10. Now let variations in the _amount of sterility_ and in
the _disinclination to crossed unions_ continue to occur--also
in certain parts of the area: exactly the same result must
recur, and the progeny of this new physiological variety will in
time occupy the whole area.
11. There is yet another consideration that would facilitate the
process. It seems probable that the _sterility variations_
would, to some extent, concur with, and perhaps depend upon, the
_specific variations_; so that, just in proportion as the _two
forms_ diverged and became better adapted to the conditions of
existence, they would become more sterile when intercrossed. If
this were the case, then natural selection would act with double
strength; and those which were better adapted to survive both
structurally and physiologically would certainly do so.]
[Footnote 63: Cases of this kind are referred to at p. 155. It must,
however, be noted, that such sterility in first crosses appears to be
equally rare between different species of the same genus as between
individuals of the same species.
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