for instance, the comb in the Dorking fowl) or the whole breed will
cease to have a nearly uniform character. The breed will then be said
to have degenerated. 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 natural 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 especially concerns us is, that in our domestic animals
those points, which at the present time are undergoing rapid change by
continued selection, are also eminently liable to variation. Look at the
breeds of the pigeon; see what a prodigious amount of difference there
is in the beak of the different tumblers, in the beak and wattle of
the different carriers, in the carriage and tail of our fantails, etc.,
these being the points now mainly attended to by English fanciers. Even
in the sub-breeds, as in the short-faced tumbler, it is notoriously
difficult to breed them nearly to perfection, and frequently individuals
are born which depart widely from the standard. There may be truly
said to be a constant struggle going on between, on the one hand, the
tendency to reversion to a less modified state, as well as an innate
tendency to further variability of all kinds, 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 far as to breed
a bird as coarse as a common tumbler from a good short-faced strain. But
as long as selection is rapidly going on, there may always be expected
to be much variability in the structure undergoing modification. It
further deserves notice that these variable characters, produced by
man's selection, sometimes become attached, from causes quite unknown
to us, more to one sex than to the other, generally to the male sex, as
with the wattle of carriers and the enlarged crop of pouters.

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 species
branched off from the common progenitor of the genus. This period will
seldom be remote in any extreme degree, as species very rarely endure
for more than one ge