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 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.
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