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. But as the
variability of the extraordinarily-developed part or organ has been so
great and long-continued within a period not excessively remote, we
might, as a general rule, expect still to find more variability in such
parts than in other parts of the organisation, which have remained for
a much longer period nearly constant. And this, I am convinced, is the
case. That the struggle between natural selection on the one hand, and
the tendency to reversion and variability on the other hand, will in
the
course of time cease; and that the most abnormally developed organs may
be made constant, I can see no reason to doubt. Hence when an organ,
however abnormal it may be, has been transmitted in approximately the
same condition to many modified descendants, as in the case of the wing
of the bat, it must have existed, according to my theory, for an
immense period in nearly the same state; and thus it comes to be no
more variable than any other structure. It is only in those cases in
which the modification has been comparatively recent and
extraordinarily great that we ought to find the _generative
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