species have been
endowed with sterility simply to prevent their becoming confounded
in nature? I think not. 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 to be 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, chiefly in the
reproductive systems, of the species which are crossed. The differences
being of so peculiar and limited a nature, that, in reciprocal crosses
between 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. As the capacity of one plant to be
grafted or budded on another is so entirely unimportant for its welfare
in a state of nature, I presume that no one will suppose that this
capacity is a SPECIALLY endowed quality, but will admit that it is
incidental on differences in the laws of growth of the two plants. We
can sometimes see the reason why one tree will not take on another, from
differences in their rate of growth, in the hardness of their wood, in
the period of the flow or nature of their sap, etc.; but in a multitude
of cases we can assign no reason whatever. Great diversity in the size
of two plants, one being woody and the other herbaceous, one being
evergreen and the other deciduous, and adaptation to widely different
climates, does not always prevent the two gr