If now we combine with this fact the correlation of colour with
important constitutional peculiarities, and, in some cases, with
infertility; and consider, further, the curious parallelism that has
been shown to exist between the effects of changed conditions and the
intercrossing of varieties in producing either an increase or a decrease
of fertility, we shall have obtained, at all events, a starting-point
for the production of that infertility which is so characteristic a
feature of distinct species when intercrossed. All we need, now, is some
means of increasing or accumulating this initial tendency; and to a
discussion of this problem we will therefore address ourselves.
_The Influence of Natural Selection upon Sterility and Fertility._
It will occur to many persons that, as the infertility or sterility of
incipient species would be useful to them when occupying the same or
adjacent areas, by neutralising the effects of intercrossing, this
infertility might have been increased by the action of natural
selection; and this will be thought the more probable if we admit, as we
have seen reason to do, that variations in fertility occur, perhaps as
frequently as other variations.
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