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species, compared with the other species of the same genus, we may conclude that this part has undergone an extraordinary amount of
since the period when the
branched off from the common progenitor of the genus. 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
part or organ has been so great and long-continued within a period not
remote, we might, as a general rule,
to find more variability in such parts than in other parts of the
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.
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
theory, for an immense period in nearly the same state; and thus it
→comes
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
as it may be called, still present in a high
→degree. For in this case the variability will seldom as yet have been fixed by the continued selection of the individuals varying in the required manner
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