, if it has been
inherited for a very long period; for in this case it will have been
rendered constant by long-continued natural selection.

Glancing at instincts, marvellous as some are, they offer no greater
difficulty than does corporeal structure on the theory of the natural
selection of successive, slight, but profitable modifications. We can
thus understand why nature moves by graduated steps in endowing
different animals of the same class with their several instincts. I
have attempted to show how much light the principle of gradation
throws on the admirable architectural powers of the hive-bee. Habit no
doubt sometimes comes into play in modifying instincts; but it
certainly is not indispensable, as we see, in the case of neuter
insects, which leave no progeny to inherit the effects of
long-continued habit. On the view of all the species of the same genus
having descended from a common parent, and having inherited much in
common, we can understand how it is that allied species, when placed
under considerably different conditions of life, yet should follow
nearly the same instincts; why the thrush of South America, for
instance, lines her nest with mud like our British species. On the
view of instincts having been slowly acquired through natural
selection we need not marvel at some instincts being apparently not
perfect and liable to mistakes, and at many instincts causing other
animals to suffer.

If species be only well-marked and permanent varieties, we can at once
see why their crossed offspring should follow the same complex laws in
their degrees and kinds of resemblance to their parents,--in being
absorbed into each other by successive crosses, and in other such
points,--as do the crossed offspring of acknowledged varieties. On the
other hand, these would be strange facts if species have been
independently created, and varieties have been produced by secondary
laws.

If we admit that the geological record is imperfect in an extreme
degree, then such facts as the record gives, support the theory of
descent with modification. New species have come on the stage slowly
and at successive intervals; and the amount of change, after equal
intervals of time, is widely different in different groups. The
extinction of species and of whole groups of species, which has played
so conspicuous a part in the history of the organic world, almost
inevitably follows on the principle of natural selection; for old
forms will be supplanted by new and improved forms. Neither single
species nor groups of species reappear when the chain of ordinary
generation has once been broken. The gradual diffusion of dominant
forms, with the slow modification of their descendants, causes the
forms of life, after long intervals of time, to appear as if they had
changed simultaneously throughout the world. The fact of the fossil
remains of each formation being in some degree intermediate in
character between the fossils in the formations above and below, is
simply explained by their intermediate position in the chain of
descent. The grand fact that all extinct organic beings belong to the
same system with recent beings, falling either into the same or into
intermediate groups, follows from the living and the extinct being the
offspring of common parents. As the groups which have descended from
an ancient progenitor have generally diverged in character, the
progenitor with its early descendants will often be intermediate in
character in comparison with its later descendants; and thus we can
see why the more ancient a fossil is, the oftener it stands in some
degree intermediate between existing and allied groups. Recent forms
are generally looked at as being, in some vague sense, higher than
ancient and extinct forms; and they are in so far higher as the later
and more improved forms have conquered the older and less improved
organic beings in the struggle for life. Lastly, the law of the long
endurance of allied forms on the same continent,--of marsupials in
Australia, of edentata in America, and other such cases,--is
intelligible, for within a confined country, the recent and the
extinct will naturally be allied by descent.

Looking to geographical distribution, if we admit that there has been
during the long course of ages much migration from one part of the
world to another, owing to former climatal and geographical changes
and to the many occasional and unknown means of dispersal, then we can
understand, on the theory of descent with modification, most of the
great leading facts in Distribution. We can see why there should be so
striking a parallelism in the distribution of organic beings
throughout space, and in their geological succession throughout time;
for in both cases the beings have been connected by the bond of
ordinary generation, and the means of modification have been the same.
We see the full meaning of the wonderful fact, which must have struck
every traveller, namely, that on the same continent, under the most
diverse conditions, under heat and cold, on mountain and lowland, on
deserts and marshes, most of the inhabitants within each great class
are plainly related; for they will generally be descendants of the
same progenitors and early colonists. On this same principle of former
migration, combined in most cases with modification, we can
understand, by the aid of the Glacial period, the identity of some few
plants, and the close alliance of many others, on the most distant
mountains, under the most different climates; and likewise the close
alliance of some of the inhabitants of the sea in the northern and
southern temperate zones, though separated by the whole intertropical
ocean. Although two areas may present the same physical conditions of
life, we need feel no surprise at their inhabitants being widely
different, if they have been for a long period completely separated
from each other; for as the relation of organism to organism is the
most important of all relations, and as the two areas will have
received colonists from some third source or from each other, at
various periods and in different proportions, the course of
modification in the two areas will inevitably be different.

On this view of migration, with subsequent modification, we can see
why oceanic islands should be inhabited by few species, but of these,
that many should be peculiar. We can clearly see why those animals
which cannot cross wide spaces of ocean, as frogs and terrestrial
mammals, should not inhabit oceanic islands; and why, on the other
hand, new and peculiar species of bats, which can traverse the ocean,
should so often be found on islands far distant from any continent.
Such facts as the presence of peculiar species of bats, and the
absence of all other mammals, on oceanic islands, are utterly
inexplicable on the theory of independent acts of creation.

The existence of closely allied or representative species in any two
areas, implies, on the theory of descent with modification, that the
same parents formerly inhabited both areas; and we almost invariably
find that wherever many closely allied species inhabit two areas, some
identical species common to both still exist. Wherever many closely
allied yet distinct species occur, many doubtful forms and varieties
of the same species likewise occur. It is a rule of high generality
that the inhabitants of each area are related to the inhabitants of
the nearest source whence immigrants might have been derived. We see
this in nearly all the plants and animals of the Galapagos
archipelago, of Juan Fernandez, and of the other American islands
being related in the most striking manner to the plants and animals of
the neighbouring American mainland; and those of the Cape de Verde
archipelago and other African islands to the African mainland. It must
be admitted that these facts receive no explanation on the theory of
creation.

The fact, as we have seen, that all past and present organic beings
constitute one grand natural system, with group subordinate to group,
and with extinct groups often falling in between recent groups, is
intelligible on the theory of natural selection with its contingencies
of extinction and divergence of character. On these same principles we
see how it is, that the mutual affinities of the species and genera
within each class are so complex and circuitous. We see why certain
characters are far more serviceable than others for
classification;--why adaptive characters, though of paramount
importance to the being, are of hardly any importance in
classification; why characters derived from rudimentary parts, though
of no service to the being, are often of high classificatory value;
and why embryological characters are the most valuable of all. The
real affinities of all organic beings are due to inheritance or
community of descent. The natural system is a genealogical
arrangement, in which we have to discover the lines of descent by the
most permanent characters, however slight their vital importance may
be.

The framework of bones being the same in the hand of a man, wing of a
bat, fin of the porpoise, and leg of the horse,--the same number of
vertebrae forming the neck of the giraffe and of the elephant,--and
innumerable other such facts, at once explain themselves on the theory
of descent with slow and slight successive modifications. The
similarity of pattern in the wing and leg of a bat, though used for
such different purpose,--in the jaws and legs of a crab,--in the
petals, stamens, and pistils of a flower, is likewise intelligible on
the view of the gradual modification of parts or organs, which were
alike in the early progenitor of each class. On the principle of
successive variations not always supervening at an early age, and
being inherited at a corresponding not early period of life, we can
clearly see why the embryos of mammals, birds, reptiles, and fishes
should be so closely alike, and should be so unlike the adult forms.
We may cease marvelling at the embryo of an air-breathing mammal or
bird having branchial slits and arteries running in loops, like those
in a fish which has to breathe the air dissolved in water, by the aid
of well-developed branchiae.

Disuse, aided sometimes by natural selection, will often tend to
reduce an organ, when it has become useless by changed habits or under
changed conditions of life; and we can clearly understand on this view
the meaning of rudimentary organs. But disuse and selection will
generally act on each creature, when it has come to maturity and has
to play its full part in the struggle for existence, and will thus
have little power of acting on an organ during early life; hence the
organ will not be much reduced or rendered rudimentary at this early
age. The calf, for instance, has inherited teeth, which never cut
through the gums of the upper jaw, from an early progenitor having
well-developed teeth; and we may believe, that the teeth in the mature
animal were reduced, during successive generations, by disuse or by
the tongue and palate having been fitted by natural selection to
browse without their aid; whereas in the calf, the teeth have been
left untouched by selection or disuse, and on the principle of
inheritance at corresponding ages have been inherited from a remote
period to the present day. On the view of each organic being and each
separate organ having been specially created, how utterly inexplicable
it is that parts, like the teeth in the embryonic calf or like the
shrivelled wings under the soldered wing-covers of some beetles,
should thus so frequently bear the plain stamp of inutility! Nature
may be said to have taken pains to reveal, by rudimentary organs and
by homologous structures, her scheme of modification, which it seems
that we wilfully will not understand.

I have now recapitulated the chief facts and considerations which have
thoroughly convinced me that species have changed, and are still
slowly changing by the preservation and accumulation of successive
slight favourable variations. Why, it may be asked, have all the most
eminent living naturalists and geologists rejected this view of the
mutability of species? It cannot be asserted that organic beings in a
state of nature are subject to no variation; it cannot be proved that
the amount of variation in the course of long ages is a limited
quantity; no clear distinction has been, or can be, drawn between
species and well-marked varieties. It cannot be maintained that
species when intercrossed are invariably sterile, and varieties
invariably fertile; or that sterility is a special endowment and sign
of creation. The belief that species were immutable productions was
almost unavoidable as long as the history of the world was thought to
be of short duration; and now that we have acquired some idea of the
lapse of time, we are too apt to assume, without proof, that the
geological record is so perfect that it would have afforded us plain
evidence of the mutation of species, if they had undergone mutation.

But the chief cause of our natural unwillingness to admit that one
species has given birth to other and distinct species, is that we are
always slow in admitting any great change of which we do not see the
intermediate steps. The difficulty is the same as that felt by so many
geologists, when Lyell first insisted that long lines of inland cliffs
had been formed, and great valleys excavated, by the slow action of
the coast-waves. The mind cannot possibly grasp the full meaning of
the term of a hundred million years; it cannot add up and perceive the
full effects of many slight variations, accumulated during an almost
infinite number of generations.

Although I am fully convinced of the truth of the views given in this
volume under the form of an abstract, I by no means expect to convince
experienced naturalists whose minds are stocked with a multitude of
facts all viewed, during a long course of years, from a point of view
directly opposite to mine. It is so easy to hide our ignorance under
such expressions as the "plan of creation," "unity of design," etc.,
and to think that we give an explanation when we only restate a fact.
Any one whose disposition leads him to attach more weight to
unexplained difficulties than to the explanation of a certain number
of facts will certainly reject my theory. A few naturalists, endowed
with much flexibility of mind, and who have already begun to doubt on
the immutability of species, may be influenced by this volume; but I
look with confidence to the future, to young and rising naturalists,
who will be able to view both sides of the question with impartiality.
Whoever is led to believe that species are mutable will do good
service by conscientiously expressing his conviction; for only thus
can the load of prejudice by which this subject is overwhelmed be
removed.

Several eminent naturalists have of late published their belief that a
multitude of reputed species in each genus are not real species; but
that other species are real, that is, have been independently created.
This seems to me a strange conclusion to arrive at. They admit that a
multitude of forms, which till lately they themselves thought were
special creations, and which are still thus looked at by the majority
of naturalists, and which consequently have every external
characteristic feature of true species,--they admit that these have
been produced by variation, but they refuse to extend the same view to
other and very slightly different forms. Nevertheless they do not
pretend that they can define, or even conjecture, which are the
created forms of life, and which are those produced by secondary laws.
They admit variation as a vera causa in one case, they arbitrarily
reject it in another, without assigning any distinction in the two
cases. The day will come when this will be given as a curious
illustration of the blindness of preconceived opinion. These authors
seem no more startled at a miraculous act of creation than at an
ordinary birth. But do they really believe that at innumerable periods
in the earth's history certain elemental atoms have been commanded
suddenly to flash into living tissues? Do they believe that at each
supposed act of creation one individual or many were produced? Were
all the infinitely numerous kinds of animals and plants created as
eggs or seed, or as full grown? and in the case of mammals, were they
created bearing the false marks of nourishment from the mother's womb?
Although naturalists very properly demand a full explanation of every
difficulty from those who believe in the mutability of species, on
their own side they ignore the whole subject of the first appearance
of species in what they consider reverent silence.

It may be asked how far I extend the doctrine of the modification of
species. The question is difficult to answer, because the more
distinct the forms are which we may consider, by so much the arguments
fall away in force. But some arguments of the greatest weight extend
very far. All the members of whole classes can be connected together
by chains of affinities, and all can be classified on the same
principle, in groups subordinate to groups. Fossil remains sometimes
tend to fill up very wide intervals between existing orders. Organs in
a rudimentary condition plainly show that an early progenitor had the
organ in a fully developed state; and this in some instances
necessarily implies an enormous amount of modification in the
descendants. Throughout whole classes various structures are formed on
the same pattern, and at an embryonic age the species closely resemble
each other. Therefore I cannot doubt that the theory of descent with
modification embraces all the members of the same class. I believe
that animals have descended from at most only four or five
progenitors, and plants from an equal or lesser number.

Analogy would lead me one step further, namely, to the belief that all
animals and plants have descended from some one prototype. But analogy
may be a deceitful guide. Nevertheless all living things have much in
common, in their chemical composition, their germinal vesicles, their
cellular structure, and their laws of growth and reproduction. We see
this even in so trifling a circumstance as that the same poison often
similarly affects plants and animals; or that the poison secreted by
the gall-fly produces monstrous growths on the wild rose or oak-tree.
Therefore I should infer from analogy that probably all the organic
beings which have ever lived on this earth have descended from some
one primordial form, into which life was first breathed. When the
views entertained in this volume on the origin of species, or when
analogous views are generally admitted, we can dimly foresee that
there will be a considerable revolution in natural history.
Systematists will be able to pursue their labours as at present; but
they will not be incessantly haunted by the shadowy doubt whether this
or that form be in essence a species. This I feel sure, and I speak
after experience, will be no slight relief. The endless disputes
whether or not some fifty species of British brambles are true species
will cease. Systematists will have only to decide (not that this will
be easy) whether any form be sufficiently constant and distinct from
other forms, to be capable of definition; and if definable, whether
the differences be sufficiently important to deserve a specific name.
This latter point will become a far more essential consideration than
it is at present; for differences, however slight, between any two
forms, if not blended by intermediate gradations, are looked at by
most naturalists as sufficient to raise both forms to the rank of
species. Hereafter we shall be compelled to acknowledge that the only
distinction between species and well-marked varieties is, that the
latter are known, or believed, to be connected at the present day by
intermediate gradations, whereas species were formerly thus connected.
Hence, without quite rejecting the consideration of the present
existence of intermediate gradations between any two forms, we shall
be led to weigh more carefully and to value higher the actual amount
of difference between them. It is quite possible that forms now
generally acknowledged to be merely varieties may hereafter be thought
worthy of specific names, as with the primrose and cowslip; and in
this case scientific and common language will come into accordance. In
short, we shall have to treat species in the same manner as those
naturalists treat genera, who admit that genera are merely artificial
combinations made for convenience. This may not be a cheering
prospect; but we shall at least be freed from the vain search for the
undiscovered and undiscoverable essence of the term species.

The other and more general departments of natural history will rise
greatly in interest. The terms used by naturalists of affinity,
relationship, community of type, paternity, morphology, adaptive
characters, rudimentary and aborted organs, etc., will cease to be
metaphorical, and will have a plain signification. When we no longer
look at an organic being as a savage looks at a ship, as at something
wholly beyond his comprehension; when we regard every production of
nature as one which has had a history; when we contemplate every
complex structure and instinct as the summing up of many contrivances,
each useful to the possessor, nearly in the same way as when we look
at any great mechanical invention as the summing up of the labour, the
experience, the reason, and even the blunders of numerous workmen;
when we thus view each organic being, how far more interesting, I
speak from experience, will the study of natural history become!

A grand and almost untrodden field of inquiry will be opened, on the
causes and laws of variation, on correlation of growth, on the effects
of use and disuse, on the direct action of external conditions, and so
forth. The study of domestic productions will rise immensely in value.
A new variety raised by man will be a far more important and
interesting subject for study than one more species added to the
infinitude of already recorded species. Our classifications will come
to be, as far as they can be so made, genealogies; and will then truly
give what may be called the plan of creation. The rules for
classifying will no doubt become simpler when we have a definite
object in view. We possess no pedigrees or armorial bearings; and we
have to discover and trace the many diverging lines of descent in our
natural genealogies, by characters of any kind which have long been
inherited. Rudimentary organs will speak infallibly with respect to
the nature of long-lost structures. Species and groups of species,
which are called aberrant, and which may fancifully be called living
fossils, will aid us in forming a picture of the ancient forms of
life. Embryology will reveal to us the structure, in some degree
obscured, of the prototypes of each great class.

When we can feel assured that all the individuals of the same species,
and all the closely allied species of most genera, have within a not
very remote period descended from one parent, and have migrated from
some one birthplace; and when we better know the many means of
migration, then, by the light which geology now throws, and will
continue to throw, on former changes of climate and of the level of
the land, we shall surely be enabled to trace in an admirable manner
the former migrations of the inhabitants of the whole world. Even at
present, by comparing the differences of the inhabitants of the sea on
the opposite sides of a continent, and the nature of the various
inhabitants of that continent in relation to their apparent means of
immigration, some light can be thrown on ancient geography.

The noble science of Geology loses glory from the extreme imperfection
of the record. The crust of the earth with its embedded remains must
not be looked at as a well-filled museum, but as a poor collection
made at hazard and at rare intervals. The accumulation of each great
fossiliferous formation will be recognised as having depended on an
unusual concurrence of circumstances, and the blank intervals between
the successive stages as having been of vast duration. But we shall be
able to gauge with some security the duration of these intervals by a
comparison of the preceding and succeeding organic forms. We must be
cautious in attempting to correlate as strictly contemporaneous two
formations, which include few identical species, by the general
succession of their forms of life. As species are produced and
exterminated by slowly acting and still existing causes, and not by
miraculous acts of creation and by catastrophes; and as the most
important of all causes of organic change is one which is almost
independent of altered and perhaps suddenly altered physical
conditions, namely, the mutual relation of organism to organism,--the
improvement of one being entailing the improvement or the
extermination of others; it follows, that the amount of organic change
in the fossils of consecutive formations probably serves as a fair
measure of the lapse of actual time. A number of species, however,
keeping in a body might remain for a long period unchanged, whilst
within this same period, several of these species, by migrating into
new countries and coming into competition with foreign associates,
might become modified; so that we must not overrate the accuracy of
organic change as a measure of time. During early periods of the
earth's history, when the forms of life were probably fewer and
simpler, the rate of change was probably slower; and at the first dawn
of life, when very few forms of the simplest structure existed, the
rate of change may have been slow in an extreme degree. The whole
history of the world, as at present known, although of a length quite
incomprehensible by us, will hereafter be recognised as a mere
fragment of time, compared with the ages which have elapsed since the
first creature, the progenitor of innumerable extinct and living
descendants, was created.

In the distant future I see open fields for far more important
researches. Psychology will be based on a new foundation, that of the
necessary acquirement of each mental power and capacity by gradation.
Light will be thrown on the origin of man and his history.

Authors of the highest eminence seem to be fully satisfied with the
view that each species has been independently created. To my mind it
accords better with what we know of the laws impressed on matter by
the Creator, that the production and extinction of the past and
present inhabitants of the world should have been due to secondary
causes, like those determining the birth and death of the individual.
When I view all beings not as special creations, but as the lineal
descendants of some few beings which lived long before the first bed
of the Silurian system was deposited, they seem to me to become
ennobled. Judging from the past, we may safely infer that not one
living species will transmit its unaltered likeness to a distant
futurity. And of the species now living very few will transmit progeny
of any kind to a far distant futurity; for the manner in which all
organic beings are grouped, shows that the greater number of species
of each genus, and all the species of many genera, have left no
descendants, but have become utterly extinct. We can so far take a
prophetic glance into futurity as to foretel that it will be the
common and widely-spread species, belonging to the larger and dominant
groups, which will ultimately prevail and procreate new and dominant
species. As all the living forms of life are the lineal descendants of
those which lived long before the Silurian epoch, we may feel certain
that the ordinary succession by generation has never once been broken,
and that no cataclysm has desolated the whole world. Hence we may look
with some confidence to a secure future of equally inappreciable
length. And as natural selection works solely by and for the good of
each being, all corporeal and mental endowments will tend to progress
towards perfection.

It is interesting to contemplate an entangled bank, clothed with many
plants of many kinds, with birds singing on the bushes, with various
insects flitting about, and with worms crawling through the damp
earth, and to reflect that these elaborately constructed forms, so
different from each other, and dependent on each other in so complex a
manner, have all been produced by laws acting around us. These laws,
taken in the largest sense, being Growth with Reproduction;
Inheritance which is almost implied by reproduction; Variability from
the indirect and direct action of the external conditions of life, and
from use and disuse; a Ratio of Increase so high as to lead to a
Struggle for Life, and as a consequence to Natural Selection,
entailing Divergence of Character and the Extinction of less-improved
forms. Thus, from the war of nature, from famine and death, the most
exalted object which we are capable of conceiving, namely, the
production of the higher animals, directly follows. There is grandeur
in this view of life, with its several powers, having been originally
breathed into a few forms or into one; and that, whilst this planet
has gone cycling on according to the fixed law of gravity, from so
simple a beginning endless forms most beautiful and most wonderful
have been, and are being, evolved.



INDEX.


Aberrant groups, 429.

Abyssinia, plants of, 375.

Acclimatisation, 139.

Affinities:
of extinct species, 329.
of organic beings, 411.

Agassiz:
on Amblyopsis, 139.
on groups of species suddenly appearing, 302, 305.
on embryological succession, 338.
on the glacial period, 366.
on embryological characters, 418.
on the embryos of vertebrata, 439.
on parallelism of embryological development and geological succession,
449.

Algae of New Zealand, 376.

Alligators, males, fighting, 88.

Amblyopsis, blind fish, 139.

America, North:
productions allied to those of Europe, 371.
boulders and glaciers of, 373.
South, no modern formations on west coast, 290.

Ammonites, sudden extinction of, 321.

Anagallis, sterility of, 247.

Analogy of variations, 159.

Ancylus, 386.

Animals:
not domesticated from being variable, 17.
domestic, descended from several stocks, 19.
acclimatisation of, 141.
of Australia, 116.
with thicker fur in cold climates, 133.
blind, in caves, 137.
extinct, of Australia, 339.

Anomma, 240.

Antarctic islands, ancient flora of, 399.

Antirrhinum, 161.

Ants:
attending aphides, 211.
slave-making instinct, 219.

Ants, neuter, structure of, 236.

Aphides attended by ants, 211.

Aphis, development of, 442.

Apteryx, 182.

Arab horses, 35,

Aralo-Caspian Sea, 339.

Archiac, M. de, on the succession of species, 325.

Artichoke, Jerusalem, 142.

Ascension, plants of, 389.

Asclepias, pollen of, 193.

Asparagus, 359.

Aspicarpa, 417.

Asses, striped, 163.

Ateuchus, 135,

Audubon:
on habits of frigate-bird, 185.
on variation in birds'-nests, 212,
on heron eating seeds, 387.

Australia:
animals of, 116.
dogs of, 215.
extinct animals of, 339.
European plants in, 375.

Azara on flies destroying cattle, 72.

Azores, flora of, 363.

Babington, Mr., on British plants, 48.

Balancement of growth, 147.

Bamboo with hooks, 197.

Barberry, flowers of, 98.

Barrande, M. :
on Silurian colonies, 313.
on the succession of species, 325.
on parallelism of palaeozoic formations, 328.
on affinities of ancient species, 330.

Barriers, importance of, 347.

Batrachians on islands, 393.

Bats:
how structure acquired, 180.
distribution of, 394.

Bear, catching water-insects, 184.

Bee:
sting of, 202.
queen, killing rivals, 202.

Bees fertilising flowers, 73.

Bees:
hive, not sucking the red clover, 95.
cell-making instinct, 224.
humble, cells of, 225.
parasitic, 218.

Beetles:
wingless, in Madeira, 135.
with deficient tarsi, 135.

Bentham, Mr. :
on British plants, 48.
on classification, 419.

Berkeley, Mr., on seeds in salt-water, 358.

Bermuda, birds of, 391.

Birds:
acquiring fear, 212.
annually cross the Atlantic, 364.
colour of, on continents, 132.
fossil, in caves of Brazil, 339.
of Madeira, Bermuda, and Galapagos, 390.
song of males, 89.
transporting seeds, 361.
waders, 386.
wingless, 134, 182.
with traces of embryonic teeth, 451.

Bizcacha, 349.
affinities of, 429.

Bladder for swimming in fish, 190.

Blindness of cave animals, 137,

Blyth, Mr. :
on distinctness of Indian cattle, 18.
on striped Hemionus, 163.
on crossed geese, 253.

Boar, shoulder-pad of, 88.

Borrow, Mr., on the Spanish pointer, 35.

Bory St. Vincent on Batrachians, 393.

Bosquet, M., on fossil Chthamalus, 304.

Boulders, erratic, on the Azores, 363.

Branchiae, 190.

Brent, Mr. :
on house-tumblers, 214.
on hawks killing pigeons, 362.

Brewer, Dr., on American cuckoo, 217.

Britain, mammals of, 395.

Bronn on duration of specific forms, 293.

Brown, Robert, on classification, 414.

Buckman on variation in plants, 10.

Buzareingues on sterility of varieties, 270.

Cabbage, varieties of, crossed, 99.

Calceolaria, 251.

Canary-birds, sterility of hybrids, 252.

Cape de Verde islands, 398.

Cape of Good Hope, plants of, 110, 375.

Carrier-pigeons killed by hawks, 362.

Cassini on flowers of compositae, 145.

Catasetum, 424.

Cats:
with blue eyes, deaf, 12.
variation in habits of, 91.
curling tail when going to spring, 201.

Cattle:
destroying fir-trees, 71.
destroyed by flies in La Plata, 72.
breeds of, locally extinct, 111.
fertility of Indian and European breeds, 254.

Cave, inhabitants of, blind, 137.

Centres of creation, 352.

Cephalopodae, development of, 442.

Cervulus, 253.

Cetacea, teeth and hair, 144.

Ceylon, plants of, 375.

Chalk formation, 322.

Characters:
divergence of, 111.
sexual, variable, 156.
adaptive or analogical, 427.

Charlock, 76,

Checks:
to increase, 67.
mutual, 71.

Chickens, instinctive tameness of, 216.

Chthamalinae, 288.

Chthamalus, cretacean species of, 304.

Circumstances favourable:
to selection of domestic products, 40.
to natural selection, 101.

Cirripedes:
capable of crossing, 101.
carapace aborted, 148.
their ovigerous frena, 192.
fossil, 304.
larvae of, 440.

Classification, 413.

Clift, Mr., on the succession of types, 339.

Climate:
effects of, in checking increase of beings, 68.
adaptation of, to organisms, 139.

Cobites, intestine of, 190.

Cockroach, 76.

Collections, palaeontological, poor, 287.

Colour:
influenced by climate, 132.
in relation to attacks by flies, 198.

Columba livia, parent of domestic pigeons, 23.

Colymbetes, 386.

Compensation of growth, 147.

Compositae:
outer and inner florets of, 144.
male flowers of, 451.

Conclusion, general, 480.

Conditions, slight changes in, favourable to fertility, 267.

Coot, 185.

Coral:
islands, seeds drifted to, 360.
reefs, indicating movements of earth, 309.

Corn-crake, 185.

Correlation:
of growth in domestic productions, 11.
of growth, 143, 198.

Cowslip, 49.

Creation, single centres of, 352.

Crinum, 250.

Crosses, reciprocal, 258.

Crossing:
of domestic animals, importance in altering breeds, 20.
advantages of, 96.
unfavourable to selection, 102.

Crustacea of New Zealand, 376.

Crustacean, blind, 137.

Cryptocerus, 238.

Ctenomys, blind, 137.

Cuckoo, instinct of, 216.

Currants, grafts of, 262.

Currents of sea, rate of, 359.

Cuvier:
on conditions of existence, 206.
on fossil monkeys, 303.

Cuvier, Fred., on instinct, 208.

Dana, Professor:
on blind cave-animals, 139.
on relations of crustaceans of Japan, 372.
on crustaceans of New Zealand, 376.

De Candolle:
on struggle for existence, 62.
on umbelliferae, 146.
on general affinities, 430.

De Candolle, Alph.:
on low plants, widely dispersed, 406.
on widely-ranging plants being variable, 53.
on naturalisation, 115.
on winged seeds, 146.
on Alpine species suddenly becoming rare, 175.
on distribution of plants with large seeds, 360.
on vegetation of Australia, 379.
on fresh-water plants, 386.
on insular plants, 389.

Degradation of coast-rocks, 282.

Denudation:
rate of, 285.
of oldest rocks, 308.

Development of ancient forms, 336.

Devonian system, 334.

Dianthus, fertility of crosses, 256.

Dirt on feet of birds, 362.

Dispersal:
means of, 356.
during glacial period, 365.

Distribution:
geographical, 346.
means of, 356.

Disuse, effects of, under nature, 134.

Divergence of character, 111.

Division, physiological, of labour, 115.

Dogs:
hairless, with imperfect teeth, 12.
descended from several wild stocks, 18.
domestic instincts of, 213.
inherited civilisation of, 215.
fertility of breeds together, 254.
of crosses, 268,
proportions of, when young, 444.

Domestication, variation under, 7.

Downing, Mr., on fruit-trees in America, 85.

Downs, North and South, 285.

Dragon-flies, intestines of, 190.

Drift-timber, 360.

Driver-ant, 240.

Drones killed by other bees, 202.

Duck:
domestic, wings of, reduced, 11.
logger-headed, 182.

Duckweed, 385.

Dugong, affinities of, 414.

Dung-beetles with deficient tarsi, 135.

Dyticus, 386.

Earl, Mr. W., on the Malay Archipelago, 395.

Ears:
drooping, in domestic animals, 11.
rudimentary, 454.

Earth, seeds in roots of trees, 361.

Eciton, 238.

Economy of organisation, 147.

Edentata:
teeth and hair, 144.
fossil species of, 339.

Edwards, Milne:
on physiological divisions of labour, 115.
on gradations of structure, 194.
on embryological characters, 418.

Eggs, young birds escaping from, 87.

Electric organs, 192.

Elephant:
rate of increase, 64.
of glacial period, 141.

Embryology, 439.

Existence:
struggle for, 60.
conditions of, 206.

Extinction:
as bearing on natural selection, 109.
of domestic varieties, 111.
317.

Eye:
structure of, 187.
correction for aberration, 202.

Eyes reduced in moles, 137.

Fabre, M., on parasitic sphex, 218.

Falconer, Dr. :
on naturalization of plants in India, 65.
on fossil crocodile, 313.
on elephants and mastodons, 334,
and Cautley on mammals of sub-Himalayan beds, 340.

Falkland Island, wolf of, 393.

Faults, 285.

Faunas, marine, 348.

Fear, instinctive, in birds, 212.

Feet of birds, young molluscs adhering to, 385.

Fertility:
of hybrids, 249.
from slight changes in conditions, 267.
of crossed varieties, 267.

Fir-trees:
destroyed by cattle, 71.
pollen of, 203.

Fish:
flying, 182.
teleostean, sudden appearance of, 305.
eating seeds, 362, 387.
fresh-water, distribution of, 384.

Fishes:
ganoid, now confined to fresh water, 107.
electric organs of, 192.
ganoid, living in fresh water, 321.
of southern hemisphere, 376.

Flight, powers of, how acquired, 182.

Flowers:
structure of, in relation to crossing, 97.
of compositae and umbelliferae, 144.

Forbes, E. :
on colours of shells, 132.
on abrupt range of shells in depth, 175.
on poorness of palaeontological collections, 287.
on continuous succession of genera, 316.
on continental extensions, 357.
on distribution during glacial period, 366,
on parallelism in time and space, 409.

Forests, changes in, in America, 74.

Formation, Devonian, 334.

Formations:
thickness of, in Britain, 284.
intermittent, 290.

Formica rufescens, 219.

Formica sanguinea, 219.

Formica flava, neuter of, 239.

Frena, ovigerous, of cirripedes, 192.

Fresh-water productions, dispersal of, 383.

Fries on species in large genera being closely allied to other
species, 57.

Frigate-bird, 185.

Frogs on islands, 393.

Fruit-trees:
gradual improvement of, 37.
in United States, 85.
varieties of, acclimatised in United States, 142.

Fuci, crossed, 258.

Fur, thicker in cold climates, 133.

Furze, 439.

Galapagos Archipelago:
birds of, 390.
productions of, 398, 400.

Galeopithecus, 181.

Game, increase of, checked by vermin, 68.

Gartner:
on sterility of hybrids, 247, 255.
on reciprocal crosses, 258.
on crossed maize and verbascum, 270.
on comparison of hybrids and mongrels, 272.

Geese:
fertility when crossed, 253.
upland, 185.

Genealogy important in classification, 425.

Geoffrey St. Hilaire:
on balancement, 147.
on homologous organs, 434.

Geoffrey St. Hilaire, Isidore:
on variability of repeated parts, 149.
on correlation in monstrosities, 11.
on correlation, 144.
on variable parts being often monstrous, 155.

Geographical distribution, 346.

Geography, ancient, 487.

Geology:
future progress of, 487.
imperfection of the record, 279.

Giraffe, tail of, 195.

Glacial period, 365.

Gmelin on distribution, 365.

Gnathodon, fossil, 368.

Godwin-Austen, Mr., on the Malay Archipelago, 299.

Goethe on compensation of growth, 147.

Gooseberry, grafts of, 262.

Gould, Dr. A., on land-shells, 397.

Gould, Mr.:
on colours of birds, 132.
on birds of the Galapagos, 398.
on distribution of genera of birds, 404.

Gourds, crossed, 270.

Grafts, capacity of, 261.

Grasses, varieties of, 113.

Gray, Dr. Asa:
on trees of United States, 100.
on naturalised plants in the United States, 115.
on rarity of intermediate varieties, 176.
on Alpine plants, 365.

Gray, Dr. J. E., on striped mule, 165.

Grebe, 185.

Groups, aberrant, 429.

Grouse:
colours of, 84.
red, a doubtful species, 49.

Growth:
compensation of, 147.
correlation of, in domestic products, 11.
correlation of, 143.

Habit:
effect of, under domestication, 11.
effect of, under nature, 134.
diversified, of same species, 183.

Hair and teeth, correlated, 144.

Harcourt, Mr. E. V., on the birds of Madeira, 391.

Hartung, M., on boulders in the Azores, 363.

Hazel-nuts, 359.

Hearne on habits of bears, 184.

Heath, changes in vegetation, 72,

Heer, O., on plants of Madeira, 107.

Helix pomatia, 397.

Helosciadium, 359.

Hemionus, striped, 163.

Herbert, W. :
on struggle for existence, 62.
on sterility of hybrids, 249.

Hermaphrodites crossing, 96.

Heron eating seed, 387.

Heron, Sir R., on peacocks, 89.

Heusinger on white animals not poisoned by certain plants, 12.

Hewitt, Mr., on sterility of first crosses. 264.

Himalaya:
glaciers of, 373.
plants of, 375.

Hippeastrum, 250.

Holly-trees, sexes of, 93.

Hollyhock, varieties of, crossed, 271.

Hooker, Dr., on trees of New Zealand, 100.

Hooker, Dr. :
on acclimatisation of Himalayan trees, 140.
on flowers of umbelliferae, 145.
on glaciers of Himalaya, 373.
on algae of New Zealand, 376.
on vegetation at the base of the Himalaya, 378.
on plants of Tierra del Fuego, 374, 378.
on Australian plants, 375, 399.
on relations of flora of South America, 379.
on flora of the Antarctic lands, 381, 399.
on the plants of the Galapagos, 391, 398.

Hooks:
on bamboos, 197.
to seeds on islands, 392.

Horner, Mr., on the antiquity of Egyptians, 18.

Horns, rudimentary, 454.

Horse, fossil, in La Plata, 318.

Horses:
destroyed by flies in La Plata, 72.
striped, 163.
proportions of, when young, 445.

Horticulturists, selection applied by, 32.

Huber on cells of bees, 230.

Huber, P.:
on reason blended with instinct, 208.
on habitual nature of instincts, 208.
on slave making ants, 219.
on Melipona domestica, 225.

Humble-bees, cells of, 225.

Hunter, J., on secondary sexual characters, 150.

Hutton, Captain, on crossed geese, 253.

Huxley, Professor:
on structure of hermaphrodites, 101.
on embryological succession, 338.
on homologous organs, 438.
on the development of aphis, 442.

Hybrids and mongrels compared, 272.

Hybridism, 245.

Hydra, structure of, 190.

Ibla, 148.

Icebergs transporting seeds, 363.

Increase, rate of, 63.

Individuals:
numbers favourable to selection, 102.
many, whether simultaneously created, 356.

Inheritance:
laws of, 12.
at corresponding ages, 14, 86.

Insects:
colour of, fitted for habitations, 84.
sea-side, colours of, 132.
blind, in caves, 138.
luminous, 193.
neuter, 236.

Instinct, 207.

Instincts, domestic, 213.

Intercrossing, advantages of, 96.

Islands, oceanic, 388.

Isolation favourable to selection, 104.

Japan, productions of, 372.

Java, plants of, 375.

Jones, Mr. J. M., on the birds of Bermuda, 391.

Jussieu on classification, 417.

Kentucky, caves of, 137.

Kerguelen-land, flora of, 381, 399.

Kidney-bean, acclimatisation of, 142.

Kidneys of birds, 144.

Kirby on tarsi deficient in beetles, 135.

Knight, Andrew, on cause of variation, 7.

Kolreuter:
on the barberry, 98.
on sterility of hybrids, 247.
on reciprocal crosses, 258.
on crossed varieties of nicotiana, 271.
on crossing male and hermaphrodite flowers, 451.

Lamarck on adaptive characters, 427.

Land-shells:
distribution of, 397.
of Madeira, naturalised, 402.

Languages, classification of, 422.

Lapse, great, of time, 282.

Larvae, 440.

Laurel, nectar secreted by the leaves, 92.

Laws of variation, 131.

Leech, varieties of, 76.

Leguminosae, nectar secreted by glands, 92.

Lepidosiren, 107, 330.

Life, struggle for, 60.

Lingula, Silurian, 306.

Linnaeus, aphorism of, 413.

Lion:
mane of, 88.
young of, striped, 439.

Lobelia fulgens, 73, 98,

Lobelia, sterility of crosses, 250.

Loess of the Rhine, 384.

Lowness of structure connected with variability, 149.

Lowness, related to wide distribution, 406.

Lubbock, Mr., on the nerves of coccus, 46.

Lucas, Dr. P.:
on inheritance, 12.
on resemblance of child to parent, 275.

Lund and Clausen on fossils of Brazil, 339.

Lyell, Sir C.:
on the struggle for existence, 62.
on modern changes of the earth, 95.
on measure of denudation, 283.
on a carboniferous land-shell, 289.
on fossil whales, 303.
on strata beneath Silurian system, 307.
on the imperfection of the geological record, 310.
on the appearance of species, 312.
on Barrande's colonies, 313.
on tertiary formations of Europe and North America, 323.
on parallelism of tertiary formations, 328.
on transport of seeds by icebergs, 363.
on great alternations of climate, 382.
on the distribution of fresh-water shells, 385.
on land-shells of Madeira, 402.

Lyell and Dawson on fossilized trees in Nova Scotia, 296.

Macleay on analogical characters, 427.

Madeira:
plants of, 107.
beetles of, wingless, 135.
fossil land-shells of, 339.
birds of, 390.

Magpie tame in Norway, 212.

Maize, crossed, 270.

Malay Archipelago:
compared with Europe, 299.
mammals of, 395.

Malpighiaceae, 417.

Mammae, rudimentary, 451.

Mammals:
fossil, in secondary formation, 303.
insular, 393.

Man, origin of races of, 199.

Manatee, rudimentary nails of, 454.

Marsupials:
of Australia, 116.
fossil species of, 339.

Martens, M., experiment on seeds, 360.

Martin, Mr. W. C., on striped mules, 165.

Matteuchi on the electric organs of rays, 193.

Matthiola, reciprocal crosses of, 258.

Means of dispersal, 356.

Melipona domestica, 225.

Metamorphism of oldest rocks 308.

Mice:
destroying bees, 74.
acclimatisation of, 141.

Migration, bears on first appearance of fossils, 296.

Miller, Professor, on the cells of bees, 226.

Mirabilis, crosses of, 258.

Missel-thrush, 76.

Misseltoe, complex relations of, 3.

Mississippi, rate of deposition at mouth, 284.

Mocking-thrush of the Galapagos, 402.

Modification of species, how far applicable, 483.

Moles, blind, 137.

Mongrels:
fertility and sterility of, 267.
and hybrids compared, 272.

Monkeys, fossil, 303,

Monocanthus, 424.

Mons, Van, on the origin of fruit-trees, 29, 39.

Moquin-Tandon on sea-side plants, 132.

Morphology, 434.

Mozart, musical powers of, 209.

Mud, seeds in, 386.

Mules, striped, 165.

Muller, Dr. F., on Alpine Australian plants, 375.

Murchison, Sir R.:
on the formations of Russia, 289.
on azoic formations, 307.
on extinction, 317.

Mustela vison, 179.

Myanthus, 424.

Myrmecocystus, 238.

Myrmica, eyes of, 240.

Nails, rudimentary, 453.

Natural history:
future progress of, 484.
selection, 80.
system, 413.

Naturalisation:
of forms distinct from the indigenous species, 115.
in New Zealand, 201.

Nautilus, Silurian, 306.

Nectar of plants, 92.

Nectaries, how formed, 92.

Nelumbium luteum, 387.

Nests, variation in, 212.

Neuter insects, 236.

Newman, Mr., on humble-bees, 74.

New Zealand:
productions of, not perfect, 201.
naturalised products of, 337.
fossil birds of, 339.
glacial action in, 373,
crustaceans of, 376.
algae of, 376.
number of plants of, 389.
flora of, 399.

Nicotiana:
crossed varieties of, 271.
certain species very sterile, 257.

Noble, Mr., on fertility of Rhododendron, 251.

Nodules, phosphatic, in azoic rocks, 307,

Oak, varieties of, 50.

Onites apelles, 135.

Orchis, pollen of, 193,

Organs:
of extreme perfection, 186,
electric, of fishes, 192.
of little importance, 194.
homologous, 434.
rudiments of, 450.

Ornithorhynchus, 107, 416.

Ostrich:
not capable of flight, 134.
habit of laying eggs together, 218.
American, two species of, 349.

Otter, habits of, how acquired, 179.

Ouzel, water, 185.

Owen, Professor:
on birds not flying, 134.
on vegetative repetition, 149.
on variable length of arms in ourang-outang, 150.
on the swim-bladder of fishes, 191.
on electric organs, 192.
on fossil horse of La Plata, 319.
on relations of ruminants and pachyderms, 329.
on fossil birds of New Zealand, 339.
on succession of types, 339.
on affinities of the dugong, 414.
on homologous organs, 435.
on the metamorphosis of cephalopods and spiders, 442.

Pacific Ocean, faunas of, 348.

Paley on no organ formed to give pain, 201.

Pallas on the fertility of the wild stocks of domestic animals, 253.

Paraguay, cattle destroyed by flies, 72.

Parasites, 217.

Partridge, dirt on feet, 362.

Parts:
greatly developed, variable, 150.
degrees of utility of, 201.

Parus major, 183.

Passiflora, 251.

Peaches in United States, 85.

Pear, grafts of, 261.

Pelargonium:
flowers of, 145.
sterility of, 251.

Pelvis of women, 144,

Peloria, 145.

Period, glacial, 365.

Petrels, habits of, 184.

Phasianus, fertility of hybrids, 253.

Pheasant, young, wild, 216.

Philippi on tertiary species in Sicily, 312.

Pictet, Professor:
on groups of species suddenly appearing, 302, 305.
on rate of organic change, 313.
on continuous succession of genera, 316.
on close alliance of fossils in consecutive formations, 335.
on embryological succession, 338.

Pierce, Mr., on varieties of wolves, 91.

Pigeons:
with feathered feet and skin between toes, 12.
breeds described, and origin of, 20.
breeds of, how produced, 39, 42.
tumbler, not being able to get out of egg, 87.
reverting to blue colour, 160.
instinct of tumbling, 214.
carriers, killed by hawks, 362.
young of, 445.

Pistil, rudimentary, 451.

Plants:
poisonous, not affecting certain coloured animals, 12.
selection applied to, 32.
gradual improvement of, 37.
not improved in barbarous countries, 38.
destroyed by insects, 67.
in midst of range, have to struggle with other plants, 77.
nectar of, 92,
fleshy, on sea-shores, 132.
fresh-water, distribution of, 386.
low in scale, widely distributed, 406.

Plumage, laws of change in sexes of birds, 89.

Plums in the United States, 85.

Pointer dog:
origin of, 35.
habits of, 213.

Poison not affecting certain coloured animals, 12.

Poison, similar effect of, on animals and plants, 484.

Pollen of fir-trees, 203,

Poole, Col., on striped hemionus, 163.

Potamogeton, 387.

Prestwich, Mr., on English and French eocene formations, 328.

Primrose, 49.
sterility of, 247.

Primula, varieties of, 49.

Proteolepas, 148.

Proteus, 139.

Psychology, future progress of, 488.

Quagga, striped, 165.

Quince, grafts of, 261.

Rabbit, disposition of young, 215.

Races, domestic, characters of, 16.

Race-horses:
Arab, 35.
English, 356.

Ramond on plants of Pyrenees, 368.

Ramsay, Professor:
on thickness of the British formations, 284.
on faults, 285.

Ratio of increase, 63.

Rats:
supplanting each other, 76.
acclimatisation of, 141.
blind in cave, 137.

Rattle-snake, 201.

Reason and instinct, 208.

Recapitulation, general, 459.

Reciprocity of crosses, 258.

Record, geological, imperfect, 279.

Rengger on flies destroying cattle, 72.

Reproduction, rate of, 63.

Resemblance to parents in mongrels and hybrids, 273.

Reversion:
law of inheritance, 14.
in pigeons to blue colour, 160.

Rhododendron, sterility of, 251.

Richard, Professor, on Aspicarpa, 417.

Richardson, Sir J.:
on structure of squirrels, 180.
on fishes of the southern hemisphere, 376.

Robinia, grafts of, 262.

Rodents, blind, 137.

Rudimentary organs, 450.

Rudiments important for classification, 416.

Sageret on grafts, 262.

Salmons, males fighting, and hooked jaws of, 88.

Salt-water, how far injurious to seeds, 358.

Saurophagus sulphuratus, 183.

Schiodte on blind insects, 138.

Schlegel on snakes, 144

Sea-water, how far injurious to seeds, 358.

Sebright, Sir J.:
on crossed animals, 20.
on selection of pigeons, 31.

Sedgwick, Professor, on groups of species suddenly appearing, 302.

Seedlings destroyed by insects, 67.

Seeds:
nutriment in, 77.
winged, 146.
power of resisting salt-water, 358.
in crops and intestines of birds, 361.
eaten by fish, 362, 387.
in mud, 386.
hooked, on islands, 392.

Selection:
of domestic products, 29.
principle not of recent origin, 33.
unconscious, 34.
natural, 80.
sexual, 87.
natural, circumstances favourable to, 101,

Sexes, relations of, 87.

Sexual:
characters variable, 156.
selection, 87.

Sheep:
Merino, their selection, 31.
two sub-breeds unintentionally produced, 36.
mountain, varieties of, 76.

Shells:
colours of, 132.
littoral, seldom embedded, 288.
fresh-water, dispersal of, 385.
of Madeira, 391,
land, distribution of, 397.

Silene, fertility of crosses, 257.

Silliman, Professor, on blind rat, 137.

Skulls of young mammals, 197, 437.

Slave-making instinct, 219.

Smith, Col. Hamilton, on striped horses, 164.

Smith, Mr. Fred.:
on slave-making ants, 219.
on neuter ants, 239.

Smith, Mr., of Jordan Hill, on the degradation of coast-rocks, 283.

Snap-dragon, 161.

Somerville, Lord, on selection of sheep, 31.

Sorbus, grafts of, 262.

Spaniel, King Charles's breed, 35.

Species:
polymorphic, 46.
common, variable, 53.
in large genera variable, 54.
groups of, suddenly appearing, 302, 306.
beneath Silurian formations, 306.
successively appearing, 312.
changing simultaneously throughout the world, 322.

Spencer, Lord, on increase in size of cattle, 35.

Sphex, parasitic, 218.

Spiders, development of, 442.

Spitz-dog crossed with fox, 268.

Sports in plants, 9.

Sprengel, C. C.:
on crossing, 98.
on ray-florets, 145.

Squirrels, gradations in structure, 180.

Staffordshire, heath, changes in, 72.

Stag-beetles, fighting, 88.

Sterility:
from changed conditions of life, 9.
of hybrids, 246.
laws of, 254.
causes of, 263.
from unfavourable conditions, 265.
of certain varieties, 269.

St. Helena, productions of, 389.

St. Hilaire, Aug., on classification, 418.

St. John, Mr., on habits of cats, 91.

Sting of bee, 202.

Stocks, aboriginal, of domestic animals, 18,

Strata, thickness of, in Britain, 284.

Stripes on horses, 163.

Structure, degrees of utility of, 201.

Struggle for existence, 60.

Succession, geological, 312.

Succession of types in same areas, 338.

Swallow, one species supplanting another, 76.

Swim-bladder, 190.

System, natural, 413.

Tail:
of giraffe, 195.
of aquatic animals, 196.
rudimentary, 454.

Tarsi deficient, 135.

Tausch on umbelliferous flowers, 146.

Teeth and hair:
correlated, 144.
embryonic, traces of, in birds, 451.
rudimentary, in embryonic calf, 450, 480.

Tegetmeier, Mr., on cells of bees, 228, 233.

Temminck on distribution aiding classification, 419.

Thouin on grafts, 262.

Thrush:
aquatic species of, 185.
mocking, of the Galapagos, 402.
young of, spotted, 439.
nest of, 243.

Thuret, M., on crossed fuci, 258.

Thwaites, Mr., on acclimatisation, 140.

Tierra del Fuego:
dogs of, 215.
plants of, 374, 378.

Timber-drift, 360.

Time, lapse of, 282.

Titmouse, 183.

Toads on islands, 393.

Tobacco, crossed varieties of, 271.

Tomes, Mr., on the distribution of bats, 394.

Transitions in varieties rare, 172.

Trees:
on islands belong to peculiar orders, 392.
with separated sexes, 99.

Trifolium pratense, 73, 94.

Trifolium incarnatum, 94.

Trigonia, 321.

Trilobites, 306.
sudden extinction of, 321,

Troglodytes, 243.

Tucutucu, blind, 137.

Tumbler pigeons:
habits of, hereditary, 214.
young of, 446,

Turkey-cock, brush of hair on breast, 90.

Turkey:
naked skin on head, 197.
young, wild, 216.

Turnip and cabbage, analogous variations of, 159.

Type, unity of, 206.

Types, succession of, in same areas, 338.

Udders:
enlarged by use, 11.
rudimentary, 451.

Ulex, young leaves of, 439.

Umbelliferae, outer and inner florets of, 144.

Unity of type, 206.

Use:
effects of, under domestication, 11.
effects of, in a state of nature, 134.

Utility, how far important in the construction of each part, 199.

Valenciennes on fresh-water fish, 384.

Variability of mongrels and hybrids, 274.

Variation:
under domestication, 7.
caused by reproductive system being affected by conditions of life, 8.
under nature, 44.
laws of, 131.

Variations:
appear at corresponding ages, 14, 86.
analogous in distinct species, 159.

Varieties:
natural, 44.
struggle between, 75.
domestic, extinction of, 111.
transitional, rarity of, 172.
when crossed, fertile, 267.
when crossed, sterile, 269.
classification of, 423.

Verbascum:
sterility of, 251.
varieties of, crossed, 270.

Verneuil, M. de, on the succession of species, 325.

Viola tricolor, 73.

Volcanic islands, denudation of, 284.

Vulture, naked skin on head, 197.

Wading-birds, 386.

Wallace, Mr.:
on origin of species, 2.
on law of geographical distribution, 355.
on the Malay Archipelago, 395.

Wasp, sting of, 202.

Water, fresh, productions of, 383.

Water-hen, 185.

Waterhouse, Mr.:
on Australian marsupials, 116.
on greatly developed parts being variable, 150.
on the cells of bees, 225.
on general affinities, 429.

Water-ouzel, 185.

Watson, Mr. H. C.:
on range of varieties of British plants, 58.
on acclimatisation, 140.
on flora of Azores, 363.
on Alpine plants, 367, 376.
on rarity of intermediate varieties, 176.

Weald, denudation of, 285.

Web of feet in water-birds, 185.

West Indian islands, mammals of, 395.

Westwood:
on species in large genera being closely allied to others, 57.
on the tarsi of Engidae, 157.
on the antennae of hymenopterous insects, 416.

Whales, fossil, 303.

Wheat, varieties of, 113.

White Mountains, flora of, 365.

Wings, reduction of size, 134.

Wings:
of insects homologous with branchiae, 191.
rudimentary, in insects, 451.

Wolf:
crossed with dog, 214.
of Falkland Isles, 393.

Wollaston, Mr.:
on varieties of insects, 48.
on fossil varieties of land-shells in Madeira, 52.
on colours of insects on sea-shore, 132.
on wingless beetles, 135.
on rarity of intermediate varieties, 176.
on insular insects, 389.
on land-shells of Madeira, naturalised, 402.

Wolves, varieties of, 90.

Woodpecker:
habits of, 184.
green colour of, 197.

Woodward, Mr.:
on the duration of specific forms, 293.
on the continuous succession of genera, 316.
on the succession of types, 339.

World, species changing simultaneously throughout, 322.

Wrens, nest of, 243.

Youatt, Mr.:
on selection, 31.
on sub-breeds of sheep, 36.
on rudimentary horns in young cattle, 454.

Zebra, stripes on, 163.


THE END.










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