HYBRIDISM 233 



Considering the several rules now given, which 

 govern the fertility of first crosses and of hybrids, we 

 see that when forms, which must be considered as good 

 and distinct species, are united, their fertility graduates 

 from zero to perfect fertility, or even to fertility under 

 certain conditions in excess. That their fertility, 

 besides being eminently susceptible to favourable and 

 unfavourable 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 

 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 differ- 

 ence, and occasionally the widest possible difference, in 

 the facility of effecting a union. The hybrids, more- 

 over, produced 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 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 

 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 per- 

 mitted ? to grant to species the special power of produc- 

 ing hybrids, and then to stop their further propagation 



