On the Origin of Species by Charles Darwin

On the Origin of Species

by Charles Darwin

'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

Contents

Introduction

Chapter I

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 II

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 III

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 IV

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 V

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 VI

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 VII

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 VIII

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 IX

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 X

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 XI

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 XII

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 XIII

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 XIV

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, &c., 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 I

Variation under Domestication

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, &c., 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, &c.,
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 deciding 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, &c.; 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, &c.,
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, &c., 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, &c., but
that each of these kingdoms possesses several peculiar breeds of cattle,
sheep, &c., 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, &c.--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,
&c., 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 Hon. W. Elliot from India, and by the Hon. C.
Murray from Persia. Many treatises in different languages have been
published on pigeons, and some of them are very important, as being of
considerably 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,
&c., 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, &c.,
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, &c., 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 II

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 any
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, &c., 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 whenever 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 III

Struggle for Existence

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, &c., 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 hundreds 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 IV

Natural Selection

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, &c., 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, &c.), 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, &c., 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, &c.; 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 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 V

Laws of Variation

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, &c., 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, &c.: 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, &c., 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, &c.),
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 Prof. 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, &c., 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 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, &c., 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, &c.,
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 VI

Difficulties on Theory

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, &c., 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

On the Origin of Species by Charles Darwin Hypertext Meanings and Commentaries from the Encyclopedia of the Self by Mark Zimmerman

On the Origin of Species
by Charles Darwin
Hypertext Meanings and Commentaries
from the Encyclopedia of the Self
by Mark Zimmerman