art (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, &c., 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 {153} 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 th