an 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 {154} 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.

The principle included in these remarks may be extended. It is notorious
that specific characters are more variable than generic. To explain by a
simple example what is meant. If some species in a large genus of plants
had blue flowers and some had red, the colour would be only a specific
character, and no one would be surprised at one of the blue species varying
into red, or conversely; but if all the species had blue flowers, the
colour would become a generic character, and its variation would be a more
unusual circumstance. I have chosen this example because an explanation is
not in this case applicable, which most naturalists would advance, namely,
that specific characters are more variable than generic, because they are
taken from parts of less physiological importance than those comm