CHAPTER 1.
VARIATION UNDER DOMESTICATION.
Causes of Variability. Effects of Habit. Correlation of Growth. Inheritance. Character of Domestic Varieties.
Difficulty of distinguishing between Varieties and Species. Origin of Domestic Varieties from one or more
Species. Domestic Pigeons, their Differences and Origin. Principle of Selection anciently followed, its Effects.
Methodical and Unconscious Selection. Unknown Origin of our Domestic Productions. Circumstances
favourable to Man's power of Selection.
When we look to the individuals of the same variety or sub-variety of our older cultivated plants and animals,
one of the first points which strikes us, is, that they generally differ much more from each other, than do the
individuals of any one species or variety in a state of nature. When we reflect on the vast diversity of the
plants and animals which have been cultivated, and which have varied during all ages under the most different
climates and treatment, I think we are driven to conclude that this greater variability is simply due to our
domestic productions having been raised under conditions of life not so uniform as, and somewhat different
from, those to which the parent-species have been exposed under nature. There is, also, I think, some
probability in the view propounded by Andrew Knight, that this variability may be partly connected with
excess of food. It seems pretty clear that organic beings must be exposed during several generations to the
new conditions of life to cause any appreciable amount of variation; and that when the organisation has once
begun to vary, it generally continues to vary for many generations. No case is on record of a variable being
ceasing to be variable under cultivation. Our oldest cultivated plants, such as wheat, still often yield new
varieties: our oldest domesticated animals are still capable of rapid improvement or modification.
It has been disputed at what period of life the causes of variability, whatever they may be, generally act;
whether during the early or late period of development of the embryo, or at the instant of conception.
Geoffroy St. Hilaire's experiments show that unnatural treatment of the embryo causes monstrosities; and
monstrosities cannot be separated by any clear line of distinction from mere variations. But I am strongly
inclined to suspect that the most frequent cause of variability may be attributed to the male and female
reproductive elements having been affected prior to the act of conception. Several reasons make me believe in
this; but the chief one is the remarkable effect which confinement or cultivation has on the functions of the
reproductive system; this system appearing to be far more susceptible than any other part of the organisation,
to the action of any change in the conditions of life. Nothing is more easy than to tame an animal, and few
things more difficult than to get it to breed freely under confinement, even in the many cases when the male
and female unite. How many animals there are which will not breed, though living long under not very close
confinement in their native country! This is generally attributed to vitiated instincts; but how many cultivated
plants display the utmost vigour, and yet rarely or never seed! In some few such cases it has been found out
that very trifling changes, such as a little more or less water at some particular period of growth, will
determine whether or not the plant sets a seed. I cannot here enter on the copious details which I have
collected on this curious subject; but to show how singular the laws are which determine the reproduction of
animals under confinement, I may just mention that carnivorous animals, even from the tropics, breed in this
country pretty freely under confinement, with the exception of the plantigrades or bear family; whereas,
carnivorous birds, with the rarest exceptions, hardly ever lay fertile eggs. Many exotic plants have pollen
utterly worthless, in the same exact condition as in the most sterile hybrids. When, on the one hand, we see
domesticated animals and plants, though often weak and sickly, yet breeding quite freely under confinement;
and when, on the other hand, we see individuals, though taken young from a state of nature, perfectly tamed,
long-lived, and healthy (of which I could give numerous instances), yet having their reproductive system so
seriously affected by unperceived causes as to fail in acting, we need not be surprised at this system, when it
does act under confinement, acting not quite regularly, and producing offspring not perfectly like their parents
or variable.
Sterility has been said to be the bane of horticulture; but on this view we owe variability to the same cause
which produces sterility; and variability is the source of all the choicest productions of the garden. I may add,
that as some organisms will breed most freely under the most unnatural conditions (for instance, the rabbit and
ferret kept in hutches), showing that their reproductive system has not been thus affected; so will some
animals and plants withstand domestication or cultivation, and vary very slightly--perhaps hardly more than in
a state of nature.
A long list could easily be given of "sporting plants;" by this term gardeners mean a single bud or offset,
which suddenly assumes a new and sometimes very different character from that of the rest of the plant. Such
buds can be propagated by grafting, etc., and sometimes by seed. These "sports" are extremely rare under
nature, but far from rare under cultivation; and in this case we see that the treatment of the parent has affected
a bud or offset, and not the ovules or pollen. But it is the opinion of most physiologists that there is no
essential difference between a bud and an ovule in their earliest stages of formation; so that, in fact, "sports"
support my view, that variability may be largely attributed to the ovules or pollen, or to both, having been
affected by the treatment of the parent prior to the act of conception. These cases anyhow show that variation
is not necessarily connected, as some authors have supposed, with the act of generation.
Seedlings from the same fruit, and the young of the same litter, sometimes differ considerably from each
other, though both the young and the parents, as Muller has remarked, have apparently been exposed to
exactly the same conditions of life; and this shows how unimportant the direct effects of the conditions of life
are in comparison with the laws of reproduction, and of growth, and of inheritance; for had the action of the
conditions been direct, if any of the young had varied, all would probably have varied in the same manner. To
judge how much, in the case of any variation, we should attribute to the direct action of heat, moisture, light,
food, etc., is most difficult: my impression is, that with animals such agencies have produced very little direct
effect, though apparently more in the case of plants. Under this point of view, Mr. Buckman's recent
experiments on plants seem extremely valuable. When all or nearly all the individuals exposed to certain
conditions are affected in the same way, the change at first appears to be directly due to such conditions; but
in some cases it can be shown that quite opposite conditions produce similar changes of structure.
Nevertheless some slight amount of change may, I think, be attributed to the direct action of the conditions of
life--as, in some cases, increased size from amount of food, colour from particular kinds of food and from
light, and perhaps the thickness of fur from climate.
Habit also has a decided influence, as in the period of flowering with plants when transported from one
climate to another. In animals it has a more marked effect; for instance, I find in the domestic duck that the
bones of the wing weigh less and the bones of the leg more, in proportion to the whole skeleton, than do the
same bones in the wild-duck; and I presume that this change may be safely attributed to the domestic duck
flying much less, and walking more, than its wild parent. The great and inherited development of the udders in
cows and goats in countries where they are habitually milked, in comparison with the state of these organs in
other countries, is another instance of the effect of use. Not a single domestic animal can be named which has
not in some country drooping ears; and the view suggested by some authors, that the drooping is due to the
disuse of the muscles of the ear, from the animals not being much alarmed by danger, seems probable.
There are many laws regulating variation, some few of which can be dimly seen, and will be hereafter briefly
mentioned. I will here only allude to what may be called correlation of growth. Any change in the embryo or
larva will almost certainly entail changes in the mature animal. In monstrosities, the correlations between
quite distinct parts are very curious; and many instances are given in Isidore Geoffroy St. Hilaire's great work
on this subject. Breeders believe that long limbs are almost always accompanied by an elongated head. Some
instances of correlation are quite whimsical; thus cats with blue eyes are invariably deaf; colour and
constitutional peculiarities go together, of which many remarkable cases could be given amongst animals and
plants. From the facts collected by Heusinger, it appears that white sheep and pigs are differently affected
from coloured individuals by certain vegetable poisons. Hairless dogs have imperfect teeth; long-haired and
coarse-haired animals are apt to have, as is asserted, long or many horns; pigeons with feathered feet have skin between their outer toes; pigeons with short beaks have small feet, and those with long beaks large feet.
Hence, if man goes on selecting, and thus augmenting, any peculiarity, he will almost certainly unconsciously
modify other parts of the structure, owing to the mysterious laws of the correlation of growth.
The result of the various, quite unknown, or dimly seen laws of variation is infinitely complex and diversified.
It is well worth while carefully to study the several treatises published on some of our old cultivated plants, as
on the hyacinth, potato, even the dahlia, etc.; and it is really surprising to note the endless points in structure
and constitution in which the varieties and sub-varieties differ slightly from each other. The whole
organisation seems to have become plastic, and tends to depart in some small degree from that of the parental
type.
Any variation which is not inherited is unimportant for us. But the number and diversity of inheritable
deviations of structure, both those of slight and those of considerable physiological importance, is endless. Dr.
Prosper Lucas's treatise, in two large volumes, is the fullest and the best on this subject. No breeder doubts
how strong is the tendency to inheritance: like produces like is his fundamental belief: doubts have been
thrown on this principle by theoretical writers alone. When a deviation appears not unfrequently, and we see it
in the father and child, we cannot tell whether it may not be due to the same original cause acting on both; but
when amongst individuals, apparently exposed to the same conditions, any very rare deviation, due to some
extraordinary combination of circumstances, appears in the parent--say, once amongst several million
individuals--and it reappears in the child, the mere doctrine of chances almost compels us to attribute its
reappearance to inheritance. Every one must have heard of cases of albinism, prickly skin, hairy bodies, etc.,
appearing in several members of the same family. If strange and rare deviations of structure are truly
inherited, less strange and commoner deviations may be freely admitted to be inheritable. Perhaps the correct
way of viewing the whole subject, would be, to look at the inheritance of every character whatever as the rule,
and non-inheritance as the anomaly.
The laws governing inheritance are quite unknown; no one can say why the same peculiarity in different
individuals of the same species, and in individuals of different species, is sometimes inherited and sometimes
not so; why the child often reverts in certain characters to its grandfather or grandmother or other much more
remote ancestor; why a peculiarity is often transmitted from one sex to both sexes or to one sex alone, more
commonly but not exclusively to the like sex. It is a fact of some little importance to us, that peculiarities
appearing in the males of our domestic breeds are often transmitted either exclusively, or in a much greater
degree, to males alone. A much more important rule, which I think may be trusted, is that, at whatever period
of life a peculiarity first appears, it tends to appear in the offspring at a corresponding age, though sometimes
earlier. In many cases this could not be otherwise: thus the inherited peculiarities in the horns of cattle could
appear only in the offspring when nearly mature; peculiarities in the silkworm are known to appear at the
corresponding caterpillar or cocoon stage. But hereditary diseases and some other facts make me believe that
the rule has a wider extension, and that when there is no apparent reason why a peculiarity should appear at
any particular age, yet that it does tend to appear in the offspring at the same period at which it first appeared
in the parent. I believe this rule to be of the highest importance in explaining the laws of embryology. These
remarks are of course confined to the first APPEARANCE of the peculiarity, and not to its primary cause,
which may have acted on the ovules or male element; in nearly the same manner as in the crossed offspring
from a short-horned cow by a long-horned bull, the greater length of horn, though appearing late in life, is
clearly due to the male element.
Having alluded to the subject of reversion, I may here refer to a statement often made by naturalists--namely,
that our domestic varieties, when run wild, gradually but certainly revert in character to their aboriginal
stocks. Hence it has been argued that no deductions can be drawn from domestic races to species in a state of
nature. I have in vain endeavoured to discover on what decisive facts the above statement has so often and so
boldly been made. There would be great difficulty in proving its truth: we may safely conclude that very many
of the most strongly-marked domestic varieties could not possibly live in a wild state. In many cases we do
not know what the aboriginal stock was, and so could not tell whether or not nearly perfect reversion had ensued. It would be quite necessary, in order to prevent the effects of intercrossing, that only a single variety
should be turned loose in its new home. Nevertheless, as our varieties certainly do occasionally revert in some
of their characters to ancestral forms, it seems to me not improbable, that if we could succeed in naturalising,
or were to cultivate, during many generations, the several races, for instance, of the cabbage, in very poor soil
(in which case, however, some effect would have to be attributed to the direct action of the poor soil), that
they would to a large extent, or even wholly, revert to the wild aboriginal stock. Whether or not the
experiment would succeed, is not of great importance for our line of argument; for by the experiment itself the
conditions of life are changed. If it could be shown that our domestic varieties manifested a strong tendency to
reversion,--that is, to lose their acquired characters, whilst kept under unchanged conditions, and whilst kept
in a considerable body, so that free intercrossing might check, by blending together, any slight deviations of
structure, in such case, I grant that we could deduce nothing from domestic varieties in regard to species. But
there is not a shadow of evidence in favour of this view: to assert that we could not breed our cart and
race-horses, long and short-horned cattle, and poultry of various breeds, and esculent vegetables, for an almost
infinite number of generations, would be opposed to all experience. I may add, that when under nature the
conditions of life do change, variations and reversions of character probably do occur; but natural selection, as
will hereafter be explained, will determine how far the new characters thus arising shall be preserved.
When we look to the hereditary varieties or races of our domestic animals and plants, and compare them with
species closely allied together, we generally perceive in each domestic race, as already remarked, less
uniformity of character than in true species. Domestic races of the same species, also, often have a somewhat
monstrous character; by which I mean, that, although differing from each other, and from the other species of
the same genus, in several trifling respects, they often differ in an extreme degree in some one part, both when
compared one with another, and more especially when compared with all the species in nature to which they
are nearest allied. With these exceptions (and with that of the perfect fertility of varieties when crossed,--a
subject hereafter to be discussed), domestic races of the same species differ from each other in the same
manner as, only in most cases in a lesser degree than, do closely-allied species of the same genus in a state of
nature. I think this must be admitted, when we find that there are hardly any domestic races, either amongst
animals or plants, which have not been ranked by some competent judges as mere varieties, and by other competent judges as the descendants of aboriginally distinct species. If any marked distinction existed between domestic races and species, this source of doubt could not so perpetually recur. It has often been stated that domestic races do not differ from each other in characters of generic value. I think it could be shown that this statement is hardly correct; but naturalists differ most widely in determining what characters are of generic value; all such valuations being at present empirical. Moreover, on the view of the origin of genera which I shall presently give, we have no right to expect often to meet with generic differences in our domesticated productions.
When we attempt to estimate the amount of structural difference between the domestic races of the same species, we are soon involved in doubt, from not knowing whether they have descended from one or several parent-species. This point, if it could be cleared up, would be interesting; if, for instance, it could be shown that the greyhound, bloodhound, terrier, spaniel, and bull-dog, which we all know propagate their kind so truly, were the offspring of any single species, then such facts would have great weight in making us doubt about the immutability of the many very closely allied and natural species--for instance, of the many foxes--inhabiting different quarters of the world. I do not believe, as we shall presently see, that all our dogs have descended from any one wild species; but, in the case of some other domestic races, there is presumptive, or even strong, evidence in favour of this view.
It has often been assumed that man has chosen for domestication animals and plants having an extraordinary inherent tendency to vary, and likewise to withstand diverse climates. I do not dispute that these capacities have added largely to the value of most of our domesticated productions; but how could a savage possibly know, when he first tamed an animal, whether it would vary in succeeding generations, and whether it would endure other climates? Has the little variability of the ass or guinea-fowl, or the small power of endurance of warmth by the rein-deer, or of cold by the common camel, prevented their domestication? I cannot doubt that if other animals and plants, equal in number to our domesticated productions, and belonging to equally diverse classes and countries, were taken from a state of nature, and could be made to breed for an equal number of generations under domestication, they would vary on an average as largely as the parent species of our existing domesticated productions have varied.
In the case of most of our anciently domesticated animals and plants, I do not think it is possible to come to any definite conclusion, whether they have descended from one or several species. The argument mainly relied on by those who believe in the multiple origin of our domestic animals is, that we find in the most ancient records, more especially on the monuments of Egypt, much diversity in the breeds; and that some of the breeds closely resemble, perhaps are identical with, those still existing. Even if this latter fact were found more strictly and generally true than seems to me to be the case, what does it show, but that some of our breeds originated there, four or five thousand years ago? But Mr. Horner's researches have rendered it in some degree probable that man sufficiently civilized to have manufactured pottery existed in the valley of the Nile thirteen or fourteen thousand years ago; and who will pretend to say how long before these ancient periods, savages, like those of Tierra del Fuego or Australia, who possess a semi-domestic dog, may not have existed in Egypt?
The whole subject must, I think, remain vague; nevertheless, I may, without here entering on any details, state that, from geographical and other considerations, I think it highly probable that our domestic dogs have descended from several wild species. In regard to sheep and goats I can form no opinion. I should think, from facts communicated to me by Mr. Blyth, on the habits, voice, and constitution, etc., of the humped Indian cattle, that these had descended from a different aboriginal stock from our European cattle; and several competent judges believe that these latter have had more than one wild parent. With respect to horses, from reasons which I cannot give here, I am doubtfully inclined to believe, in opposition to several authors, that all the races have descended from one wild stock. Mr. Blyth, whose opinion, from his large and varied stores of knowledge, I should value more than that of almost any one, thinks that all the breeds of poultry have proceeded from the common wild Indian fowl (Gallus bankiva). In regard to ducks and rabbits, the breeds of which differ considerably from each other in structure, I do not doubt that they all have descended from the common wild duck and rabbit.
The doctrine of the origin of our several domestic races from several aboriginal stocks, has been carried to an absurd extreme by some authors. They believe that every race which breeds true, let the distinctive characters be ever so slight, has had its wild prototype. At this rate there must have existed at least a score of species of wild cattle, as many sheep, and several goats in Europe alone, and several even within Great Britain. One author believes that there formerly existed in Great Britain eleven wild species of sheep peculiar to it! When we bear in mind that Britain has now hardly one peculiar mammal, and France but few distinct from those of Germany and conversely, and so with Hungary, Spain, etc., but that each of these kingdoms possesses several peculiar breeds of cattle, sheep, etc., we must admit that many domestic breeds have originated in Europe; for whence could they have been derived, as these several countries do not possess a number of peculiar species as distinct parent-stocks? So it is in India. Even in the case of the domestic dogs of the whole world, which I fully admit have probably descended from several wild species, I cannot doubt that there has been an immense amount of inherited variation. Who can believe that animals closely resembling the Italian greyhound, the bloodhound, the bull-dog, or Blenheim spaniel, etc.--so unlike all wild Canidae--ever existed freely in a state of nature? It has often been loosely said that all our races of dogs have been produced by the crossing of a few aboriginal species; but by crossing we can get only forms in some degree intermediate between their parents; and if we account for our several domestic races by this process, we must admit the former existence of the most extreme forms, as the Italian greyhound, bloodhound, bull-dog, etc., in the wild state. Moreover, the possibility of making distinct races by crossing has been greatly exaggerated. There can be no doubt that a race may be modified by occasional crosses, if aided by the careful selection of those individual mongrels, which present any desired character; but that a race could be obtained nearly intermediate between two extremely different races or species, I can hardly believe. Sir J. Sebright expressly experimentised for this object, and failed. The offspring from the first cross between two pure breeds is tolerably and sometimes (as I have found with pigeons) extremely uniform, and everything seems simple enough; but when these mongrels are crossed one with another for several generations, hardly two of them will be alike, and then the extreme difficulty, or rather utter hopelessness, of the task becomes apparent. Certainly, a breed intermediate between TWO VERY DISTINCT breeds could not be got without extreme care and long-continued selection; nor can I find a single case on record of a permanent race having been thus formed.
VARIATION UNDER DOMESTICATION.
Causes of Variability. Effects of Habit. Correlation of Growth. Inheritance. Character of Domestic Varieties.
Difficulty of distinguishing between Varieties and Species. Origin of Domestic Varieties from one or more
Species. Domestic Pigeons, their Differences and Origin. Principle of Selection anciently followed, its Effects.
Methodical and Unconscious Selection. Unknown Origin of our Domestic Productions. Circumstances
favourable to Man's power of Selection.
When we look to the individuals of the same variety or sub-variety of our older cultivated plants and animals,
one of the first points which strikes us, is, that they generally differ much more from each other, than do the
individuals of any one species or variety in a state of nature. When we reflect on the vast diversity of the
plants and animals which have been cultivated, and which have varied during all ages under the most different
climates and treatment, I think we are driven to conclude that this greater variability is simply due to our
domestic productions having been raised under conditions of life not so uniform as, and somewhat different
from, those to which the parent-species have been exposed under nature. There is, also, I think, some
probability in the view propounded by Andrew Knight, that this variability may be partly connected with
excess of food. It seems pretty clear that organic beings must be exposed during several generations to the
new conditions of life to cause any appreciable amount of variation; and that when the organisation has once
begun to vary, it generally continues to vary for many generations. No case is on record of a variable being
ceasing to be variable under cultivation. Our oldest cultivated plants, such as wheat, still often yield new
varieties: our oldest domesticated animals are still capable of rapid improvement or modification.
It has been disputed at what period of life the causes of variability, whatever they may be, generally act;
whether during the early or late period of development of the embryo, or at the instant of conception.
Geoffroy St. Hilaire's experiments show that unnatural treatment of the embryo causes monstrosities; and
monstrosities cannot be separated by any clear line of distinction from mere variations. But I am strongly
inclined to suspect that the most frequent cause of variability may be attributed to the male and female
reproductive elements having been affected prior to the act of conception. Several reasons make me believe in
this; but the chief one is the remarkable effect which confinement or cultivation has on the functions of the
reproductive system; this system appearing to be far more susceptible than any other part of the organisation,
to the action of any change in the conditions of life. Nothing is more easy than to tame an animal, and few
things more difficult than to get it to breed freely under confinement, even in the many cases when the male
and female unite. How many animals there are which will not breed, though living long under not very close
confinement in their native country! This is generally attributed to vitiated instincts; but how many cultivated
plants display the utmost vigour, and yet rarely or never seed! In some few such cases it has been found out
that very trifling changes, such as a little more or less water at some particular period of growth, will
determine whether or not the plant sets a seed. I cannot here enter on the copious details which I have
collected on this curious subject; but to show how singular the laws are which determine the reproduction of
animals under confinement, I may just mention that carnivorous animals, even from the tropics, breed in this
country pretty freely under confinement, with the exception of the plantigrades or bear family; whereas,
carnivorous birds, with the rarest exceptions, hardly ever lay fertile eggs. Many exotic plants have pollen
utterly worthless, in the same exact condition as in the most sterile hybrids. When, on the one hand, we see
domesticated animals and plants, though often weak and sickly, yet breeding quite freely under confinement;
and when, on the other hand, we see individuals, though taken young from a state of nature, perfectly tamed,
long-lived, and healthy (of which I could give numerous instances), yet having their reproductive system so
seriously affected by unperceived causes as to fail in acting, we need not be surprised at this system, when it
does act under confinement, acting not quite regularly, and producing offspring not perfectly like their parents
or variable.
Sterility has been said to be the bane of horticulture; but on this view we owe variability to the same cause
which produces sterility; and variability is the source of all the choicest productions of the garden. I may add,
that as some organisms will breed most freely under the most unnatural conditions (for instance, the rabbit and
ferret kept in hutches), showing that their reproductive system has not been thus affected; so will some
animals and plants withstand domestication or cultivation, and vary very slightly--perhaps hardly more than in
a state of nature.
A long list could easily be given of "sporting plants;" by this term gardeners mean a single bud or offset,
which suddenly assumes a new and sometimes very different character from that of the rest of the plant. Such
buds can be propagated by grafting, etc., and sometimes by seed. These "sports" are extremely rare under
nature, but far from rare under cultivation; and in this case we see that the treatment of the parent has affected
a bud or offset, and not the ovules or pollen. But it is the opinion of most physiologists that there is no
essential difference between a bud and an ovule in their earliest stages of formation; so that, in fact, "sports"
support my view, that variability may be largely attributed to the ovules or pollen, or to both, having been
affected by the treatment of the parent prior to the act of conception. These cases anyhow show that variation
is not necessarily connected, as some authors have supposed, with the act of generation.
Seedlings from the same fruit, and the young of the same litter, sometimes differ considerably from each
other, though both the young and the parents, as Muller has remarked, have apparently been exposed to
exactly the same conditions of life; and this shows how unimportant the direct effects of the conditions of life
are in comparison with the laws of reproduction, and of growth, and of inheritance; for had the action of the
conditions been direct, if any of the young had varied, all would probably have varied in the same manner. To
judge how much, in the case of any variation, we should attribute to the direct action of heat, moisture, light,
food, etc., is most difficult: my impression is, that with animals such agencies have produced very little direct
effect, though apparently more in the case of plants. Under this point of view, Mr. Buckman's recent
experiments on plants seem extremely valuable. When all or nearly all the individuals exposed to certain
conditions are affected in the same way, the change at first appears to be directly due to such conditions; but
in some cases it can be shown that quite opposite conditions produce similar changes of structure.
Nevertheless some slight amount of change may, I think, be attributed to the direct action of the conditions of
life--as, in some cases, increased size from amount of food, colour from particular kinds of food and from
light, and perhaps the thickness of fur from climate.
Habit also has a decided influence, as in the period of flowering with plants when transported from one
climate to another. In animals it has a more marked effect; for instance, I find in the domestic duck that the
bones of the wing weigh less and the bones of the leg more, in proportion to the whole skeleton, than do the
same bones in the wild-duck; and I presume that this change may be safely attributed to the domestic duck
flying much less, and walking more, than its wild parent. The great and inherited development of the udders in
cows and goats in countries where they are habitually milked, in comparison with the state of these organs in
other countries, is another instance of the effect of use. Not a single domestic animal can be named which has
not in some country drooping ears; and the view suggested by some authors, that the drooping is due to the
disuse of the muscles of the ear, from the animals not being much alarmed by danger, seems probable.
There are many laws regulating variation, some few of which can be dimly seen, and will be hereafter briefly
mentioned. I will here only allude to what may be called correlation of growth. Any change in the embryo or
larva will almost certainly entail changes in the mature animal. In monstrosities, the correlations between
quite distinct parts are very curious; and many instances are given in Isidore Geoffroy St. Hilaire's great work
on this subject. Breeders believe that long limbs are almost always accompanied by an elongated head. Some
instances of correlation are quite whimsical; thus cats with blue eyes are invariably deaf; colour and
constitutional peculiarities go together, of which many remarkable cases could be given amongst animals and
plants. From the facts collected by Heusinger, it appears that white sheep and pigs are differently affected
from coloured individuals by certain vegetable poisons. Hairless dogs have imperfect teeth; long-haired and
coarse-haired animals are apt to have, as is asserted, long or many horns; pigeons with feathered feet have skin between their outer toes; pigeons with short beaks have small feet, and those with long beaks large feet.
Hence, if man goes on selecting, and thus augmenting, any peculiarity, he will almost certainly unconsciously
modify other parts of the structure, owing to the mysterious laws of the correlation of growth.
The result of the various, quite unknown, or dimly seen laws of variation is infinitely complex and diversified.
It is well worth while carefully to study the several treatises published on some of our old cultivated plants, as
on the hyacinth, potato, even the dahlia, etc.; and it is really surprising to note the endless points in structure
and constitution in which the varieties and sub-varieties differ slightly from each other. The whole
organisation seems to have become plastic, and tends to depart in some small degree from that of the parental
type.
Any variation which is not inherited is unimportant for us. But the number and diversity of inheritable
deviations of structure, both those of slight and those of considerable physiological importance, is endless. Dr.
Prosper Lucas's treatise, in two large volumes, is the fullest and the best on this subject. No breeder doubts
how strong is the tendency to inheritance: like produces like is his fundamental belief: doubts have been
thrown on this principle by theoretical writers alone. When a deviation appears not unfrequently, and we see it
in the father and child, we cannot tell whether it may not be due to the same original cause acting on both; but
when amongst individuals, apparently exposed to the same conditions, any very rare deviation, due to some
extraordinary combination of circumstances, appears in the parent--say, once amongst several million
individuals--and it reappears in the child, the mere doctrine of chances almost compels us to attribute its
reappearance to inheritance. Every one must have heard of cases of albinism, prickly skin, hairy bodies, etc.,
appearing in several members of the same family. If strange and rare deviations of structure are truly
inherited, less strange and commoner deviations may be freely admitted to be inheritable. Perhaps the correct
way of viewing the whole subject, would be, to look at the inheritance of every character whatever as the rule,
and non-inheritance as the anomaly.
The laws governing inheritance are quite unknown; no one can say why the same peculiarity in different
individuals of the same species, and in individuals of different species, is sometimes inherited and sometimes
not so; why the child often reverts in certain characters to its grandfather or grandmother or other much more
remote ancestor; why a peculiarity is often transmitted from one sex to both sexes or to one sex alone, more
commonly but not exclusively to the like sex. It is a fact of some little importance to us, that peculiarities
appearing in the males of our domestic breeds are often transmitted either exclusively, or in a much greater
degree, to males alone. A much more important rule, which I think may be trusted, is that, at whatever period
of life a peculiarity first appears, it tends to appear in the offspring at a corresponding age, though sometimes
earlier. In many cases this could not be otherwise: thus the inherited peculiarities in the horns of cattle could
appear only in the offspring when nearly mature; peculiarities in the silkworm are known to appear at the
corresponding caterpillar or cocoon stage. But hereditary diseases and some other facts make me believe that
the rule has a wider extension, and that when there is no apparent reason why a peculiarity should appear at
any particular age, yet that it does tend to appear in the offspring at the same period at which it first appeared
in the parent. I believe this rule to be of the highest importance in explaining the laws of embryology. These
remarks are of course confined to the first APPEARANCE of the peculiarity, and not to its primary cause,
which may have acted on the ovules or male element; in nearly the same manner as in the crossed offspring
from a short-horned cow by a long-horned bull, the greater length of horn, though appearing late in life, is
clearly due to the male element.
Having alluded to the subject of reversion, I may here refer to a statement often made by naturalists--namely,
that our domestic varieties, when run wild, gradually but certainly revert in character to their aboriginal
stocks. Hence it has been argued that no deductions can be drawn from domestic races to species in a state of
nature. I have in vain endeavoured to discover on what decisive facts the above statement has so often and so
boldly been made. There would be great difficulty in proving its truth: we may safely conclude that very many
of the most strongly-marked domestic varieties could not possibly live in a wild state. In many cases we do
not know what the aboriginal stock was, and so could not tell whether or not nearly perfect reversion had ensued. It would be quite necessary, in order to prevent the effects of intercrossing, that only a single variety
should be turned loose in its new home. Nevertheless, as our varieties certainly do occasionally revert in some
of their characters to ancestral forms, it seems to me not improbable, that if we could succeed in naturalising,
or were to cultivate, during many generations, the several races, for instance, of the cabbage, in very poor soil
(in which case, however, some effect would have to be attributed to the direct action of the poor soil), that
they would to a large extent, or even wholly, revert to the wild aboriginal stock. Whether or not the
experiment would succeed, is not of great importance for our line of argument; for by the experiment itself the
conditions of life are changed. If it could be shown that our domestic varieties manifested a strong tendency to
reversion,--that is, to lose their acquired characters, whilst kept under unchanged conditions, and whilst kept
in a considerable body, so that free intercrossing might check, by blending together, any slight deviations of
structure, in such case, I grant that we could deduce nothing from domestic varieties in regard to species. But
there is not a shadow of evidence in favour of this view: to assert that we could not breed our cart and
race-horses, long and short-horned cattle, and poultry of various breeds, and esculent vegetables, for an almost
infinite number of generations, would be opposed to all experience. I may add, that when under nature the
conditions of life do change, variations and reversions of character probably do occur; but natural selection, as
will hereafter be explained, will determine how far the new characters thus arising shall be preserved.
When we look to the hereditary varieties or races of our domestic animals and plants, and compare them with
species closely allied together, we generally perceive in each domestic race, as already remarked, less
uniformity of character than in true species. Domestic races of the same species, also, often have a somewhat
monstrous character; by which I mean, that, although differing from each other, and from the other species of
the same genus, in several trifling respects, they often differ in an extreme degree in some one part, both when
compared one with another, and more especially when compared with all the species in nature to which they
are nearest allied. With these exceptions (and with that of the perfect fertility of varieties when crossed,--a
subject hereafter to be discussed), domestic races of the same species differ from each other in the same
manner as, only in most cases in a lesser degree than, do closely-allied species of the same genus in a state of
nature. I think this must be admitted, when we find that there are hardly any domestic races, either amongst
animals or plants, which have not been ranked by some competent judges as mere varieties, and by other competent judges as the descendants of aboriginally distinct species. If any marked distinction existed between domestic races and species, this source of doubt could not so perpetually recur. It has often been stated that domestic races do not differ from each other in characters of generic value. I think it could be shown that this statement is hardly correct; but naturalists differ most widely in determining what characters are of generic value; all such valuations being at present empirical. Moreover, on the view of the origin of genera which I shall presently give, we have no right to expect often to meet with generic differences in our domesticated productions.
When we attempt to estimate the amount of structural difference between the domestic races of the same species, we are soon involved in doubt, from not knowing whether they have descended from one or several parent-species. This point, if it could be cleared up, would be interesting; if, for instance, it could be shown that the greyhound, bloodhound, terrier, spaniel, and bull-dog, which we all know propagate their kind so truly, were the offspring of any single species, then such facts would have great weight in making us doubt about the immutability of the many very closely allied and natural species--for instance, of the many foxes--inhabiting different quarters of the world. I do not believe, as we shall presently see, that all our dogs have descended from any one wild species; but, in the case of some other domestic races, there is presumptive, or even strong, evidence in favour of this view.
It has often been assumed that man has chosen for domestication animals and plants having an extraordinary inherent tendency to vary, and likewise to withstand diverse climates. I do not dispute that these capacities have added largely to the value of most of our domesticated productions; but how could a savage possibly know, when he first tamed an animal, whether it would vary in succeeding generations, and whether it would endure other climates? Has the little variability of the ass or guinea-fowl, or the small power of endurance of warmth by the rein-deer, or of cold by the common camel, prevented their domestication? I cannot doubt that if other animals and plants, equal in number to our domesticated productions, and belonging to equally diverse classes and countries, were taken from a state of nature, and could be made to breed for an equal number of generations under domestication, they would vary on an average as largely as the parent species of our existing domesticated productions have varied.
In the case of most of our anciently domesticated animals and plants, I do not think it is possible to come to any definite conclusion, whether they have descended from one or several species. The argument mainly relied on by those who believe in the multiple origin of our domestic animals is, that we find in the most ancient records, more especially on the monuments of Egypt, much diversity in the breeds; and that some of the breeds closely resemble, perhaps are identical with, those still existing. Even if this latter fact were found more strictly and generally true than seems to me to be the case, what does it show, but that some of our breeds originated there, four or five thousand years ago? But Mr. Horner's researches have rendered it in some degree probable that man sufficiently civilized to have manufactured pottery existed in the valley of the Nile thirteen or fourteen thousand years ago; and who will pretend to say how long before these ancient periods, savages, like those of Tierra del Fuego or Australia, who possess a semi-domestic dog, may not have existed in Egypt?
The whole subject must, I think, remain vague; nevertheless, I may, without here entering on any details, state that, from geographical and other considerations, I think it highly probable that our domestic dogs have descended from several wild species. In regard to sheep and goats I can form no opinion. I should think, from facts communicated to me by Mr. Blyth, on the habits, voice, and constitution, etc., of the humped Indian cattle, that these had descended from a different aboriginal stock from our European cattle; and several competent judges believe that these latter have had more than one wild parent. With respect to horses, from reasons which I cannot give here, I am doubtfully inclined to believe, in opposition to several authors, that all the races have descended from one wild stock. Mr. Blyth, whose opinion, from his large and varied stores of knowledge, I should value more than that of almost any one, thinks that all the breeds of poultry have proceeded from the common wild Indian fowl (Gallus bankiva). In regard to ducks and rabbits, the breeds of which differ considerably from each other in structure, I do not doubt that they all have descended from the common wild duck and rabbit.
The doctrine of the origin of our several domestic races from several aboriginal stocks, has been carried to an absurd extreme by some authors. They believe that every race which breeds true, let the distinctive characters be ever so slight, has had its wild prototype. At this rate there must have existed at least a score of species of wild cattle, as many sheep, and several goats in Europe alone, and several even within Great Britain. One author believes that there formerly existed in Great Britain eleven wild species of sheep peculiar to it! When we bear in mind that Britain has now hardly one peculiar mammal, and France but few distinct from those of Germany and conversely, and so with Hungary, Spain, etc., but that each of these kingdoms possesses several peculiar breeds of cattle, sheep, etc., we must admit that many domestic breeds have originated in Europe; for whence could they have been derived, as these several countries do not possess a number of peculiar species as distinct parent-stocks? So it is in India. Even in the case of the domestic dogs of the whole world, which I fully admit have probably descended from several wild species, I cannot doubt that there has been an immense amount of inherited variation. Who can believe that animals closely resembling the Italian greyhound, the bloodhound, the bull-dog, or Blenheim spaniel, etc.--so unlike all wild Canidae--ever existed freely in a state of nature? It has often been loosely said that all our races of dogs have been produced by the crossing of a few aboriginal species; but by crossing we can get only forms in some degree intermediate between their parents; and if we account for our several domestic races by this process, we must admit the former existence of the most extreme forms, as the Italian greyhound, bloodhound, bull-dog, etc., in the wild state. Moreover, the possibility of making distinct races by crossing has been greatly exaggerated. There can be no doubt that a race may be modified by occasional crosses, if aided by the careful selection of those individual mongrels, which present any desired character; but that a race could be obtained nearly intermediate between two extremely different races or species, I can hardly believe. Sir J. Sebright expressly experimentised for this object, and failed. The offspring from the first cross between two pure breeds is tolerably and sometimes (as I have found with pigeons) extremely uniform, and everything seems simple enough; but when these mongrels are crossed one with another for several generations, hardly two of them will be alike, and then the extreme difficulty, or rather utter hopelessness, of the task becomes apparent. Certainly, a breed intermediate between TWO VERY DISTINCT breeds could not be got without extreme care and long-continued selection; nor can I find a single case on record of a permanent race having been thus formed.
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