Transcriber's note: A few typographical errors have been corrected: they
are listed at the end of the text.
* * * * *
ON THE
ORIGIN OF SPECIES.
* * * * *
"But with regard to the material world, we can at least go so far as
this--we can perceive that events are brought about not by insulated
interpositions of Divine power, exerted in each particular case, but by the
establishment of general laws."
WHEWELL: _Bridgewater Treatise_.
"The only distinct meaning of the word 'natural' is _stated_, _fixed_, or
_settled_; since what is natural as much requires and presupposes an
intelligent agent to render it so, _i.e._ to effect it continually or at
stated times, as what is supernatural or miraculous does to effect it for
once."
BUTLER: _Analogy of Revealed Religion_.
"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._ (_1st Thousand_).
* * * * *
ON
THE ORIGIN OF SPECIES
BY MEANS OF NATURAL SELECTION,
OR THE
PRESERVATION OF FAVOURED RACES IN THE STRUGGLE
FOR LIFE.
BY CHARLES DARWIN, M.A.,
FELLOW OF THE ROYAL, GEOLOGICAL, LINNEAN, ETC., SOCIETIES;
AUTHOR OF 'JOURNAL OF RESEARCHES DURING H. M. S. BEAGLE'S VOYAGE
ROUND THE WORLD.'
_FIFTH THOUSAND._
LONDON:
JOHN MURRAY, ALBEMARLE STREET.
1860.
_The right of Translation is reserved._
* * * * *
LONDON: PRINTED BY W. CLOWES AND SONS, STAMFORD STREET,
AND CHARING CROSS.
* * * * *
{v}
CONTENTS.
* * * * *
INTRODUCTION
Page 1
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
7-43
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
44-59
{vi}
CHAPTER III.
STRUGGLE FOR EXISTENCE.
Its bearing 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
60-79
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
80-130
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
131-170
{vii}
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
171-206
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
207-244
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
245-278
{viii}
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 palæontological 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
279-311
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
312-345
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
346-382
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
383-410
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
411-458
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
459-490
* * * * *
{1}
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 {2} 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 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, {3}
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 {4} generations, some bird had given birth to a
woodpecker, and some plant to the missletoe, 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 the high
geometrical ratio of their {5} 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 leads to 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 {6} 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.
* * * * *
{7}
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.
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 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 great
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. {8} 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
function 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 discovered 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 {9} 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.
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 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. {10}
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 Müller 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, 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 are
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 {11} 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 or from light, and perhaps the thickness of fur from climate.
Habit also has a decided influence, as in the period of flowering with
plants when transported from one climate to another. In animals it has a
more marked effect; for instance, I find in the domestic duck that the
bones of the wing weigh less and the bones of the leg more, in proportion
to the whole skeleton, than do the same bones in the wild-duck; and I
presume that this change may be safely attributed to the domestic duck
flying much less, and walking more, than its wild parent. The great and
inherited development of the udders in cows and goats in countries where
they are habitually milked, in comparison with the state of these organs in
other countries, is another instance of the effect of use. Not a single
domestic animal can be named which has not in some country drooping ears;
and the view suggested by some authors, that the drooping is due to the
disuse of the muscles of the ear, from the animals not being much alarmed
by danger, seems probable.
There are many laws regulating variation, some few of which can be dimly
seen, and will be hereafter briefly mentioned. I will here only allude to
what may be called correlation of growth. Any change in the embryo or larva
will almost certainly entail changes in the mature animal. In
monstrosities, the correlations between quite distinct parts are very
curious; and many instances are given in Isidore Geoffroy St. Hilaire's
great work on this subject. Breeders believe that long limbs are almost
always accompanied by an elongated head. Some instances of correlation are
quite whimsical: thus {12} 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 any deviation of structure
often appears, and we see it in the {13} father and child, we cannot tell
whether it may not be due to the same cause having acted 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 a
peculiarity in different individuals of the same species, or 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 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 {14} 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 {15} 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 the same conditions, and
whilst kept in a considerable body, so that free intercrossing might check,
by blending together, any slight deviations in their 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 closely allied species, 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 {16} from each other, and from 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 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 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 {17} 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 natural species--for instance, of the many foxes--inhabiting
different quarters of the world. I do not believe, as we shall presently
see, that the whole amount of difference between the several breeds of the
dog has been produced under domestication; I believe that some small part
of the difference is due to their being descended from distinct species. In
the case of some other domesticated species, there is presumptive, or even
strong evidence, that all the breeds have descended from a single wild
stock.
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 reindeer, 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 wild species. The argument mainly relied
on by those who believe in the multiple origin {18} 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. Knowing, as we do, that savages are
very fond of taming animals, it seems to me unlikely, in the case of the
dog-genus, which is distributed in a wild state throughout the world, that
since man first appeared one single species alone should have been
domesticated. 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 {19} 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 have all 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
Canidæ--ever existed freely in a state of nature? It has often been loosely
said that all our races of dogs have {20} been produced by the crossing of
a few aboriginal species; but by crossing we can only get 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 {21} considerable
antiquity. I have associated with several eminent fanciers, and have been
permitted to join two of the London Pigeon Clubs. The diversity of the
breeds is something astonishing. Compare the English carrier and the
short-faced tumbler, and see the wonderful difference in their beaks,
entailing corresponding differences in their skulls. The carrier, more
especially the male bird, is also remarkable from the wonderful development
of the carunculated skin about the head, and this is accompanied by greatly
elongated eyelids, very large external orifices to the nostrils, and a wide
gape of mouth. The short-faced tumbler has a beak in outline almost like
that of a finch; and the common tumbler has the singular 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 {22} carried so erect that in
good birds the head and tail touch; the oil-gland is quite aborted. Several
other less distinct breeds might be 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 vertebræ
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
scutellæ 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 {23} 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 {24} 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 Columbidæ 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.
{25}
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 subspecies, 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, {26} 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 {27} 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
Columbidæ, 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
varieties, 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 Ægyptian 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 {28} 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 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. {29} 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 {30} 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 {31} 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 agriculturists 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; {32} 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 {33} 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 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 {34} 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
encyclopædia. 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,
{35} 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, {36} 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
{37} 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 the 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 {38} 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 {39} 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 {40} 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 civilization 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 {41} 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 {42} 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 {43} 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.
* * * * *
{44}