be applied, that part (for
instance, the comb in the Dorking fowl) or the whole breed will cease to
have a uniform character: and the breed may be said to be degenerating.
In rudimentary organs, and in those which have been but little
specialised for any particular purpose, and perhaps in polymorphic
groups, we see a nearly parallel case; for in such cases natural
selection either has not or cannot come into full play, and thus the
organisation is left in a fluctuating condition. But what here more
particularly concerns us is, that those points in our domestic animals,
which at the present time are undergoing rapid change by continued
selection, are also eminently liable to variation. Look at the
individuals of the same breed of the pigeon; and see what a prodigious
amount of difference there is in the beak of tumblers, in the beak and
wattle of carriers, in the carriage and tail of fantails, etc., these
being the points now mainly attended to by English fanciers. Even in the
same sub-breed, as in that of the short-faced tumbler, it is notoriously
difficult to breed nearly perfect birds, many departing widely from the
standard. There may truly be said to be a constant struggle going on
between, on the one hand, the tendency to reversion to a less perfect
state, as well as an innate tendency to new variations, and, on the
other hand, the power of steady selection to keep the breed true. In
the long run selection gains the day, and we do not expect to fail so
completely as to breed a bird as coarse as a common tumbler pigeon from
a good short-faced strain. But as long as selection is rapidly going
on, much variability in the parts undergoing modification may always be
expected.

Now let us turn to nature. When a part has been developed in an
extraordinary manner in any one species, compared with the other species
of the same genus, we may conclude that this part has undergone an
extraordinary amount of modification since the period when the several
species branched off from the common progenitor of the genus. This
period will seldom be remote in any extreme degree, as species rarely
endure for more than one geological period. An extraordinary amount of
modification implies an unusually large and long-continued amount of
variability, which has continually been accumulated by natural
selection for the benefit of the species. But as the variability of
the extraordinarily developed part or organ has been so great and
long-c