Isolation. 23 



divergent evolution going on at the same time (and 

 here on the same area) as there are forms of isolation 

 affecting the oceanic colony. And this because each 

 of the forms of isolation has given rise to a different 

 case of isolation. 



Now, inasmuch as different forms of isolation, when 

 thus superadded one to another, constitute different 

 cases of isolation, we may lay down the following 

 general law as applying to all the forms of isola- 

 tion namely, The number of possible directions in 

 ivhich divergent evolution can occur \ is never greater 

 than, thoitgh it may be equal to. the number of cases 

 of efficient isolation or the number of efficiently 

 separated groups of intergenerants. 



6. We have now to consider with some care the 

 particular and highly important form of isolation 

 that is presented by natural selectioa For while 

 this form of isolation resembles all the other forms 

 of the discriminate kind in that it secures homo- 

 gamy, there are two points in which it differs from 

 all of them, and one point in which it differs from 

 most of them. 



Natural selection differs from all the other known 

 forms of isolation (whether discriminate or indis- 

 criminate) in that it has exclusive reference to 

 adaptations on the one hand, and, on the other hand, 

 necessitates not only the elimination, but the de- 

 struction of the excluded individuals. Again, natural 

 selection differs from most of the other forms of 

 isolation in that, unless assisted by some other 

 form, it can never lead to polytypic, but only 

 to monotypic evolution. The first two points of 

 difference are here immaterial ; but the last is one 



