It is, therefore, directly
under the control of natural selection, which acts both by the
self-preservation of fertile and the self-destruction of infertile
stocks--except always where correlated as above, when they become
useful, and therefore subject to be increased by natural selection.
_Summary and Concluding Remarks on Hybridity._
The facts which are of the greatest importance to a comprehension of
this very difficult subject are those which show the extreme
susceptibility of the reproductive system both in plants and animals. We
have seen how both these classes of organisms may be rendered infertile,
by a change of conditions which does not affect their general health, by
captivity, or by too close interbreeding. We have seen, also, that
infertility is frequently correlated with a difference of colour, or
with other characters; that it is not proportionate to divergence of
structure; that it varies in reciprocal crosses between pairs of the
same species; while in the cases of dimorphic and trimorphic plants the
different crosses between the same pair of individuals may be fertile or
sterile at the same time.
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