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In a bird which has been kept and studied like the pigeon, it is difficult to believe that any remarkable spontaneous variations would pass unnoticed by breeders, or that they would fail to be attended to and developed by some one fancier or other. On the hypothesis of indefinite variability, it is then hard to say why pigeons with bills like toucans, or with certain feathers lengthened like those of trogans, or those of birds of paradise, have never been produced. This, however, is a question which may be settled by experiment. Let a pigeon be bred with a bill like a toucan's, and with the two middle tail-feathers lengthened like those of the king bird of paradise, or even let individuals be produced which exhibit any marked tendency of the kind, and the claim to indefinite variability shall be at once conceded.
As yet all the changes which have taken place in pigeons are of a few definite kinds only, such as may be well conceived to be compatible with a species possessed of a certain inherent capacity for considerable yet definite variation, a capacity for the ready production of certain degrees of abnormality which once attained cannot be further increased.
Mr. Darwin himself has already acquiesced in the proposition here maintained, inasmuch as he distinctly affirms the existence of a marked internal barrier to change in certain cases. And if this is admitted in one case, the principle1 is conceded, and it immediately becomes pro
1 Mr. Darwin, in his "Descent of Man," just published, distinctly admits the existence of such internal powers. Thus, in vol. i. p. 154, he says, of the exciting causes of modification, "they relate much more closely to the constitution of the varying organism, than to the nature of the conditions to which it has been subjected." In a note on page 223 he speaks of "incidental results of certain unknown differences in the con
bable that such internal barriers exist in all, although enclosing a much larger field for variation in some cases than in others. Mr. Darwin abundantly demonstrates the variability of dogs, horses, fowls, and pigeons, but he none the less shows clearly the very small extent to which the goose, the peacock, and the guinea-fowl have varied.1 Mr. Darwin attempts to explain this fact as regards the goose by the animal being valued only for food and feathers, and from no pleasure having been felt in it on other accounts. He adds, however, at the end the striking remark," which concedes the whole position, "but the goose seems to have a singularly inflexible organization." This is not the only place in which such expressions are used. He elsewhere makes use of phrases which quite harmonize with the conception of a normal specific constancy, but varying greatly and suddenly at intervals. Thus he speaks of a whole organization seeming to have become plastic, and tending to depart from the parental type. That different organisms should have different degrees of variability, is only what might have been expected a priori from the existence of parallel differences in inorganic species, some of these having but a single form, and others being polymorphic.
To return to the goose, however, it may be remarked that it is at least as probable that its fixity of character is
stitution of the reproductive system of the species crossed ;" and in vol. ii. at p. 388 may be read the following passage:-"In the greater number of cases we can only say that the cause of each slight variation and of each monstrosity lies much more in the nature or constitution of the organism than in the nature of the surrounding conditions; though new and changed conditions certainly play an important part in exciting organic changes of all kinds."
"Animals and Plants under Domestication," vol. i. pp. 289–295. Origin of Species," 5th edition, 1869, p. 45,
3 Ibid. p. 13.
the cause of the neglect, as the reverse. It is by no means unfair to assume that had the goose shown a tendency to vary similar in degree to the tendency to variation of the fowl or pigeon, it would have received attention at once on that account.
As to the peacock, it is excused on the pleas (1), that the individuals maintained are so few in number, and (2), that its beauty is so great it can hardly be improved. But the individuals maintained have not been too few for the independent origin of the black-shouldered form, or for the supplanting of the commoner one by it. As to any neglect in selection, it can hardly be imagined that with regard to this bird (kept as it is all but exclusively for its beauty), any spontaneous beautiful variation in colour or form would have been neglected. On the contrary, it
would have been seized upon with avidity and preserved with anxious care. Yet apart from the black-shouldered and white varieties, no tendency to change has been known to show itself. As to its being too beautiful for improvement, that is a proposition which can hardly be maintained. Many consider the Javan bird much handsomer than the common peacock, and it would be easy to suggest a score of improvements as regards either species.
The guinea-fowl is excused, as being "no general favourite, and scarcely more common than the peacock;" but Mr. Darwin himself shows and admits that it is a noteworthy instance of constancy under very varied. conditions.
These instances alone (and there are yet others) seem sufficient to establish the assertion that degree of change is different in different domestic animals. It is, then, somewhat unwarrantable in any Darwinian to assume that
all wild animals have a capacity for change similar to that existing in some of the domestic ones. It seems more reasonable to maintain the opposite, namely, that if, as Mr. Darwin says, the capacity for change is different in different domestic animals, it must surely be limited in those domestic animals which have it least, and a fortiori limited in wild animals.
Indeed, it cannot be reasonably maintained that wild species certainly vary as much as do domestic races; it is possible that they may do so, but at least this has not been yet shown. Indeed, the much greater degree of variation amongst domestic animals than amongst wild ones is asserted over and over again by Mr. Darwin, and his assertions are supported by an overwhelming mass of facts and instances.
Of course, it may be maintained that a tendency to indefinite change exists in all cases, and that it is only the circumstances and conditions of life which modify the effects of this tendency to change so as to produce such different results in different cases. But assertion is not proof, and this assertion has not been proved. Indeed, it may be equally asserted (and the statement is more consonant with some of the facts given), that domestication in certain animals induces and occasions a capacity for change which is wanting in wild animals the introduction of new causes occasioning new effects. For, though a certain degree of variability (normally, in all probability, only oscillation) exists in all organisms, yet domestic ones are exposed to new and different causes of variability, resulting in such striking divergencies as have been observed. Not even in this latter case, however, is it necessary to believe that the variability is indefinite, but only that the small oscillations become in certain instances intensified into
large and conspicuous ones. Moreover, it is possible that some of our domestic animals have been in part chosen and domesticated through possessing variability in an eminent degree.
That each species exhibits certain oscillations of structure is admitted on all hands. Mr. Darwin asserts that this is the exhibition of a tendency to vary which is absolutely indefinite. If this indefinite variability does exist, of course no more need be said. But we have seen that there are arguments a priori and a posteriori against it, while the occurrence of variations in certain domestic animals greater in degree than the differences between many wild species, is no argument in favour of its existence, until it can be shown that the causes of variability in the one case are the same as in the other. An argument against indefinite variability, however, may be drawn from the fact, that certain animals, though placed under the influence of those exceptional causes of variation to which domestic animals are subject, have yet never been known to vary, even in a degree equal to that in which certain wild kinds have been ascertained to vary.
In addition to this immutability of character in some animals, it is undeniable that domestic varieties have little stability, and much tendency to reversion, whatever be the true explanation of such phenomena.
In controverting the generally received opinion as to "reversion," Mr. Darwin has shown that it is not all breeds which in a few years revert to the original form; but he has shown no more. Thus, the feral rabbits of Porto Santo, Jamaica, and the Falkland Islands, have not yet so reverted in those several parts of the world.1 Nevertheless, a Porto 1 "Animals and Plants under Domestication," vol. i. p. 115.