Seite - 310 - in The Origin of Species

310 ORIGIN OF SPECIES able 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 be- ing of so peculiar and limited a nature, that, in reciprocal crosses between the same two species, the male sexual ele- ment 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 littlemore fullybyan 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, &c. ; but in a multi- tude of cases we can assign no reason whatever. Great di- versity in the sizeoftwo plants, onebeingwoodyandthe other herbaceous, one being evergreen and the other decidu- ous, an adaptation to widely different climates, do not always prevent the two grafting together. As in hybridisa- tion, so with grafting, the capacity is limited by systematic affinity, for no one has been able to graft together trees be- longing 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