lity 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 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