VAIUATION OF SPECIFIC CHAEACTEES. 359 



of sexual union. Hence we may further infer that sexual union 

 is in itself an avixiliary to the interruption of the constancy of 

 species, even when it is effected between two individuals of the 

 same species ; only in this case it is worked out by nature with 

 far less violence, as I may say, than in cases of true hybridisa- 

 tion. And this agrees with the results of investigation in the 

 domain of botany, for Sachs, in his ' Text-book of Botany,' 

 expressly says that there is no essential difference between the 

 self-fertilisation of ipwce species or varieties, and fertilisation by 

 other species or varieties, and that in the case of true hybridisa- 

 tion many peculiar characters attributable to sexual differentia- 

 tion and agreement are brought into greater prominence. Prom 

 all that we know at present the interruption of the constancy of 

 specific characters may be regarded as one of the most conspicu- 

 ous of these peculiarities. 



