Chap. IX.] OP FIRST CROSSES AND OF HYBRIDS. 17 



that species have been endowed with sterility simply 

 to prevent their becoming confounded in nature? I 

 think not. For why should the sterility be so extremely 

 different in degree, when various species are crossed, 

 all of which we must suppose it would be equally im- 

 portant to keep from blending together? Why should 

 the degree of sterility be innately variable in the in- 

 dividuals of the same species? Why should some spe- 

 cies 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 re- 

 ciprocal 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 lim- 

 ited 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 ad- 

 visable 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 un- 

 important for their welfare in a state of nature, I pre- 



