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 VARIABILITY, 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 and degree, and by the continued rejection of
those tending to revert to a former and less modified condition.
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