174 
DARWINISM 
CHAP. 
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 in¬ 
cipient 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. If these two forms freely intercross 
with each other, and produce mongrel offspring which are 
quite fertile inter se, then the further differentiation of the 
forms into two distinct species will be retarded, or perhaps 
entirely prevented ; for the offspring of the crossed unions 
will be, perhaps, more vigorous on account of the cross, 
although less perfectly adapted to the conditions of existence 
than either of the pure breeds; and this would certainly estab¬ 
lish a powei'ful antagonistic influence to the further differentia¬ 
tion of the two forms. 
