ral infertility might arise in correlation with the different modes
of life of two varieties or incipient species. A considerable body of
facts already adduced renders it probable that this _is_ the mode in
which any widespread infertility would arise; and, if so, it has been
shown that, by the influence of natural selection and the known laws
which affect varieties, the infertility would be gradually increased.
But, if we suppose the infertility to arise sporadically within the two
forms, and to affect only a small proportion of the individuals in any
area, it will be difficult, if not impossible, to show that such
infertility would have any tendency to increase, or would produce any
but a prejudicial effect. If, for example, five per cent of each form
thus varied so as to be infertile with the other form, the result would
be hardly perceptible, because the individuals which formed cross-unions
and produced hybrids would constitute a very small portion of the whole
species; and the hybrid offspring, being at a disadvantage in the
struggle for existence and being themselves infertile, would soon die
out, while the much more numerous fertile portion of the two forms would
increase rapidly, and furnish a sufficient number of pure-bred offspring
of each form to take the place of the somewhat inferior hybrids between
them whenever the struggle for existence became severe. We must suppose
that the normal fertile forms would transmit their fertility to their
progeny, and the few infertile forms their infertility; but the latter
would necessarily lose half their proper increase by the sterility of
their hybrid offspring whenever they crossed with the other form, and
when they bred with their own form the tendency to sterility would die
out except in the very minute proportion of the five per cent
(one-twentieth) that chance would lead to pair together. Under these
circumstances the incipient sterility between the two forms would
rapidly be eliminated, and could never rise much above the numbers which
were produced by sporadic variation each year.

It was, probably, by a consideration of some such case as this that Mr.
Darwin came to the conclusion that infertility arising between incipient
species could not be increased by natural selection; and this is the
more likely, as he was always disposed to minimise both the frequency
and the amount even of structural variations.

We have yet to notice another mode of action of natural