HYBRIDISM 249 



fertility under certain conditions in excess; that their fertility, 

 besides being eminently susceptible to favorable and unfavorable 

 conditions, is innately variable; that it is by no means always the 

 same in degree in the first cross and in the hybrids produced from 

 this cross; that the fertility of hybrids is not related to the degree 

 in which they resemble in external appearance either parent; and 

 lastly, that the facility of making a first cross between any two 

 species is not always governed by their systematic affinity or 

 degree of resemblance to each other. This latter statement is 

 clearly proved by the difference in the result of reciprocal crosses 

 between the same two species, for, according as the one species 

 or the other is used as the father or the mother, there is generally 

 some difference, and occasionally the widest possible difference, 

 in the facility of effecting an union. The hybrids, moreover, pro- 

 duced from reciprocal crosses often differ in fertility. 



Now, do these complex and singular rules indicate that species 

 have been endowed with sterility simply to prevent their becom- 

 ing 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 individuals of the same 

 species? Why should some species cross with facility, and yet 

 produce very sterile hybrids; and other species cross with ex- 

 treme 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 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 re- 

 versed 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 



