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 may be shown that natural selection
_is_, in some probable cases at all events, able to accumulate
variations in infertility between incipient species.
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