28 THE ORIGIN OF SPECIES 



tility. 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 dis- 

 tinct 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 favorable and unfavorable condi- 

 tions, 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 ex- 

 ternal appearance either parent; and lastly, that the facil- 

 ity 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 recipro- 

 cal 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 oc- 

 casionally the widest possible difference, in the facility 

 of effecting a union. The hj^brids, 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 dif- 

 ferent in degree, when various species are crossed, all of 

 which we must suppose it would be equally important 

 to keep from blending together? Why should the de- 



