6. We may fairly suppose, also, that as soon as any sterility
appears some disinclination to _cross unions_ will appear, 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.
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