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 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, &c. ; but in a multi- 

 tude of cases we can assign no reason whatever. Great di- 

 versity in the size of two plants, one being woody and the 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 



