HYBRIDISM 23 



gree of sterility be innately variable 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 hy- 

 brids, and then to stop their further propagation by dif- 

 ferent 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 limited 

 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 dif- 

 ferences, 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 inci- 

 dental 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 



