Isolation. 21 



dissimilar, and that only in as far as it does this 

 can it be a factor in organic evolution, it follows that 

 the resulting segregation, even though cumulative, 

 can only lead to divergence of organic types in as 

 many directions as there are cases of isolation. For 

 any one group of intergenerants only serial trans- 

 formation is possible, even though the transformation 

 be cumulative through successive generations in the 

 single line of change. But there is always a probability 

 that during the course of such serial transformation 

 in time, some other case of isolation may supervene, 

 so as to divide the previously isolated group of inter- 

 generants into two or more further isolated groups. 

 Then, of course, opportunity will be furnished for 

 divergent transformation in space and this in as 

 many different lines as there are now different 

 homogamous groups. 



That this must be so is further evident, if we 

 reflect that the evolutionary power of isolation 

 depends, not only on the preventing of intercross- 

 ing between the isolated portion of a species and 

 the rest of that species, but also upon the permitting 

 of intercrossing between all individuals of the isolated 

 portion, whereby the peculiar average of qualities which 

 they as a whole present may be allowed to assert itself 

 in their progeny or, if the isolation has been from the 

 first discriminate, whereby the resulting homogamy 

 may thus be allowed to assert itself. Hence any 

 one case of either species of isolation, discriminate or 

 indiscriminate, can only give 'rise to what Mr. Gulick 

 has aptly called fi monotypic evolution," or a chain- 

 like series of types arising successively in time, as 

 distinguished from what he has called " polytypic 



