388 LAWS OOVERNma THE 8TEBILITT 



character between their two parents, always closely resem- 

 ble one of them; and such hybrids, though externally so 

 like one of their pure parent-species, are with rare excep- 

 tions extremely sterile. So again among 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 con- 

 ditions in excess; that their fertility, besides being emi- 

 nently 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 dif- 

 ference 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 

 difiereuce, and occasionally the widest possible dif- 

 ference, 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 differ- 

 ent in degree, when various species are crossed, all of which 

 we must suppose it would be equallv important to keep 

 from blending together? Why should the degree of 

 sterility be innately variable in the individuals of the same 



