For why should the sterility be so extremely
different in degree, when various species are crossed, all of which we must
suppose it would be equally important to keep from blending together? Why
should the degree of sterility be innately variable in the individuals of
the same species? Why should some species cross with facility and yet
produce very sterile hybrids; and other species cross with extreme
difficulty, and yet produce fairly fertile hybrids? Why should there often
be so great a difference in the result of a reciprocal cross between the
same two species? Why, it may even be asked, has the production of hybrids
been permitted? To grant to species the special power of producing
hybrids, and then to stop their further propagation by different degrees of
sterility, not strictly related to the facility of the first union between
their parents, seems a strange arrangement.
The foregoing rules and facts, on the other hand, appear to me clearly to
indicate that the sterility, both of first crosses and of hybrids, is
simply incidental or dependent on unknown differences in their reproductive
systems; the differences being of so peculiar and limited a nature, that,
in reciprocal crosses between the same two species, the male sexual element
of the one will often freely act on the female sexual element of the other,
but not in a reversed direction. It will be advisable to explain a little
more fully, by an example, what I mean by sterility being incidental on
other differences, and not a specially endowed quality.
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