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 grafting together.
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