and
this will further tend to the diminution of the production of
hybrids.

7. In the other part of the area, however, where hybridism
occurs with perfect freedom, hybrids of various degrees may
increase till they equal or even exceed in number the pure
species--that is, the incipient species will be liable to be
swamped by intercrossing.

8. The first result, then, of a partial sterility of crosses
appearing in one part of the area occupied by the two forms,
will be--that the great majority of the individuals will there
consist of the two pure forms only, while in the remaining part
these will be in a minority,--which is the same as saying that
the new _physiological variety_ of the two forms will be better
suited to the conditions of existence than the remaining portion
which has not varied physiologically.

9. But when the struggle for existence becomes severe, that
variety which is best adapted to the conditions of existence
always supplants that which is imperfectly adapted; therefore,
_by natural selection_ the _varieties_ which are _sterile_ when
crossed will become established as the only ones.

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. Mules and other hybrids are freely
produced between very distinct species, but are themselves infertile or
quite ster