owe variability to the same cause which produces sterility; and variability
is the source of all the choicest productions of the garden. I may add,
that as some organisms will breed most freely under the most unnatural
conditions (for instance, the rabbit and ferret kept in hutches), showing
that their reproductive system has not been thus affected; so will some
animals and plants withstand domestication or cultivation, and vary very
slightly--perhaps hardly more than in a state of nature.
A long list could easily be given of 'sporting plants;' by this term
gardeners mean a single bud or offset, which suddenly assumes a new and
sometimes very different character from that of the rest of the plant.
Such buds can be propagated by grafting, &c., and sometimes by seed. These
'sports' are extremely rare under nature, but far from rare under
cultivation; and in this case we see that the treatment of the parent has
affected a bud or offset, and not the ovules or pollen. But it is the
opinion of most physiologists that there is no essential difference between
a bud and an ovule in their earliest stages of formation; so that, in fact,
'sports' support my view, that variability may be largely attributed to the
ovules or pollen, or to both, having been affected by the treatment of the
parent prior to the act of conception. These cases anyhow show that
variation is not necessarily connected, as some authors have supposed, with
the act of generation.
Seedlings from the same fruit, and the young of the same litter, sometimes
differ considerably from each other, though both the young and the parents,
as Muller has remarked, have apparently been exposed to exactly the same
conditions of life; and this shows how unimportant the direct effects of
the conditions of life are in comparison with the laws of reproduction, and
of growth, and of inheritance; for had the action of the conditions been
direct, if any of the young had varied, all would probably have varied in
the same manner. To judge how much, in the case of any variation, we
should attribute to the direct action of heat, moisture, light, food, &c.,
is most difficult: my impression is, that with animals such agencies have
produced very little direct effect, though apparently more in the case of
plants. Under this point of view, Mr. Buckman's recent experiments on
plants seem extremely valuable. When all or nearly all the individuals
exposed to certain conditions are affected in the same way, the change at
first appears to be directly due to such conditions; but in some cases it
can be shown that quite opposite conditions produce similar changes of
structure. Nevertheless some slight amount of change may, I think, be
attributed to the direct action of the conditions of life--as, in some
cases, increased size from amount of food, colour from particular kinds of
food and from light, and perhaps the thickness of fur from climate.
Habit also has a deciding influence, as in the period of flowering with
plants when transported from one climate to another. In animals it has a
more marked effect; for instance, I find in the domestic duck that the
bones of the wing weigh less and the bones of the leg more, in proportion
to the whole skeleton, than do the same bones in the wild-duck; and I
presume that this change may be safely attributed to the domestic duck
flying much less, and walking more, than its wild parent. The great and
inherited development of the udders in cows and goats in countries where
they are habitually milked, in comparison with the state of these organs in
other countries, is another instance of the effect of use. Not a single
domestic animal can be named which has not in some country drooping ears;
and the view suggested by some authors, that the drooping is due to the
disuse of the muscles of the ear, from the animals not being much alarmed
by danger, seems probable.
There are many laws regulating variation, some few of which can be dimly
seen, and will be hereafter briefly mentioned. I will here only allude to
what may be called correlation of growth. Any change in the embryo or
larva will almost certainly entail changes in the mature animal. In
monstrosities, the correlations between quite distinct parts are very
curious; and many instances are given in Isidore Geoffroy St. Hilaire's
great work on this subject. Breeders believe that long limbs are almost
always accompanied by an elongated head. Some instances of correlation are
quite whimsical; thus cats with blue eyes are invariably deaf; colour and
could be given amongst animals and plants. From the facts collected by
Heusinger, it appears that white sheep and pigs are differently affected
from coloured individuals by certain vegetable poisons. Hairless dogs have
imperfect teeth; long-haired and coarse-haired animals are apt to have, as
is asserted, long or many horns; pigeons with feathered feet have skin
between their outer toes; pigeons with short beaks have small feet, and
those with long beaks large feet. Hence, if man goes on selecting, and
thus augmenting, any peculiarity, he will almost certainly unconsciously
modify other parts of the structure, owing to the mysterious laws of the
correlation of growth.
The result of the various, quite unknown, or dimly seen laws of variation
is infinitely complex and diversified. It is well worth while carefully to
study the several treatises published on some of our old cultivated plants,
as on the hyacinth, potato, even the dahlia, &c.; and it is really
surprising to note the endless points in structure and constitution in
which the varieties and sub-varieties differ slightly from each other. The
whole organisation seems to have become plastic, and tends to depart in
some small degree from that of the parental type.
Any variation which is not inherited is unimportant for us. But the number
and diversity of inheritable deviations of structure, both those of slight
and those of considerable physiological importance, is endless. Dr.
Prosper Lucas's treatise, in two large volumes, is the fullest and the best
on this subject. No breeder doubts how strong is the tendency to
inheritance: like produces like is his fundamental belief: doubts have
been thrown on this principle by theoretical writers alone. When a
deviation appears not unfrequently, and we see it in the father and child,
we cannot tell whether it may not be due to the same original cause acting
on both; but when amongst individuals, apparently exposed to the same
conditions, any very rare deviation, due to some extraordinary combination
of circumstances, appears in the parent--say, once amongst several million
individuals--and it reappears in the child, the mere doctrine of chances
almost compels us to attribute its reappearance to inheritance. Every one
must have heard of cases of albinism, prickly skin, hairy bodies, &c.,
appearing in several members of the same family. If strange and rare
deviations of structure are truly inherited, less strange and commoner
&nbs
p; 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
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 a
nd natural species--for instance, of the many foxes--inhabiting
different quarters of the world. I do not believe, as we shall presently
see, that all our dogs have descended from any one wild species; but, in
the case of some other domestic races, there is presumptive, or even
strong, evidence in favour of this view.
It has often been assumed that man has chosen for domestication animals and
plants having an extraordinary inherent tendency to vary, and likewise to
withstand diverse climates. I do not dispute that these capacities have
added largely to the value of most of our domesticated productions; but how
could a savage possibly know, when he first tamed an animal, whether it
would vary in succeeding generations, and whether it would endure other
climates? Has the little variability of the ass or guinea-fowl, or the
small power of endurance of warmth by the rein-deer, or of cold by the
common camel, prevented their domestication? I cannot doubt that if other
animals and plants, equal in number to our domesticated productions, and
belonging to equally diverse classes and countries, were taken from a state
of nature, and could be made to breed for an equal number of generations
under domestication, they would vary on an average as largely as the parent
species of our existing domesticated productions have varied.
In the case of most of our anciently domesticated animals and plants, I do
not think it is possible to come to any definite conclusion, whether they
have descended from one or several species. The argument mainly relied on
by those who believe in the multiple origin of our domestic animals is,
that we find in the most ancient records, more especially on the monuments
of Egypt, much diversity in the breeds; and that some of the breeds closely
resemble, perhaps are identical with, those still existing. Even if this
latter fact were found more strictly and generally true than seems to me to
be the case, what does it show, but that some of our breeds originated
there, four or five thousand years ago? But Mr. Horner's researches have
rendered it in some degree probable that man sufficiently civilized to have
manufactured pottery existed in the valley of the Nile thirteen or fourteen
thousand years ago; and who will pretend to say how long before these
ancient periods, savages, like those of Tierra del Fuego or Australia, who
possess a semi-domestic dog, may not have existed in Egypt?
The whole subject must, I think, remain vague; nevertheless, I may, without
here entering on any details, state that, from geographical and other
considerations, I think it highly probable that our domestic dogs have
descended from several wild species. In regard to sheep and goats I can
form no opinion. I should think, from facts communicated to me by Mr.
Blyth, on the habits, voice, and constitution, &c., of the humped Indian
cattle, that these had descended from a different aboriginal stock from our
European cattle; and several competent judges believe that these latter
have had more than one wild parent. With respect to horses, from reasons
which I cannot give here, I am doubtfully inclined to believe, in
opposition to several authors, that all the races have descended from one
wild stock. Mr. Blyth, whose opinion, from his large and varied stores of
knowledge, I should value more than that of almost any one, thinks that all
the breeds of poultry have proceeded from the common wild Indian fowl
(Gallus bankiva). In regard to ducks and rabbits, the breeds of which
differ considerably from each other in structure, I do not doubt that they
all have descended from the common wild duck and rabbit.
The doctrine of the origin of our several domestic races from several
aboriginal stocks, has been carried to an absurd extreme by some authors.
They believe that every race which breeds true, let the distinctive