ological 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 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 commonly used for