|
Charles Darwin: The origin of life
For those without javascript use this menu
CONTENTS.
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 organic beings inhabiting South America,
and in the geological relations of the present to the past inhabitants of
that continent. These facts, as will be seen in the latter chapters of
this volume, seemed 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 (1859) nearly finished; but as it will take me many 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. In 1858 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 may 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
is here impossible.
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 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 justly excites our
admiration. Naturalists continually refer to external conditions, such as
climate, food, etc., as the only possible cause of variation. In one
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 mistletoe, 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.
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 among all
organic beings throughout the world, which inevitably follows from the high
geometrical ratio of their increase, will be considered. 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. In
the five succeeding chapters, the most apparent and gravest difficulties in
accepting the theory will be given: namely, first, the difficulties of
transitions, or how a simple being or a simple organ can be changed and
perfected into a highly developed being or into an 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 twelfth and
thirteenth, their geographical distribution throughout space; in the
fourteenth, 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 make due allowance for
our profound ignorance in regard to the mutual relations of the many 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 until recently entertained, 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 most important, but not the exclusive, means of modification.
CHAPTER I.
VARIATION UNDER DOMESTICATION.
Causes of Variability -- Effects of Habit and the use and disuse of Parts
-- Correlated Variation -- 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 -- Principles of Selection, anciently followed,
their Effects -- Methodical and Unconscious Selection -- Unknown Origin of
our Domestic Productions -- Circumstances favourable to Man's power of
Selection.
CAUSES OF VARIABILITY.
When we compare 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. And if 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, we are driven to conclude that this great
variability is 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 had been exposed under nature. There is, also,
some probability in the view propounded by Andrew Knight, that this
variability may be partly connected with excess of food. It seems clear
that organic beings must be exposed during several generations to new
conditions to cause any great amount of variation; and that, when the
organisation has once begun to vary, it generally continues varying for
many generations. No case is on record of a variable organism ceasing to
vary under cultivation. Our oldest cultivated plants, such as wheat, still
yield new varieties: our oldest domesticated animals are still capable of
rapid improvement or modification.
As far as I am able to judge, after long attending to the subject, the
conditions of life appear to act in two ways--directly on the whole
organisation or on certain parts alone and in directly by affecting the
reproductive system. With respect to the direct action, we must bear in
mind that in every case, as Professor Weismann has lately insisted, and as
I have incidently shown in my work on "Variation under Domestication,"
there are two factors: namely, the nature of the organism and the nature
of the conditions. The former seems to be much the more important; for
nearly similar variations sometimes arise under, as far as we can judge,
dissimilar conditions; and, on the other hand, dissimilar variations arise
under conditions which appear to be nearly uniform. The effects on the
offspring are either definite or in definite. They may be considered as
definite when all or nearly all the offspring of individuals exposed to
certain conditions during several generations are modified in the same
manner. It is extremely difficult to come to any conclusion in regard to
the extent of the changes which have been thus definitely induced. There
can, however, be little doubt about many slight changes, such as size from
the amount of food, colour from the nature of the food, thickness of the
skin and hair from climate, etc. Each of the endless variations which we
see in the plumage of our fowls must have had some efficient cause; and if
the same cause were to act uniformly during a long series of generations on
many individuals, all probably would be modified in the same manner. Such
facts as the complex and extraordinary out growths which variably follow
from the insertion of a minute drop of poison by a gall-producing insect,
shows us what singular modifications might result in the case of plants
from a chemical change in the nature of the sap.
In definite variability is a much more common result of changed conditions
than definite variability, and has probably played a more important part in
the formation of our domestic races. We see in definite variability in the
endless slight peculiarities which distinguish the individuals of the same
species, and which cannot be accounted for by inheritance from either
parent or from some more remote ancestor. Even strongly-marked differences
occasionally appear in the young of the same litter, and in seedlings from
the same seed-capsule. At long intervals of time, out of millions of
individuals reared in the same country and fed on nearly the same food,
deviations of structure so strongly pronounced as to deserve to be called
monstrosities arise; but monstrosities cannot be separated by any distinct
line from slighter variations. All such changes of structure, whether
extremely slight or strongly marked, which appear among many individuals
living together, may be considered as the in definite effects of the
conditions of life on each individual organism, in nearly the same manner
as the chill effects different men in an in definite manner, according to
their state of body or constitution, causing coughs or colds, rheumatism,
or inflammation of various organs.
With respect to what I have called the in direct action of changed
conditions, namely, through the reproductive system of being affected, we
may infer that variability is thus induced, partly from the fact of this
system being extremely sensitive to any change in the conditions, and
partly from the similarity, as Kolreuter and others have remarked, between
the variability which follows from the crossing of distinct species, and
that which may be observed with plants and animals when reared under new or
unnatural conditions. Many facts clearly show how eminently susceptible
the reproductive system is to very slight changes in the surrounding
conditions. Nothing is more easy than to tame an animal, and few things
more difficult than to get it to breed freely under confinement, even when
the male and female unite. How many animals there are which will not
breed, though kept in an almost free state in their native country! This
is generally, but erroneously attributed to vitiated instincts. Many
cultivated plants display the utmost vigour, and yet rarely or never seed!
In some few cases it has been discovered that a very trifling change, such
as a little more or less water at some particular period of growth, will
determine whether or not a plant will produce seeds. I cannot here give
the details which I have collected and elsewhere published on this curious
subject; but to show how singular the laws are which determine the
reproduction of animals under confinement, I may 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, which
seldom produce young; whereas, carnivorous birds, with the rarest
exception, hardly ever lay fertile eggs. Many exotic plants have pollen
utterly worthless, in the same condition as in the most sterile hybrids.
When, on the one hand, we see domesticated animals and plants, though often
weak and sickly, breeding 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 to act, we need not be surprised at this
system, when it does act under confinement, acting irregularly, and
producing offspring somewhat unlike their parents. I may add that as some
organisms breed freely under the most unnatural conditions--for instance,
rabbits and ferrets kept in hutches--showing that their reproductive organs
are not easily affected; so will some animals and plants withstand
domestication or cultivation, and vary very slightly--perhaps hardly more
than in a state of nature.
Some naturalists have maintained that all variations are connected with the
act of sexual reproduction; but this is certainly an error; for I have
given in another work a long list of "sporting plants;" as they are called
by gardeners; that is, of plants which have suddenly produced a single bud
with a new and sometimes widely different character from that of the other
buds on the same plant. These bud variations, as they may be named, can be
propagated by grafts, offsets, etc., and sometimes by seed. They occur
rarely under nature, but are far from rare under culture. As a single bud
out of many thousands produced year after year on the same tree under
uniform conditions, has been known suddenly to assume a new character; and
as buds on distinct trees, growing under different conditions, have
sometimes yielded nearly the same variety--for instance, buds on peach-
trees producing nectarines, and buds on common roses producing moss-roses--
we clearly see that the nature of the conditions is of subordinate
importance in comparison with the nature of the organism in determining
each particular form of variation; perhaps of not more importance than the
nature of the spark, by which a mass of combustible matter is ignited, has
in determining the nature of the flames.
EFFECTS OF HABIT AND OF THE USE OR DISUSE OF PARTS; CORRELATED VARIATION;
INHERITANCE.
Changed habits produce an inherited effect as in the period of the
flowering of plants when transported from one climate to another. With
animals the increased use or disuse of parts has had a more marked
influence; thus 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 this change may be
safely attributed to the domestic duck flying much less, and walking more,
than its wild parents. The great and inherited development of the udders
in cows and goats in countries where they are habitually milked, in
comparison with these organs in other countries, is probably another
instance of the effects of use. Not one of our domestic animals can be
named which has not in some country drooping ears; and the view which has
been suggested that the drooping is due to disuse of the muscles of the
ear, from the animals being seldom much alarmed, seems probable.
Many laws regulate variation, some few of which can be dimly seen, and will
hereafter be briefly discussed. I will here only allude to what may be
called correlated variation. Important changes in the embryo or larva will
probably 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 which are entirely white and have blue eyes are generally deaf; but it
has been lately stated by Mr. Tait that this is confined to the males.
Colour and constitutional peculiarities go together, of which many
remarkable cases could be given among animals and plants. From facts
collected by Heusinger, it appears that white sheep and pigs are injured by
certain plants, while dark-coloured individuals escape: Professor Wyman
has recently communicated to me a good illustration of this fact; on asking
some farmers in Virginia how it was that all their pigs were black, they
informed him that the pigs ate the paint-root (Lachnanthes), which coloured
their bones pink, and which caused the hoofs of all but the black varieties
to drop off; and one of the "crackers" (i.e. Virginia squatters) added, "we
select the black members of a litter for raising, as they alone have a good
chance of living." 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 modify unintentionally other parts of the structure,
owing to the mysterious laws of correlation.
The results of the various, unknown, or but dimly understood laws of
variation are infinitely complex and diversified. It is well worth while
carefully to study the several treatises 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 of structure and constitution in
which the varieties and sub-varieties differ slightly from each other. The
whole organisation seems to have become plastic, and departs in a slight
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, are endless. Dr.
Prosper Lucas' treatise, in two large volumes, is the fullest and the best
on this subject. No breeder doubts how strong is the tendency to
inheritance; that like produces like is his fundamental belief: doubts
have been thrown on this principle only by theoretical writers. When any
deviation of structure often appears, and we see it in the father and
child, we cannot tell whether it may not be due to the same cause having
acted on both; but when among individuals, apparently exposed to the same
conditions, any very rare deviation, due to some extraordinary combination
of circumstances, appears in the parent--say, once among 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 for the most part unknown; no one can
say why the same peculiarity in different individuals of the same species,
or in different species, is sometimes inherited and sometimes not so; why
the child often reverts in certain characteristics to its grandfather or
grandmother or 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 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 the 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
reappear 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 silk-worm 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 the primary cause which may have acted on the
ovules or on the male element; in nearly the same manner as the increased
length of the horns in the offspring from a short-horned cow by a
long-horned bull, 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 invariably 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 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 DEFINITE 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, while kept under the same conditions and while 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
unlimited number of generations, would be opposed to all experience.
CHARACTER OF DOMESTIC VARIETIES; DIFFICULTY OF DISTINGUISHING BETWEEN
VARIETIES AND SPECIES; ORIGIN OF DOMESTIC VARIETIES FROM ONE OR MORE
SPECIES.
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 often have a somewhat
monstrous character; by which I mean, that, although differing 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 the
species under 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 do the closely allied
species of the same genus in a state of nature, but the differences in most
cases are less in degree. This must be admitted as true, for the domestic
races of many animals and plants have been ranked by some competent judges
as the descendants of aboriginally distinct species, and by other competent
judges as mere varieties. If any well marked distinction existed between a
domestic race and a species, this source of doubt would not so perpetually
recur. It has often been stated that domestic races do not differ from
each other in characters of generic value. It can be shown that this
statement is not correct; but naturalists differ much in determining what
characters are of generic value; all such valuations being at present
empirical. When it is explained how genera originate under nature, it will
be seen that we have no right to expect often to find a generic amount of
difference in our domesticated races.
In attempting to estimate the amount of structural difference between
allied domestic races, we are soon involved in doubt, from not knowing
whether they are 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 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 closely allied natural species--for
instance, of the many foxes--inhabiting the 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 a small part of the difference is due to
their being descended from distinct species. In the case of strongly
marked races of some other domesticated species, there is presumptive or
even strong evidence that all are 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 and goose, 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 on an average vary 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, it is
not possible to come to any definite conclusion, whether they are descended
from one or several wild 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 times, on the monuments of Egypt, and in the lake-
habitations of Switzerland, much diversity in the breeds; and that some of
these ancient breeds closely resemble, or are even identical with, those
still existing. But this only throws far backward the history of
civilisation, and shows that animals were domesticated at a much earlier
period than has hitherto been supposed. The lake-inhabitants of
Switzerland cultivated several kinds of wheat and barley, the pea, the
poppy for oil and flax; and they possessed several domesticated animals.
They also carried on commerce with other nations. All this clearly shows,
as Heer has remarked, that they had at this early age progressed
considerably in civilisation; and this again implies a long continued
previous period of less advanced civilisation, during which the
domesticated animals, kept by different tribes in different districts,
might have varied and given rise to distinct races. Since the discovery of
flint tools in the superficial formations of many parts of the world, all
geologists believe that barbarian men existed at an enormously remote
period; and we know that at the present day there is hardly a tribe so
barbarous as not to have domesticated at least the dog.
The origin of most of our domestic animals will probably forever remain
vague. But I may here state that, looking to the domestic dogs of the
whole world, I have, after a laborious collection of all known facts, come
to the conclusion that several wild species of Canidae have been tamed, and
that their blood, in some cases mingled together, flows in the veins of our
domestic breeds. In regard to sheep and goats I can form no decided
opinion. From facts communicated to me by Mr. Blyth, on the habits, voice,
constitution and structure of the humped Indian cattle, it is almost
certain that they are descended from a different aboriginal stock from our
European cattle; and some competent judges believe that these latter have
had two or three wild progenitors, whether or not these deserve to be
called species. This conclusion, as well as that of the specific
distinction between the humped and common cattle, may, indeed, be looked
upon as established by the admirable researches of Professor Rutimeyer.
With respect to horses, from reasons which I cannot here give, I am
doubtfully inclined to believe, in opposition to several authors, that all
the races belong to the same species. Having kept nearly all the English
breeds of the fowl alive, having bred and crossed them, and examined their
skeletons, it appears to me almost certain that all are the descendants of
the wild Indian fowl, Gallus bankiva; and this is the conclusion of Mr.
Blyth, and of others who have studied this bird in India. In regard to
ducks and rabbits, some breeds of which differ much from each other, the
evidence is clear that they are all descended from the common duck and wild
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 eleven wild
species of sheep peculiar to Great Britain! When we bear in mind that
Britain has now not one peculiar mammal, and France but few distinct from
those of Germany, 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 must have originated in Europe; for whence
otherwise could they have been derived? So it is in India. Even in the
case of the breeds of the domestic dog throughout the world, which I admit
are descended from several wild species, it cannot be doubted that there
has been an immense amount of inherited variation; for who will believe
that animals closely resembling the Italian greyhound, the bloodhound, the
bull-dog, pug-dog, or Blenheim spaniel, etc.--so unlike all wild
Canidae--ever existed 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 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, etc.,
in the wild state. Moreover, the possibility of making distinct races by
crossing has been greatly exaggerated. Many cases are on record showing
that a race may be modified by occasional crosses if aided by the careful
selection of the individuals which present the desired character; but to
obtain a race intermediate between two quite distinct races would be very
difficult. Sir J. Sebright expressly experimented with this object and
failed. The offspring from the first cross between two pure breeds is
tolerably and sometimes (as I have found with pigeons) quite uniform in
character, and every thing seems simple enough; but when these mongrels are
crossed one with another for several generations, hardly two of them are
alike, and then the difficulty of the task becomes manifest.
BREEDS OF THE DOMESTIC PIGEON, THEIR DIFFERENCES AND ORIGIN.
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 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 long beak, has a very short and 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 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, 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 the members of the
great pigeon family: 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 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 caudal and sacral vertebrae vary in
number; 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 the wing and
tail to each other and to the body; the relative length of the leg and
foot; the number of scutellae on the toes, the development of skin between
the toes, are all points of structure which are variable. The period at
which the perfect plumage is acquired varies, as does the state of the down
with which the nestling birds are clothed when hatched. The shape and size
of the eggs vary. The manner of flight, and in some breeds the voice and
disposition, differ remarkably. Lastly, in certain breeds, the males and
females have come to differ in 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 be ranked by him as well-defined species. Moreover, I do not
believe that any ornithologist would in this case place the English
carrier, the short-faced tumbler, the runt, the barb, pouter, and fantail
in the same genus; more especially as in each of these breeds several
truly-inherited sub-breeds, or species, as he would call them, could be
shown him.
Great as are the differences between the breeds of the pigeon, I am fully
convinced that the common opinion of naturalists is correct, namely, that
all are 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, they did not
breed or willingly perch 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 improbable; or they must have become extinct in the wild
state. But birds breeding on precipices, and good flyers, 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 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 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 difficult to get wild animals 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 of great weight, and applicable in several other cases, is,
that the above-specified breeds, though agreeing generally with the wild
rock-pigeon in constitution, habits, voice, colouring, and in most parts of
their structure, yet are certainly highly abnormal in other parts; we may
look in vain through the whole great family of Columbidae for a beak like
that of the English carrier, or that of the short-faced tumbler, or barb;
for reversed feathers like those of the Jacobin; for a crop like that of
the pouter; for tail-feathers like those of the fantail. Hence it must be
assumed, not only that half-civilized man succeeded in thoroughly
domesticating several species, but that he intentionally or by chance
picked out extraordinarily abnormal species; and further, that these very
species have since all become extinct or unknown. So many strange
contingencies are 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, with white loins; but
the Indian sub-species, C. intermedia of Strickland, has this part bluish.
The tail has a terminal dark bar, with the outer feathers externally edged
at the base with white. The wings have two black bars. Some semi-domestic
breeds, and some 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 birds belonging to two or more distinct breeds
are crossed, none of which are blue or have any of the above-specified
marks, the mongrel offspring are very apt suddenly to acquire these
characters. To give one instance out of several which I have observed: I
crossed some white fantails, which breed very true, with some black barbs--
and it so happens that blue varieties of barbs are so rare that I never
heard of an instance in England; and the mongrels were black, brown and
mottled. I also crossed a barb with a spot, which is a white bird with a
red tail and red spot on the forehead, and which notoriously breeds very
true; the mongrels were dusky and mottled. I then crossed one of the
mongrel barb-fantails with a mongrel barb-spot, and they produced a bird of
as beautiful a blue colour, with the white loins, 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 are descended from the
rock-pigeon. But if we deny this, we must make one of the two following
highly improbable suppositions. Either, first, 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 no
instance is known of crossed descendants reverting to an ancestor of
foreign blood, removed by a greater number of generations. In a breed
which has been crossed only once the tendency to revert to any character
derived from such a 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, and there is a tendency in the breed to revert to
a character which was 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 of
reversion are often confounded together by those who have written on
inheritance.
Lastly, the hybrids or mongrels from between all the breeds of the pigeon
are perfectly fertile, as I can state from my own observations, purposely
made, on the most distinct breeds. Now, hardly any cases have been
ascertained with certainty of hybrids from two quite distinct species of
animals being perfectly fertile. Some authors believe that long-continued
domestication eliminates this strong tendency to sterility in species.
From the history of the dog, and of some other domestic animals, this
conclusion is probably quite correct, if applied to species closely related
to each other. But to extend it 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 made seven or eight supposed species of pigeons to breed freely
under domestication--these supposed species being quite unknown in a wild
state, and their not having become anywhere feral--these species presenting
certain very abnormal characters, as compared with all other Columbidae,
though so like the rock-pigeon in most other respects--the occasional
reappearance of the blue colour and various black marks in all the breeds,
both when kept pure and when crossed--and lastly, the mongrel offspring
being perfectly fertile--from these several reasons, taken together, we may
safely conclude that all our domestic breeds are descended from the rock-
pigeon or Columba livia with its geographical sub-species.
In favour of this view, I may add, firstly, that the wild C. livia 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, that although an English carrier or a
short-faced tumbler differs immensely in certain characters from the
rock-pigeon, yet that by comparing the several sub-breeds of these two
races, more especially those brought from distant countries, we can make,
between them and the rock-pigeon, an almost perfect series; so we can in
some other cases, but not with all the breeds. Thirdly, those characters
which are mainly distinctive of each breed are in each eminently variable,
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; and
the explanation of this fact will be obvious when we 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 likewise be obvious when we
treat of selection. We shall then, also, see how it is that the several
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, well knowing how truly they breed, I felt fully as much
difficulty in believing that since they had been domesticated they had all
proceeded from a common parent, as any naturalist could in coming to a
similar conclusion in regard to the many species of finches, or other
groups of birds, in nature. One circumstance has struck me much; namely,
that nearly all the breeders of the various domestic animals and the
cultivators of plants, with whom I have 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, or both from a common parent-
stock, 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 are 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?
PRINCIPLES OF SELECTION ANCIENTLY FOLLOWED, AND THEIR EFFECTS.
Let us now briefly consider the steps by which domestic races have been
produced, either from one or from several allied species. Some effect may
be attributed to the direct and definite action of the external conditions
of life, and some to habit; but he would be a bold man who would account by
such agencies for the differences between 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
teasel, with its hooks, which can not 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 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 can not suppose that all the breeds were suddenly produced
as perfect and as useful as we now see them; indeed, in many 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
have made 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 their 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 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 animals, 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." 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 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 among 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 several 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, likewise
followed; for hardly any one is so careless as to breed from his worst
animals.
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--I speak
after careful observation--perhaps never, the case. The law of correlated
variation, the importance of which should never be overlooked, will ensure
some differences; but, as a general rule, it cannot be doubted 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 works
of high antiquity, in which the full importance of the principle is
acknowledged. In rude and barbarous periods of English history choice
animals were often imported, and laws were passed to prevent their
exportation: the destruction of horses under a certain size was ordered,
and this may be compared to the "roguing" of plants by nurserymen. The
principle of selection I find distinctly given in an ancient Chinese
encyclopaedia. Explicit rules are laid down by some of the Roman classical
writers. From passages in Genesis, it is clear that the colour of domestic
animals was at that early period attended to. Savages now sometimes cross
their dogs with wild canine animals, to improve the breed, and they
formerly did so, as is attested by passages in Pliny. The savages in South
Africa match their draught cattle by colour, as do some of the Esquimaux
their teams of dogs. Livingstone states that good domestic breeds are
highly valued by the negroes in 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.
UNCONSCIOUS SELECTION.
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 of the kind in the country. But, for our purpose, a form 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 we may infer 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 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 have been made long ago, which may serve for comparison. In some
cases, however, unchanged, or but little changed, individuals of the same
breed exist in less civilised districts, where the breed has been less
improved. There is reason to believe that King Charles' 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 foxhound; 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, English race-
horses have come to surpass in fleetness and size the parent Arabs, so that
the latter, by the regulations for the Goodwood Races, are favoured in the
weights which 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 various old treatises of the former and present state of carrier
and tumbler pigeons in Britain, India, and Persia, we can 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 unconscious, in so far that the
breeders could never have expected, or even wished, to produce the result
which ensued--namely, the production of the 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 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 chanced to appear, selecting it, and so onwards.
But the gardeners of the classical period, who cultivated the best pears
which 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, thus slowly and unconsciously accumulated,
explains, as I believe, the well-known fact, that in a 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
acquired by 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 why the varieties kept by savages, as has been
remarked by some authors, have more of the character of true species than
the varieties kept in civilised countries.
On the view here given of the 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
have been set on any slight differences in the individuals of the same
species, be judged of by the value which is now set on them, after several
breeds have fairly been established. It is known that with pigeons many
slight variations now occasionally appear, but these are rejected as faults
or deviations from the standard of perfection in each breed. The common
goose has not given rise to any marked varieties; hence the Toulouse and
the common breed, which differ only in colour, that most fleeting of
characters, have lately been exhibited as distinct at our poultry-shows.
These views appear to explain what has sometimes been noticed, namely, that
we know hardly anything 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 a distinct 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 animals slowly spread in the immediate neighbourhood. But they
will as yet hardly have a distinct name, and from being only slightly
valued, their history will have been disregarded. When further improved by
the same slow and gradual process, they will spread more widely, and will
be 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 of a new sub-breed will be a slow
process. As soon as the points of value are once 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.
CIRCUMSTANCES FAVOURABLE TO MAN'S POWER OF SELECTION.
I will 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. Hence
number is of the highest importance for success. On this principle
Marshall formerly remarked, with respect to the sheep of part of Yorkshire,
"As they generally belong to poor people, and are mostly IN SMALL LOTS,
they never can be improved." On the other hand, nurserymen, from keeping
large stocks of the same plant, are generally far more successful than
amateurs in raising new and valuable varieties. A large number of
individuals of an animal or plant can be reared only where the conditions
for its propagation are favourable. When the individuals are scanty all
will be allowed to breed, whatever their quality may be, and this will
effectually prevent selection. But probably the most important element is
that the animal or plant should be so highly valued by man, that the
closest attention is paid to even the slightest deviations in its qualities
or structure. 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 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 (with some aid by crossing distinct species) those
many admirable varieties of the strawberry were raised which have appeared
during the last half-century.
With animals, facility in preventing crosses is an important element 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
improved and kept true, though mingled in the same aviary; and this
circumstance must have largely favoured the 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,
can not be easily matched, and, although so much valued by women and
children, we rarely see a distinct breed long kept up; such breeds as we do
sometimes see are almost always imported from some other country. 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; for recently in certain parts of Spain and of the
United States this animal has been surprisingly modified and improved by
careful selection; 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; but the goose, under the conditions to
which it is exposed when domesticated, seems to have a singularly
inflexible organisation, though it has varied to a slight extent, as I have
elsewhere described.
Some authors have maintained that the amount of variation in our domestic
productions is soon reached, and can never afterward be exceeded. It would
be somewhat rash to assert that the limit has been attained in any one
case; for almost all our animals and plants have been greatly improved in
many ways within a recent period; and this implies variation. It would be
equally rash to assert that characters now increased to their utmost limit,
could not, after remaining fixed for many centuries, again vary under new
conditions of life. No doubt, as Mr. Wallace has remarked with much truth,
a limit will be at last reached. For instance, there must be a limit to
the fleetness of any terrestrial animal, as this will be determined by the
friction to be overcome, the weight of the body to be carried, and the
power of contraction in the muscular fibres. But what concerns us is that
the domestic varieties of the same species differ from each other in almost
every character, which man has attended to and selected, more than do the
distinct species of the same genera. Isidore Geoffroy St. Hilaire has
proved this in regard to size, and so it is with colour, and probably with
the length of hair. With respect to fleetness, which depends on many
bodily characters, Eclipse was far fleeter, and a dray-horse is comparably
stronger, than any two natural species belonging to the same genus. So
with plants, the seeds of the different varieties of the bean or maize
probably differ more in size than do the seeds of the distinct species in
any one genus in the same two families. The same remark holds good in
regard to the fruit of the several varieties of the plum, and still more
strongly with the melon, as well as in many other analogous cases.
To sum up on the origin of our domestic races of animals and plants.
Changed conditions of life are of the highest importance in causing
variability, both by acting directly on the organisation, and indirectly by
affecting the reproductive system. It is not probable that variability is
an inherent and necessary contingent, under all circumstances. The greater
or less force of inheritance and reversion determine whether variations
shall endure. Variability is governed by many unknown laws, of which
correlated growth is probably the most important. Something, but how much
we do not know, may be attributed to the definite action of the conditions
of life. Some, perhaps a great, effect may be attributed to the increased
use or disuse of parts. The final result is thus rendered infinitely
complex. In some cases the intercrossing of aboriginally distinct species
appears to have played an important part in the origin of our breeds. When
several breeds have once been formed in any country, their occasional
intercrossing, with the aid of selection, has, no doubt, largely aided in
the formation of new sub-breeds; but the importance of crossing has been
much exaggerated, both in regard to animals and to those plants which are
propagated by seed. With plants which are temporarily propagated by
cuttings, buds, etc., the importance of crossing is immense; for the
cultivator may here disregard the extreme variability both of hybrids and
of mongrels, and the sterility of hybrids; but plants not propagated by
seed are of little importance to us, for their endurance is only temporary.
Over all these causes of change, the accumulative action of selection,
whether applied methodically and quickly, or unconsciously and slowly, but
more efficiently, seems to have been the predominant power.
|
