For
it would clearly be advantageous to two varieties or incipient species if
they could be kept from blending, on the same principle that, when man is
selecting at the same time two varieties, it is necessary that he should
keep them separate. In the first place, it may be remarked that species
inhabiting distinct regions are often sterile when crossed; now it could
clearly have been of no advantage to such separated species to have been
rendered mutually sterile, and consequently this could not have been
effected through natural selection; but it may perhaps be argued, that, if
a species was rendered sterile with some one compatriot, sterility with
other species would follow as a necessary contingency. In the second
place, it is almost as much opposed to the theory of natural selection as
to that of special creation, that in reciprocal crosses the male element of
one form should have been rendered utterly impotent on a second form, while
at the same time the male element of this second form is enabled freely to
fertilise the first form; for this peculiar state of the reproductive
system could hardly have been advantageous to either species.
In considering the probability of natural selection having come into
action, in rendering species mutually sterile, the greatest difficulty will
be found to lie in the existence of many graduated steps, from slightly
lessened fertility to absolute sterility.
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