16 LAWS GOVERNING THE STERILITY [Chap. IX. 



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 independent 

 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 fertility, or even to fertility under 

 certain conditions in excess; that their fertility, besides 

 being eminently susceptible to favourable and unfa- 

 vourable 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 syste- 

 matic affinity or degree of resemblance to each other. 

 This latter statement is clearly proved by the differ- 

 ence 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 gen- 

 erally some difference, and occasionally the widest pos- 

 sible 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 



