ately 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. As the capacity of one plant to be grafted or budded on
another is unimportant for their 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
grafting together. As in hybridisation, so with grafting, the capacity
is limited by systematic affinity, for no one has been able to graft
together trees belonging to quite distinct families; and, on the other
hand, closely allied species and varieties of the same species, can
usually, but not invariably, be grafted with ease. But this capacity,
as in hybridisation, is by no means abso