| The foregoing rules and facts, on the other hand,
to me clearly to indicate that the sterility both of first crosses and of hybrids is simply incidental or dependent on unknown
in
reproductive
differences being of so peculiar and limited a nature, that, in reciprocal crosses between
→two
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
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, &c.; 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,
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
belonging to quite distinct families; and, on the other hand, closely allied species, and varieties of the same species, can usually,
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