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-continued within a period not
excessively remote, we might, as a general rule, still expect 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 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 our theory, for an immense
period in nearly the same state; and thus it has come not to be 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 VARIABILITY, as it may be called, still
present in a high degree.
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