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1228 1859, First Edition
22764 1860, Second Edition
2009 1872, Sixth Edition, considered the definitive edition.
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, Linnæan, Etc., Societies;
Author Of ‘Journal Of Researches During H.M.S. Beagle’s Voyage
Round The World.’
LONDON:
JOHN MURRAY, ALBEMARLE STREET.
1859.
“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_, 1_st_, 1859.
Contents
INTRODUCTION.
1. VARIATION UNDER DOMESTICATION.
2. VARIATION UNDER NATURE.
3. STRUGGLE FOR EXISTENCE.
4. NATURAL SELECTION.
5. LAWS OF VARIATION.
6. DIFFICULTIES ON THEORY.
7. INSTINCT.
8. HYBRIDISM.
9. ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.
10. ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS.
11. GEOGRAPHICAL DISTRIBUTION.
12. GEOGRAPHICAL DISTRIBUTION—_continued_.
13. MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY:
14. RECAPITULATION AND CONCLUSION.
INDEX
DETEAILED CONTENTS. ON THE ORIGIN OF SPECIES.
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 2. VARIATION UNDER NATURE.
Variability.
Individual Differences.
Doubtful species.
Wide ranging, much diffused, and common species vary most.
Species of the larger genera in any country vary more than the
species of the smaller genera.
Many of the species of the larger genera resemble varieties in being
very closely, but unequally, related to each other, and in having
restricted ranges.
CHAPTER 3. STRUGGLE FOR EXISTENCE.
Bears on natural selection.
The term used in a wide sense.
Geometrical powers of increase.
Rapid increase of naturalised animals and plants.
Nature of the checks to increase.
Competition universal.
Effects of climate.
Protection from the number of individuals.
Complex relations of all animals and plants throughout nature.
Struggle for life most severe between individuals and varieties of
the same species; often severe between species of the same genus.
The relation of organism to organism the most important of all
relations.
CHAPTER 4. NATURAL SELECTION.
Natural Selection: its power compared with man’s selection, its power
on characters of trifling importance, its power at all ages and on
both sexes.
Sexual Selection.
On the generality of intercrosses between individuals of the same
species.
Circumstances favourable and unfavourable to Natural Selection,
namely, intercrossing, isolation, number of individuals.
Slow action.
Extinction caused by Natural Selection.
Divergence of Character, related to the diversity of inhabitants of
any small area, and to naturalisation.
Action of Natural Selection, through Divergence of Character and
Extinction, on the descendants from a common parent.
Explains the Grouping of all organic beings.
CHAPTER 5. LAWS OF VARIATION.
Effects of external conditions.
Use and disuse, combined with natural selection; organs of flight and
of vision.
Acclimatisation.
Correlation of growth.
Compensation and economy of growth.
False correlations.
Multiple, rudimentary, and lowly organised structures variable.
Parts developed in an unusual manner are highly variable: specific
characters more variable than generic: secondary sexual characters
variable.
Species of the same genus vary in an analogous manner.
Reversions to long-lost characters.
Summary.
CHAPTER 6. DIFFICULTIES ON THEORY.
Difficulties on the theory of descent with modification.
Transitions.
Absence or rarity of transitional varieties.
Transitions in habits of life.
Diversified habits in the same species.
Species with habits widely different from those of their allies.
Organs of extreme perfection.
Means of transition.
Cases of difficulty.
Natura non facit saltum.
Organs of small importance.
Organs not in all cases absolutely perfect.
The law of Unity of Type and of the Conditions of Existence embraced
by the theory of Natural Selection.
CHAPTER 7. INSTINCT.
Instincts comparable with habits, but different in their origin.
Instincts graduated.
Aphides and ants.
Instincts variable.
Domestic instincts, their origin.
Natural instincts of the cuckoo, ostrich, and parasitic bees.
Slave-making ants.
Hive-bee, its cell-making instinct.
Difficulties on the theory of the Natural Selection of instincts.
Neuter or sterile insects.
Summary.
CHAPTER 8. HYBRIDISM.
Distinction between the sterility of first crosses and of hybrids.
Sterility various in degree, not universal, affected by close
interbreeding, removed by domestication.
Laws governing the sterility of hybrids.
Sterility not a special endowment, but incidental on other
differences.
Causes of the sterility of first crosses and of hybrids.
Parallelism between the effects of changed conditions of life and
crossing.
Fertility of varieties when crossed and of their mongrel offspring
not universal.
Hybrids and mongrels compared independently of their fertility.
Summary.
CHAPTER 9. ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.
On the absence of intermediate varieties at the present day.
On the nature of extinct intermediate varieties; on their number.
On the vast lapse of time, as inferred from the rate of deposition
and of denudation.
On the poorness of our 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.
CHAPTER 10. ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS.
On the slow and successive appearance of new species.
On their different rates of change.
Species once lost do not reappear.
Groups of species follow the same general rules in their appearance
and disappearance as do single species.
On Extinction.
On simultaneous changes in the forms of life throughout the world.
On the affinities of extinct species to each other and to living
species.
On the state of development of ancient forms.
On the succession of the same types within the same areas.
Summary of preceding and present chapters.
CHAPTER 11. GEOGRAPHICAL DISTRIBUTION.
Present distribution cannot be accounted for by differences in
physical conditions.
Importance of barriers.
Affinity of the productions of the same continent.
Centres of creation.
Means of dispersal, by changes of climate and of the level of the
land, and by occasional means.
Dispersal during the Glacial period co-extensive with the world.
CHAPTER 12. GEOGRAPHICAL DISTRIBUTION—_continued_.
Distribution of fresh-water productions.
On the inhabitants of oceanic islands.
Absence of Batrachians and of terrestrial Mammals.
On the relation of the inhabitants of islands to those of the nearest
mainland.
On colonisation from the nearest source with subsequent modification.
Summary of the last and present chapters.
CHAPTER 13. MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY:
EMBRYOLOGY: RUDIMENTARY ORGANS.
CLASSIFICATION, groups subordinate to groups.
Natural system.
Rules and difficulties in classification, explained on the theory of
descent with modification.
Classification of varieties.
Descent always used in classification.
Analogical or adaptive characters.
Affinities, general, complex and radiating.
Extinction separates and defines groups.
MORPHOLOGY, between members of the same class, between parts of the
same individual.
EMBRYOLOGY, laws of, explained by variations not supervening at an
early age, and being inherited at a corresponding age.
RUDIMENTARY ORGANS; their origin explained.
Summary.
CHAPTER 14. RECAPITULATION AND CONCLUSION.
Recapitulation of the difficulties on the theory of Natural
Selection.
Recapitulation of the general and special circumstances in its
favour.
Causes of the general belief in the immutability of species.
How far the theory of natural selection may be extended.
Effects of its adoption on the study of Natural history.
Concluding remarks.
ON THE ORIGIN OF SPECIES.
INTRODUCTION.
When on board H.M.S. ‘Beagle,’ as naturalist, I was much struck with
certain facts in the distribution of the inhabitants of South America,
and in the geological relations of the present to the past inhabitants
of that continent. These facts seemed to me to throw some light on the
origin of species—that mystery of mysteries, as it has been called by
one of our greatest philosophers. On my return home, it occurred to me,
in 1837, that something might perhaps be made out on this question by
patiently accumulating and reflecting on all sorts of facts which could
possibly have any bearing on it. After five years’ work I allowed
myself to speculate on the subject, and drew up some short notes; these
I enlarged in 1844 into a sketch of the conclusions, which then seemed
to me probable: from that period to the present day I have steadily
pursued the same object. I hope that I may be excused for entering on
these personal details, as I give them to show that I have not been
hasty in coming to a decision.
My work is now nearly finished; but as it will take me two or three
more years to complete it, and as my health is far from strong, I have
been urged to publish this Abstract. I have more especially been
induced to do this, as Mr. Wallace, who is now studying the
natural history of the Malay archipelago, has arrived at almost exactly
the same general conclusions that I have on the origin of species. Last
year he sent to me a memoir on this subject, with a request that I
would forward it to Sir Charles Lyell, who sent it to the Linnean
Society, and it is published in the third volume of the Journal of that
Society. Sir C. Lyell and Dr. Hooker, who both knew of my work—the
latter having read my sketch of 1844—honoured me by thinking it
advisable to publish, with Mr. Wallace’s excellent memoir, some brief
extracts from my manuscripts.
This Abstract, which I now publish, must necessarily be imperfect. I
cannot here give references and authorities for my several statements;
and I must trust to the reader reposing some confidence in my accuracy.
No doubt errors will have crept in, though I hope I have always been
cautious in trusting to good authorities alone. I can here give only
the general conclusions at which I have arrived, with a few facts in
illustration, but which, I hope, in most cases will suffice. No one can
feel more sensible than I do of the necessity of hereafter publishing
in detail all the facts, with references, on which my conclusions have
been grounded; and I hope in a future work to do this. For I am well
aware that scarcely a single point is discussed in this volume on which
facts cannot be adduced, often apparently leading to conclusions
directly opposite to those at which I have arrived. A fair result can
be obtained only by fully stating and balancing the facts and arguments
on both sides of each question; and this cannot possibly be here done.
I much regret that want of space prevents my having the satisfaction of
acknowledging the generous assistance which I have received from very
many naturalists, some of them personally unknown to me. I cannot,
however,
let this opportunity pass without expressing my deep obligations to Dr.
Hooker, who for the last fifteen years has aided me in every possible
way by his large stores of knowledge and his excellent judgment.
In considering the Origin of Species, it is quite conceivable that a
naturalist, reflecting on the mutual affinities of organic beings, on
their embryological relations, their geographical distribution,
geological succession, and other such facts, might come to the
conclusion that each species had not been independently created, but
had descended, like varieties, from other species. Nevertheless, such a
conclusion, even if well founded, would be unsatisfactory, until it
could be shown how the innumerable species inhabiting this world have
been modified, so as to acquire that perfection of structure and
coadaptation which most justly excites our admiration. Naturalists
continually refer to external conditions, such as climate, food, etc.,
as the only possible cause of variation. In one very limited sense, as
we shall hereafter see, this may be true; but it is preposterous to
attribute to mere external conditions, the structure, for instance, of
the woodpecker, with its feet, tail, beak, and tongue, so admirably
adapted to catch insects under the bark of trees. In the case of the
misseltoe, which draws its nourishment from certain trees, which has
seeds that must be transported by certain birds, and which has flowers
with separate sexes absolutely requiring the agency of certain insects
to bring pollen from one flower to the other, it is equally
preposterous to account for the structure of this parasite, with its
relations to several distinct organic beings, by the effects of
external conditions, or of habit, or of the volition of the plant
itself.
The author of the ‘Vestiges of Creation’ would, I presume, say that,
after a certain unknown number of
generations, some bird had given birth to a woodpecker, and some plant
to the misseltoe, and that these had been produced perfect as we now
see them; but this assumption seems to me to be no explanation, for it
leaves the case of the coadaptations of organic beings to each other
and to their physical conditions of life, untouched and unexplained.
It is, therefore, of the highest importance to gain a clear insight
into the means of modification and coadaptation. At the commencement of
my observations it seemed to me probable that a careful study of
domesticated animals and of cultivated plants would offer the best
chance of making out this obscure problem. Nor have I been
disappointed; in this and in all other perplexing cases I have
invariably found that our knowledge, imperfect though it be, of
variation under domestication, afforded the best and safest clue. I may
venture to express my conviction of the high value of such studies,
although they have been very commonly neglected by naturalists.
From these considerations, I shall devote the first chapter of this
Abstract to Variation under Domestication. We shall thus see that a
large amount of hereditary modification is at least possible, and, what
is equally or more important, we shall see how great is the power of
man in accumulating by his Selection successive slight variations. I
will then pass on to the variability of species in a state of nature;
but I shall, unfortunately, be compelled to treat this subject far too
briefly, as it can be treated properly only by giving long catalogues
of facts. We shall, however, be enabled to discuss what circumstances
are most favourable to variation. In the next chapter the Struggle for
Existence amongst all organic beings throughout the world, which
inevitably follows from their high geometrical powers of
increase, will be treated of. This is the doctrine of Malthus, applied
to the whole animal and vegetable kingdoms. As many more individuals of
each species are born than can possibly survive; and as, consequently,
there is a frequently recurring struggle for existence, it follows that
any being, if it vary however slightly in any manner profitable to
itself, under the complex and sometimes varying conditions of life,
will have a better chance of surviving, and thus be _naturally
selected_. From the strong principle of inheritance, any selected
variety will tend to propagate its new and modified form.
This fundamental subject of Natural Selection will be treated at some
length in the fourth chapter; and we shall then see how Natural
Selection almost inevitably causes much Extinction of the less improved
forms of life and induces what I have called Divergence of Character.
In the next chapter I shall discuss the complex and little known laws
of variation and of correlation of growth. In the four succeeding
chapters, the most apparent and gravest difficulties on the theory will
be given: namely, first, the difficulties of transitions, or in
understanding how a simple being or a simple organ can be changed and
perfected into a highly developed being or elaborately constructed
organ; secondly the subject of Instinct, or the mental powers of
animals, thirdly, Hybridism, or the infertility of species and the
fertility of varieties when intercrossed; and fourthly, the
imperfection of the Geological Record. In the next chapter I shall
consider the geological succession of organic beings throughout time;
in the eleventh and twelfth, their geographical distribution throughout
space; in the thirteenth, their classification or mutual affinities,
both when mature and in an embryonic condition. In the last chapter I
shall give a
brief recapitulation of the whole work, and a few concluding remarks.
No one ought to feel surprise at much remaining as yet unexplained in
regard to the origin of species and varieties, if he makes due
allowance for our profound ignorance in regard to the mutual relations
of all the beings which live around us. Who can explain why one species
ranges widely and is very numerous, and why another allied species has
a narrow range and is rare? Yet these relations are of the highest
importance, for they determine the present welfare, and, as I believe,
the future success and modification of every inhabitant of this world.
Still less do we know of the mutual relations of the innumerable
inhabitants of the world during the many past geological epochs in its
history. Although much remains obscure, and will long remain obscure, I
can entertain no doubt, after the most deliberate study and
dispassionate judgment of which I am capable, that the view which most
naturalists entertain, and which I formerly entertained—namely, that
each species has been independently created—is erroneous. I am fully
convinced that species are not immutable; but that those belonging to
what are called the same genera are lineal descendants of some other
and generally extinct species, in the same manner as the acknowledged
varieties of any one species are the descendants of that species.
Furthermore, I am convinced that Natural Selection has been the main
but not exclusive means of modification.
CHAPTER 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 much more from each other,
than do the individuals of any one species or variety in a state of
nature. When we reflect on the vast diversity of the plants and animals
which have been cultivated, and which have varied during all ages under
the most different climates and treatment, I think we are driven to
conclude that this greater variability is simply due to our domestic
productions having been raised under conditions of life not so uniform
as, and somewhat different from, those to which the parent-species have
been exposed under nature. There is, also, I think, some probability in
the view propounded by Andrew Knight, that this variability may be
partly connected with excess of food. It seems pretty clear that
organic beings must be exposed during several generations to the new
conditions of life to cause any appreciable amount of variation; and
that when the organisation has once begun to vary, it generally
continues to vary for many generations.
No case is on record of a variable being ceasing to be variable under
cultivation. Our oldest cultivated plants, such as wheat, still often
yield new varieties: our oldest domesticated animals are still capable
of rapid improvement or modification.
It has been disputed at what period of life the causes of variability,
whatever they may be, generally act; whether during the early or late
period of development of the embryo, or at the instant of conception.
Geoffroy St. Hilaire’s experiments show that unnatural treatment of the
embryo causes monstrosities; and monstrosities cannot be separated by
any clear line of distinction from mere variations. But I am strongly
inclined to suspect that the most frequent cause of variability may be
attributed to the male and female reproductive elements having been
affected prior to the act of conception. Several reasons make me
believe in this; but the chief one is the remarkable effect which
confinement or cultivation has on the functions of the reproductive
system; this system appearing to be far more susceptible than any other
part of the organisation, to the action of any change in the conditions
of life. Nothing is more easy than to tame an animal, and few things
more difficult than to get it to breed freely under confinement, even
in the many cases when the male and female unite. How many animals
there are which will not breed, though living long under not very close
confinement in their native country! This is generally attributed to
vitiated instincts; but how many cultivated plants display the utmost
vigour, and yet rarely or never seed! In some few such cases it has
been found out that very trifling changes, such as a little more or
less water at some particular period of growth, will determine whether
or not the plant sets a seed. I cannot here enter on the copious
details which I have collected on
this curious subject; but to show how singular the laws are which
determine the reproduction of animals under confinement, I may just
mention that carnivorous animals, even from the tropics, breed in this
country pretty freely under confinement, with the exception of the
plantigrades or bear family; whereas, carnivorous birds, with the
rarest exceptions, hardly ever lay fertile eggs. Many exotic plants
have pollen utterly worthless, in the same exact condition as in the
most sterile hybrids. When, on the one hand, we see domesticated
animals and plants, though often weak and sickly, yet breeding quite
freely under confinement; and when, on the other hand, we see
individuals, though taken young from a state of nature, perfectly
tamed, long-lived, and healthy (of which I could give numerous
instances), yet having their reproductive system so seriously affected
by unperceived causes as to fail in acting, we need not be surprised at
this system, when it does act under confinement, acting not quite
regularly, and producing offspring not perfectly like their parents or
variable.
Sterility has been said to be the bane of horticulture; but on this
view we owe variability to the same cause which produces sterility; and
variability is the source of all the choicest productions of the
garden. I may add, that as some organisms will breed most freely under
the most unnatural conditions (for instance, the rabbit and ferret kept
in hutches), showing that their reproductive system has not been thus
affected; so will some animals and plants withstand domestication or
cultivation, and vary very slightly—perhaps hardly more than in a state
of nature.
A long list could easily be given of “sporting plants;” by this term
gardeners mean a single bud or offset, which suddenly assumes a new and
sometimes very different character from that of the rest of the plant.
Such buds can be propagated by grafting, etc., and sometimes by seed.
These “sports” are extremely rare under nature, but far from rare under
cultivation; and in this case we see that the treatment of the parent
has affected a bud or offset, and not the ovules or pollen. But it is
the opinion of most physiologists that there is no essential difference
between a bud and an ovule in their earliest stages of formation; so
that, in fact, “sports” support my view, that variability may be
largely attributed to the ovules or pollen, or to both, having been
affected by the treatment of the parent prior to the act of conception.
These cases anyhow show that variation is not necessarily connected, as
some authors have supposed, with the act of generation.
Seedlings from the same fruit, and the young of the same litter,
sometimes differ considerably from each other, though both the young
and the parents, as 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, and of growth, and of inheritance; for had the
action of the conditions been direct, if any of the young had varied,
all would probably have varied in the same manner. To judge how much,
in the case of any variation, we should attribute to the direct action
of heat, moisture, light, food, etc., is most difficult: my impression
is, that with animals such agencies have produced very little direct
effect, though apparently more in the case of plants. Under this point
of view, Mr. Buckman’s recent experiments on plants seem extremely
valuable. When all or nearly all the individuals exposed to certain
conditions are affected in the same way, the change at first appears to
be directly due to such conditions; but in some cases it can be shown
that quite opposite conditions produce
similar changes of structure. Nevertheless some slight amount of change
may, I think, be attributed to the direct action of the conditions of
life—as, in some cases, increased size from amount of food, colour from
particular kinds of food and from light, and perhaps the thickness of
fur from climate.
Habit also has a decided influence, as in the period of flowering with
plants when transported from one climate to another. In animals it has
a more marked effect; for instance, I find in the domestic duck that
the bones of the wing weigh less and the bones of the leg more, in
proportion to the whole skeleton, than do the same bones in the
wild-duck; and I presume that this change may be safely attributed to
the domestic duck flying much less, and walking more, than its wild
parent. The great and inherited development of the udders in cows and
goats in countries where they are habitually milked, in comparison with
the state of these organs in other countries, is another instance of
the effect of use. Not a single domestic animal can be named which has
not in some country drooping ears; and the view suggested by some
authors, that the drooping is due to the disuse of the muscles of the
ear, from the animals not being much alarmed by danger, seems probable.
There are many laws regulating variation, some few of which can be
dimly seen, and will be hereafter briefly mentioned. I will here only
allude to what may be called correlation of growth. Any change in the
embryo or larva will almost certainly entail changes in the mature
animal. In monstrosities, the correlations between quite distinct parts
are very curious; and many instances are given in Isidore Geoffroy St.
Hilaire’s great work on this subject. Breeders believe that long limbs
are almost always accompanied by an elongated head. Some instances of
correlation are quite whimsical; thus
cats with blue eyes are invariably deaf; colour and constitutional
peculiarities go together, of which many remarkable cases could be
given amongst animals and plants. From the facts collected by
Heusinger, it appears that white sheep and pigs are differently
affected from coloured individuals by certain vegetable poisons.
Hairless dogs have imperfect teeth; long-haired and coarse-haired
animals are apt to have, as is asserted, long or many horns; pigeons
with feathered feet have skin between their outer toes; pigeons with
short beaks have small feet, and those with long beaks large feet.
Hence, if man goes on selecting, and thus augmenting, any peculiarity,
he will almost certainly unconsciously modify other parts of the
structure, owing to the mysterious laws of the correlation of growth.
The result of the various, quite unknown, or dimly seen laws of
variation is infinitely complex and diversified. It is well worth while
carefully to study the several treatises published on some of our old
cultivated plants, as on the hyacinth, potato, even the dahlia, etc.;
and it is really surprising to note the endless points in structure and
constitution in which the varieties and sub-varieties differ slightly
from each other. The whole organisation seems to have become plastic,
and tends to depart in some small degree from that of the parental
type.
Any variation which is not inherited is unimportant for us. But the
number and diversity of inheritable deviations of structure, both those
of slight and those of considerable physiological importance, is
endless. Dr. Prosper Lucas’s treatise, in two large volumes, is the
fullest and the best on this subject. No breeder doubts how strong is
the tendency to inheritance: like produces like is his fundamental
belief: doubts have been thrown on this principle by theoretical
writers alone. When a
deviation appears not unfrequently, and we see it in the father and
child, we cannot tell whether it may not be due to the same original
cause acting on both; but when amongst individuals, apparently exposed
to the same conditions, any very rare deviation, due to some
extraordinary combination of circumstances, appears in the parent—say,
once amongst several million individuals—and it reappears in the child,
the mere doctrine of chances almost compels us to attribute its
reappearance to inheritance. Every one must have heard of cases of
albinism, prickly skin, hairy bodies, etc., appearing in several
members of the same family. If strange and rare deviations of structure
are truly inherited, less strange and commoner deviations may be freely
admitted to be inheritable. Perhaps the correct way of viewing the
whole subject, would be, to look at the inheritance of every character
whatever as the rule, and non-inheritance as the anomaly.
The laws governing inheritance are quite unknown; no one can say why
the same peculiarity in different individuals of the same species, and
in individuals of different species, is sometimes inherited and
sometimes not so; why the child often reverts in certain characters to
its grandfather or grandmother or other much more remote ancestor; why
a peculiarity is often transmitted from one sex to both sexes or to one
sex alone, more commonly but not exclusively to the like sex. It is a
fact of some little importance to us, that peculiarities appearing in
the males of our domestic breeds are often transmitted either
exclusively, or in a much greater degree, to males alone. A much more
important rule, which I think may be trusted, is that, at whatever
period of life a peculiarity first appears, it tends to appear in the
offspring at a corresponding age, though sometimes earlier. In many
cases this could
not be otherwise: thus the inherited peculiarities in the horns of
cattle could appear only in the offspring when nearly mature;
peculiarities in the silkworm are known to appear at the corresponding
caterpillar or cocoon stage. But hereditary diseases and some other
facts make me believe that the rule has a wider extension, and that
when there is no apparent reason why a peculiarity should appear at any
particular age, yet that it does tend to appear in the offspring at the
same period at which it first appeared in the parent. I believe this
rule to be of the highest importance in explaining the laws of
embryology. These remarks are of course confined to the first
_appearance_ of the peculiarity, and not to its primary cause, which
may have acted on the ovules or male element; in nearly the same manner
as in the crossed offspring from a short-horned cow by a long-horned
bull, the greater length of horn, though appearing late in life, is
clearly due to the male element.
Having alluded to the subject of reversion, I may here refer to a
statement often made by naturalists—namely, that our domestic
varieties, when run wild, gradually but certainly revert in character
to their aboriginal stocks. Hence it has been argued that no deductions
can be drawn from domestic races to species in a state of nature. I
have in vain endeavoured to discover on what decisive facts the above
statement has so often and so boldly been made. There would be great
difficulty in proving its truth: we may safely conclude that very many
of the most strongly-marked domestic varieties could not possibly live
in a wild state. In many cases we do not know what the aboriginal stock
was, and so could not tell whether or not nearly perfect reversion had
ensued. It would be quite necessary, in order to prevent the effects of
intercrossing, that only a
single variety should be turned loose in its new home. Nevertheless, as
our varieties certainly do occasionally revert in some of their
characters to ancestral forms, it seems to me not improbable, that if
we could succeed in naturalising, or were to cultivate, during many
generations, the several races, for instance, of the cabbage, in very
poor soil (in which case, however, some effect would have to be
attributed to the direct action of the poor soil), that they would to a
large extent, or even wholly, revert to the wild aboriginal stock.
Whether or not the experiment would succeed, is not of great importance
for our line of argument; for by the experiment itself the conditions
of life are changed. If it could be shown that our domestic varieties
manifested a strong tendency to reversion,—that is, to lose their
acquired characters, whilst kept under unchanged conditions, and whilst
kept in a considerable body, so that free intercrossing might check, by
blending together, any slight deviations of structure, in such case, I
grant that we could deduce nothing from domestic varieties in regard to
species. But there is not a shadow of evidence in favour of this view:
to assert that we could not breed our cart and race-horses, long and
short-horned cattle, and poultry of various breeds, and esculent
vegetables, for an almost infinite number of generations, would be
opposed to all experience. I may add, that when under nature the
conditions of life do change, variations and reversions of character
probably do occur; but natural selection, as will hereafter be
explained, will determine how far the new characters thus arising shall
be preserved.
When we look to the hereditary varieties or races of our domestic
animals and plants, and compare them with species closely allied
together, we generally perceive in each domestic race, as already
remarked, less uniformity of character than in true species. Domestic
races of
the same species, also, often have a somewhat monstrous character; by
which I mean, that, although differing from each other, and from the
other species of the same genus, in several trifling respects, they
often differ in an extreme degree in some one part, both when compared
one with another, and more especially when compared with all the
species in nature to which they are nearest allied. With these
exceptions (and with that of the perfect fertility of varieties when
crossed,—a subject hereafter to be discussed), domestic races of the
same species differ from each other in the same manner as, only in most
cases in a lesser degree than, do closely-allied species of the same
genus in a state of nature. I think this must be admitted, when we find
that there are hardly any domestic races, either amongst animals or
plants, which have not been ranked by some competent judges as mere
varieties, and by other competent judges as the descendants of
aboriginally distinct species. If any marked distinction existed
between domestic races and species, this source of doubt could not so
perpetually recur. It has often been stated that domestic races do not
differ from each other in characters of generic value. I think it could
be shown that this statement is hardly correct; but naturalists differ
most widely in determining what characters are of generic value; all
such valuations being at present empirical. Moreover, on the view of
the origin of genera which I shall presently give, we have no right to
expect often to meet with generic differences in our domesticated
productions.
When we attempt to estimate the amount of structural difference between
the domestic races of the same species, we are soon involved in doubt,
from not knowing whether they have descended from one or several
parent-species. This point, if it could be cleared up, would be
interesting; if, for instance, it could be shown that the greyhound,
bloodhound, terrier, spaniel, and bull-dog, which we all know propagate
their kind so truly, were the offspring of any single species, then
such facts would have great weight in making us doubt about the
immutability of the many very closely allied and natural species—for
instance, of the many foxes—inhabiting different quarters of the world.
I do not believe, as we shall presently see, that all our dogs have
descended from any one wild species; but, in the case of some other
domestic races, there is presumptive, or even strong, evidence in
favour of this view.
It has often been assumed that man has chosen for domestication animals
and plants having an extraordinary inherent tendency to vary, and
likewise to withstand diverse climates. I do not dispute that these
capacities have added largely to the value of most of our domesticated
productions; but how could a savage possibly know, when he first tamed
an animal, whether it would vary in succeeding generations, and whether
it would endure other climates? Has the little variability of the ass
or guinea-fowl, or the small power of endurance of warmth by the
rein-deer, or of cold by the common camel, prevented their
domestication? I cannot doubt that if other animals and plants, equal
in number to our domesticated productions, and belonging to equally
diverse classes and countries, were taken from a state of nature, and
could be made to breed for an equal number of generations under
domestication, they would vary on an average as largely as the parent
species of our existing domesticated productions have varied.
In the case of most of our anciently domesticated animals and plants, I
do not think it is possible to come to any definite conclusion, whether
they have descended from one or several species. The argument mainly
relied on by those who believe in the multiple origin
of our domestic animals is, that we find in the most ancient records,
more especially on the monuments of Egypt, much diversity in the
breeds; and that some of the breeds closely resemble, perhaps are
identical with, those still existing. Even if this latter fact were
found more strictly and generally true than seems to me to be the case,
what does it show, but that some of our breeds originated there, four
or five thousand years ago? But Mr. Horner’s researches have rendered
it in some degree probable that man sufficiently civilized to have
manufactured pottery existed in the valley of the Nile thirteen or
fourteen thousand years ago; and who will pretend to say how long
before these ancient periods, savages, like those of Tierra del Fuego
or Australia, who possess a semi-domestic dog, may not have existed in
Egypt?
The whole subject must, I think, remain vague; nevertheless, I may,
without here entering on any details, state that, from geographical and
other considerations, I think it highly probable that our domestic dogs
have descended from several wild species. In regard to sheep and goats
I can form no opinion. I should think, from facts communicated to me by
Mr. Blyth, on the habits, voice, and constitution, etc., of the humped
Indian cattle, that these had descended from a different aboriginal
stock from our European cattle; and several competent judges believe
that these latter have had more than one wild parent. With respect to
horses, from reasons which I cannot give here, I am doubtfully inclined
to believe, in opposition to several authors, that all the races have
descended from one wild stock. Mr. Blyth, whose opinion, from his large
and varied stores of knowledge, I should value more than that of almost
any one, thinks that all the breeds of poultry have proceeded from the
common wild
Indian fowl (Gallus bankiva). In regard to ducks and rabbits, the
breeds of which differ considerably from each other in structure, I do
not doubt that they all have descended from the common wild duck and
rabbit.
The doctrine of the origin of our several domestic races from several
aboriginal stocks, has been carried to an absurd extreme by some
authors. They believe that every race which breeds true, let the
distinctive characters be ever so slight, has had its wild prototype.
At this rate there must have existed at least a score of species of
wild cattle, as many sheep, and several goats in Europe alone, and
several even within Great Britain. One author believes that there
formerly existed in Great Britain eleven wild species of sheep peculiar
to it! When we bear in mind that Britain has now hardly one peculiar
mammal, and France but few distinct from those of Germany and
conversely, and so with Hungary, Spain, etc., but that each of these
kingdoms possesses several peculiar breeds of cattle, sheep, etc., we
must admit that many domestic breeds have originated in Europe; for
whence could they have been derived, as these several countries do not
possess a number of peculiar species as distinct parent-stocks? So it
is in India. Even in the case of the domestic dogs of the whole world,
which I fully admit have probably descended from several wild species,
I cannot doubt that there has been an immense amount of inherited
variation. Who can believe that animals closely resembling the Italian
greyhound, the bloodhound, the bull-dog, or Blenheim spaniel, etc.—so
unlike all wild Canidæ—ever existed freely in a state of nature? It has
often been loosely said that all our races of dogs have been produced
by the crossing of a few aboriginal species; but by crossing we can get
only forms in some degree intermediate between their parents; and if we
account for our several domestic races by this process, we must admit
the former existence of the most extreme forms, as the Italian
greyhound, bloodhound, bull-dog, etc., in the wild state. Moreover, the
possibility of making distinct races by crossing has been greatly
exaggerated. There can be no doubt that a race may be modified by
occasional crosses, if aided by the careful selection of those
individual mongrels, which present any desired character; but that a
race could be obtained nearly intermediate between two extremely
different races or species, I can hardly believe. Sir J. Sebright
expressly experimentised for this object, and failed. The offspring
from the first cross between two pure breeds is tolerably and sometimes
(as I have found with pigeons) extremely uniform, and everything seems
simple enough; but when these mongrels are crossed one with another for
several generations, hardly two of them will be alike, and then the
extreme difficulty, or rather utter hopelessness, of the task becomes
apparent. Certainly, a breed intermediate between _two very distinct_
breeds could not be got without extreme care and long-continued
selection; nor can I find a single case on record of a permanent race
having been thus formed.
_On the Breeds of the Domestic Pigeon_.—Believing that it is always
best to study some special group, I have, after deliberation, taken up
domestic pigeons. I have kept every breed which I could purchase or
obtain, and have been most kindly favoured with skins from several
quarters of the world, more especially by the Honourable W. Elliot from
India, and by the Honourable C. Murray from Persia. Many treatises in
different languages have been published on pigeons, and some of them
are very important, as being of considerable antiquity. I have
associated with several eminent fanciers, and have been permitted to
join two
of the London Pigeon Clubs. The diversity of the breeds is something
astonishing. Compare the English carrier and the short-faced tumbler,
and see the wonderful difference in their beaks, entailing
corresponding differences in their skulls. The carrier, more especially
the male bird, is also remarkable from the wonderful development of the
carunculated skin about the head, and this is accompanied by greatly
elongated eyelids, very large external orifices to the nostrils, and a
wide gape of mouth. The short-faced tumbler has a beak in outline
almost like that of a finch; and the common tumbler has the singular
and strictly inherited habit of flying at a great height in a compact
flock, and tumbling in the air head over heels. The runt is a bird of
great size, with long, massive beak and large feet; some of the
sub-breeds of runts have very long necks, others very long wings and
tails, others singularly short tails. The barb is allied to the
carrier, but, instead of a very long beak, has a very short and very
broad one. The pouter has a much elongated body, wings, and legs; and
its enormously developed crop, which it glories in inflating, may well
excite astonishment and even laughter. The turbit has a very short and
conical beak, with a line of reversed feathers down the breast; and it
has the habit of continually expanding slightly the upper part of the
oesophagus. The Jacobin has the feathers so much reversed along the
back of the neck that they form a hood, and it has, proportionally to
its size, much elongated wing and tail feathers. The trumpeter and
laugher, as their names express, utter a very different coo from the
other breeds. The fantail has thirty or even forty tail-feathers,
instead of twelve or fourteen, the normal number in all members of the
great pigeon family; and these feathers are kept expanded, and are
carried so erect that in good birds the head and tail
touch; the oil-gland is quite aborted. Several other less distinct
breeds might have been specified.
In the skeletons of the several breeds, the development of the bones of
the face in length and breadth and curvature differs enormously. The
shape, as well as the breadth and length of the ramus of the lower jaw,
varies in a highly remarkable manner. The number of the caudal and
sacral 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 touch; the
oil-gland is quite aborted. Several other less distinct breeds might
have been specified.
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 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.
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 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
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 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, would improve and
modify any breed, in the same way as Bakewell, Collins, etc., by this
very same process, only carried on more methodically, did greatly
modify, even during their own lifetimes, the forms and qualities of
their cattle. Slow and insensible changes of this kind could never be
recognised unless actual measurements or careful drawings of the breeds
in question had been made long ago, which might serve for comparison.
In some cases, however, unchanged or but little changed individuals of
the same breed may be found in less civilised districts, where the
breed has been less improved. There is reason to believe that King
Charles’s spaniel has been unconsciously modified to a large extent
since the time of that monarch. Some highly competent authorities are
convinced that the setter is directly derived from the spaniel, and has
probably been slowly altered from it. It is known that the English
pointer has been greatly changed within the last century, and in this
case the change has, it is believed, been chiefly effected by crosses
with the fox-hound; but what concerns us is, that the change has been
effected unconsciously and gradually, and yet so effectually, that,
though the old Spanish pointer certainly came from Spain, Mr. Borrow
has not seen, as I am informed by him, any native dog in Spain like our
pointer.
By a similar process of selection, and by careful training, the whole
body of English racehorses have come to surpass in fleetness and size
the parent Arab stock, so that the latter, by the regulations for the
Goodwood Races, are favoured in the weights they carry. Lord Spencer
and others have shown how the cattle of England have increased in
weight and in early maturity, compared with the stock formerly kept in
this country. By comparing the accounts given in old pigeon treatises
of carriers
and tumblers with these breeds as now existing in Britain, India, and
Persia, we can, I think, clearly trace the stages through which they
have insensibly passed, and come to differ so greatly from the
rock-pigeon.
Youatt gives an excellent illustration of the effects of a course of
selection, which may be considered as unconsciously followed, in so far
that the breeders could never have expected or even have wished to have
produced the result which ensued—namely, the production of two distinct
strains. The two flocks of Leicester sheep kept by Mr. Buckley and Mr.
Burgess, as Mr. Youatt remarks, “have been purely bred from the
original stock of Mr. Bakewell for upwards of fifty years. There is not
a suspicion existing in the mind of any one at all acquainted with the
subject that the owner of either of them has deviated in any one
instance from the pure blood of Mr. Bakewell’s flock, and yet the
difference between the sheep possessed by these two gentlemen is so
great that they have the appearance of being quite different
varieties.”
If there exist savages so barbarous as never to think of the inherited
character of the offspring of their domestic animals, yet any one
animal particularly useful to them, for any special purpose, would be
carefully preserved during famines and other accidents, to which
savages are so liable, and such choice animals would thus generally
leave more offspring than the inferior ones; so that in this case there
would be a kind of unconscious selection going on. We see the value set
on animals even by the barbarians of Tierra del Fuego, by their killing
and devouring their old women, in times of dearth, as of less value
than their dogs.
In plants the same gradual process of improvement, through the
occasional preservation of the best individuals, whether or not
sufficiently distinct to be ranked
at their first appearance as distinct varieties, and whether or not two
or more species or races have become blended together by crossing, may
plainly be recognised in the increased size and beauty which we now see
in the varieties of the heartsease, rose, pelargonium, dahlia, and
other plants, when compared with the older varieties or with their
parent-stocks. No one would ever expect to get a first-rate heartsease
or dahlia from the seed of a wild plant. No one would expect to raise a
first-rate melting pear from the seed of a wild pear, though he might
succeed from a poor seedling growing wild, if it had come from a
garden-stock. The pear, though cultivated in classical times, appears,
from Pliny’s description, to have been a fruit of very inferior
quality. I have seen great surprise expressed in horticultural works at
the wonderful skill of gardeners, in having produced such splendid
results from such poor materials; but the art, I cannot doubt, has been
simple, and, as far as the final result is concerned, has been followed
almost unconsciously. It has consisted in always cultivating the best
known variety, sowing its seeds, and, when a slightly better variety
has chanced to appear, selecting it, and so onwards. But the gardeners
of the classical period, who cultivated the best pear they could
procure, never thought what splendid fruit we should eat; though we owe
our excellent fruit, in some small degree, to their having naturally
chosen and preserved the best varieties they could anywhere find.
A large amount of change in our cultivated plants, thus slowly and
unconsciously accumulated, explains, as I believe, the well-known fact,
that in a vast number of cases we cannot recognise, and therefore do
not know, the wild parent-stocks of the plants which have been longest
cultivated in our flower and kitchen gardens. If it has taken centuries
or thousands of years to improve
or modify most of our plants up to their present standard of usefulness
to man, we can understand how it is that neither Australia, the Cape of
Good Hope, nor any other region inhabited by quite uncivilised man, has
afforded us a single plant worth culture. It is not that these
countries, so rich in species, do not by a strange chance possess the
aboriginal stocks of any useful plants, but that the native plants have
not been improved by continued selection up to a standard of perfection
comparable with that given to the plants in countries anciently
civilised.
In regard to the domestic animals kept by uncivilised man, it should
not be overlooked that they almost always have to struggle for their
own food, at least during certain seasons. And in two countries very
differently circumstanced, individuals of the same species, having
slightly different constitutions or structure, would often succeed
better in the one country than in the other, and thus by a process of
“natural selection,” as will hereafter be more fully explained, two
sub-breeds might be formed. This, perhaps, partly explains what has
been remarked by some authors, namely, that the varieties kept by
savages have more of the character of species than the varieties kept
in civilised countries.
On the view here given of the all-important part which selection by man
has played, it becomes at once obvious, how it is that our domestic
races show adaptation in their structure or in their habits to man’s
wants or fancies. We can, I think, further understand the frequently
abnormal character of our domestic races, and likewise their
differences being so great in external characters and relatively so
slight in internal parts or organs. Man can hardly select, or only with
much difficulty, any deviation of structure excepting such as is
externally visible; and indeed he rarely cares for what is internal. He
can never act by selection, excepting on variations
which are first given to him in some slight degree by nature. No man
would ever try to make a fantail, till he saw a pigeon with a tail
developed in some slight degree in an unusual manner, or a pouter till
he saw a pigeon with a crop of somewhat unusual size; and the more
abnormal or unusual any character was when it first appeared, the more
likely it would be to catch his attention. But to use such an
expression as trying to make a fantail, is, I have no doubt, in most
cases, utterly incorrect. The man who first selected a pigeon with a
slightly larger tail, never dreamed what the descendants of that pigeon
would become through long-continued, partly unconscious and partly
methodical selection. Perhaps the parent bird of all fantails had only
fourteen tail-feathers somewhat expanded, like the present Java
fantail, or like individuals of other and distinct breeds, in which as
many as seventeen tail-feathers have been counted. Perhaps the first
pouter-pigeon did not inflate its crop much more than the turbit now
does the upper part of its oesophagus,—a habit which is disregarded by
all fanciers, as it is not one of the points of the breed.
Nor let it be thought that some great deviation of structure would be
necessary to catch the fancier’s eye: he perceives extremely small
differences, and it is in human nature to value any novelty, however
slight, in one’s own possession. Nor must the value which would
formerly be set on any slight differences in the individuals of the
same species, be judged of by the value which would now be set on them,
after several breeds have once fairly been established. Many slight
differences might, and indeed do now, arise amongst pigeons, which are
rejected as faults or deviations from the standard of perfection of
each breed. The common goose has not given rise to any marked
varieties; hence the Thoulouse and the common breed, which differ only
in colour, that
most fleeting of characters, have lately been exhibited as distinct at
our poultry-shows.
I think these views further explain what has sometimes been
noticed—namely that we know nothing about the origin or history of any
of our domestic breeds. But, in fact, a breed, like a dialect of a
language, can hardly be said to have had a definite origin. A man
preserves and breeds from an individual with some slight deviation of
structure, or takes more care than usual in matching his best animals
and thus improves them, and the improved individuals slowly spread in
the immediate neighbourhood. But as yet they will hardly have a
distinct name, and from being only slightly valued, their history will
be disregarded. When further improved by the same slow and gradual
process, they will spread more widely, and will get recognised as
something distinct and valuable, and will then probably first receive a
provincial name. In semi-civilised countries, with little free
communication, the spreading and knowledge of any new sub-breed will be
a slow process. As soon as the points of value of the new sub-breed are
once fully acknowledged, the principle, as I have called it, of
unconscious selection will always tend,—perhaps more at one period than
at another, as the breed rises or falls in fashion,—perhaps more in one
district than in another, according to the state of civilisation of the
inhabitants—slowly to add to the characteristic features of the breed,
whatever they may be. But the chance will be infinitely small of any
record having been preserved of such slow, varying, and insensible
changes.
I must now say a few words on the circumstances, favourable, or the
reverse, to man’s power of selection. A high degree of variability is
obviously favourable, as freely giving the materials for selection to
work on; not that mere individual differences are not amply
sufficient, with extreme care, to allow of the accumulation of a large
amount of modification in almost any desired direction. But as
variations manifestly useful or pleasing to man appear only
occasionally, the chance of their appearance will be much increased by
a large number of individuals being kept; and hence this comes to be of
the highest importance to success. On this principle Marshall has
remarked, with respect to the sheep of parts of Yorkshire, that “as
they generally belong to poor people, and are mostly _in small lots_,
they never can be improved.” On the other hand, nurserymen, from
raising large stocks of the same plants, are generally far more
successful than amateurs in getting new and valuable varieties. The
keeping of a large number of individuals of a species in any country
requires that the species should be placed under favourable conditions
of life, so as to breed freely in that country. When the individuals of
any species are scanty, all the individuals, whatever their quality may
be, will generally be allowed to breed, and this will effectually
prevent selection. But probably the most important point of all, is,
that the animal or plant should be so highly useful to man, or so much
valued by him, that the closest attention should be paid to even the
slightest deviation in the qualities or structure of each individual.
Unless such attention be paid nothing can be effected. I have seen it
gravely remarked, that it was most fortunate that the strawberry began
to vary just when gardeners began to attend closely to this plant. No
doubt the strawberry had always varied since it was cultivated, but the
slight varieties had been neglected. As soon, however, as gardeners
picked out individual plants with slightly larger, earlier, or better
fruit, and raised seedlings from them, and again picked out the best
seedlings and bred from them, then, there appeared (aided by some
crossing with distinct species) those many admirable varieties of the
strawberry which have been raised during the last thirty or forty
years.
In the case of animals with separate sexes, facility in preventing
crosses is an important element of success in the formation of new
races,—at least, in a country which is already stocked with other
races. In this respect enclosure of the land plays a part. Wandering
savages or the inhabitants of open plains rarely possess more than one
breed of the same species. Pigeons can be mated for life, and this is a
great convenience to the fancier, for thus many races may be kept true,
though mingled in the same aviary; and this circumstance must have
largely favoured the improvement and formation of new breeds. Pigeons,
I may add, can be propagated in great numbers and at a very quick rate,
and inferior birds may be freely rejected, as when killed they serve
for food. On the other hand, cats, from their nocturnal rambling
habits, cannot be matched, and, although so much valued by women and
children, we hardly ever see a distinct breed kept up; such breeds as
we do sometimes see are almost always imported from some other country,
often from islands. Although I do not doubt that some domestic animals
vary less than others, yet the rarity or absence of distinct breeds of
the cat, the donkey, peacock, goose, etc., may be attributed in main
part to selection not having been brought into play: in cats, from the
difficulty in pairing them; in donkeys, from only a few being kept by
poor people, and little attention paid to their breeding; in peacocks,
from not being very easily reared and a large stock not kept; in geese,
from being valuable only for two purposes, food and feathers, and more
especially from no pleasure having been felt in the display of distinct
breeds.
To sum up on the origin of our Domestic Races of animals and plants. I
believe that the conditions of life, from their action on the
reproductive system, are so far of the highest importance as causing
variability. I do not believe that variability is an inherent and
necessary contingency, under all circumstances, with all organic
beings, as some authors have thought. The effects of variability are
modified by various degrees of inheritance and of reversion.
Variability is governed by many unknown laws, more especially by that
of correlation of growth. Something may be attributed to the direct
action of the conditions of life. Something must be attributed to use
and disuse. The final result is thus rendered infinitely complex. In
some cases, I do not doubt that the intercrossing of species,
aboriginally distinct, has played an important part in the origin of
our domestic productions. When in any country several domestic breeds
have once been established, their occasional intercrossing, with the
aid of selection, has, no doubt, largely aided in the formation of new
sub-breeds; but the importance of the crossing of varieties has, I
believe, been greatly exaggerated, both in regard to animals and to
those plants which are propagated by seed. In plants which are
temporarily propagated by cuttings, buds, etc., the importance of the
crossing both of distinct species and of varieties is immense; for the
cultivator here quite disregards the extreme variability both of
hybrids and mongrels, and the frequent sterility of hybrids; but the
cases of plants not propagated by seed are of little importance to us,
for their endurance is only temporary. Over all these causes of Change
I am convinced that the accumulative action of Selection, whether
applied methodically and more quickly, or unconsciously and more
slowly, but more efficiently, is by far the predominant Power.