LAWS GOVERNING THE STERILITY 309 



of having, as is usual, an intermediate character between their 

 two parents, always closely resemble one of them; and such 

 hybrids, though externally so like one of their pure parent- 

 species, are with rare exceptions e:xtremely sterile. So again 

 amongst hybrids which are usually intermediate in structure 

 between their parents, exceptional and abnormal individuals 

 sometimes are born, which closely resemble one of their pure 

 parents ; and these hybrids are almost always utterly sterile, 

 even when the other hybrids raised from seed from the same 

 capsule have a considerable degree of fertility. These facts 

 show how completely the fertility of a hybrid may be inde- 

 pendent of its external resemblance to either pure parent. 



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 fer- 

 tility, 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 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, 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 sup- 

 pose it would be equally important to keep from blending to- 

 gether? Why should the degree of sterility be innately vari- 



