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. Mr. Darwin tells us that, at one time,
this appeared to him probable, but he found the problem to be one of
extreme complexity; and he was also influenced against the view by many
considerations which seemed to render such an origin of the sterility or
infertility of species when intercrossed very improbable. The fact that
species which occupy distinct areas, and which nowhere come in contact
with each other, are often sterile when crossed, is one of the
difficulties; but this may perhaps be overcome by the consideration
that, though now isolated, they may, and often must, have been in
contact at their origination. More important is the objection that
natural selection could not possibly have produced the difference that
often occurs between reciprocal crosses, one of these being sometimes
fertile, while the other is sterile. The extremely different amounts of
infertility or sterility between different species of the same genus,
the infertility often bearing no proportion to the difference between
the species crossed, is also an important objection. But none of these
objections would have much weight if it could be clearly shown that
natural selection _is_ able to increase the infertility variations of
incipient species, as it is certainly able to increase and develop all
useful variations of form, structure, instincts, or habits. Ample causes
of infertility have been shown to exist, in the nature of the organism
and the laws of correlation; the agency of natural selection is only
needed to accumulate the effects produced by these causes, and to render
their final results more uniform and more in accordance with the facts
that exist.

About twenty years ago I had much correspondence and discussion with Mr.
Darwin on this question. I then believed that I was able to demonstrate
the action of natural selection in accumulating infertility; but I could
not convince him, owing to the extreme complexity of the process under
the conditions which he thought most probable. I have recently returned
to the question; and, with the fuller knowledge of the facts of
variation we now possess, I think it