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 may be shown that natural selection _is_, in some probable cases at all events, able to accumulate variations in infertility between incipient species.
The simplest case to consider, will be that in which two forms or varieties of a species, occupying an extensive area, are in process of adaptation to somewhat different modes of life within the same area.