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 it, 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 {122} 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 localities.[113] Nevertheless, a Porto Santo rabbit brought to England reverted in a manner the most striking, recovering the proper colour of its fur "in rather less than four years."[114] Again, the white silk fowl, in our climate, "reverts to the ordinary colour of the common fowl in its skin and bones, due care having been taken to prevent any cross."[115] This reversion taking place in spite of careful selection, is very remarkable.
Numerous other instances of reversion are given by Mr.Darwin, both as regards plants and animals; amongst others, the singular fact of bud reversion.[116] The curiously recurring development of black sheep, in spite of the most careful breeding, may also be mentioned, though, perhaps, reversion has no part in the phenomenon.
These facts seem certainly to tell in favour of limited variability, while the cases of non-reversion do not contradict it, as it is not contended that all species have the same tendency to revert, but rather that their capacities in this respect, as well as for change, are different in different kinds, so that often reversion may only show itself at the end of very long periods indeed.
Yet some of the instances given as probable or possible causes of reversion by Mr.Darwin, can hardly be such.

He cites, for example, the occasional presence of supernumerary digits in man.[117] For this notion, however, he is not responsible, as he rests his remark on the authority of a {123} passage published by Professor Owen.