26 Gulick — Divergent Evolution and the Darwinian Theory. 



either while the individuals that are prevented from inter- 

 breeding are occupying the same limited area and exposed to 

 the same environment, or while occupying distinct areas and 

 exposed to either the same or different environments. In the 

 first case, we are told by Darwin, that exposure to uniform 

 conditions " will tend to modify all the varying individuals of 

 the same species in the same manner." In the second case, 

 as the sections of the species that are prevented from crossing 

 occupy separate areas, the advantage of freedom from competi- 

 tion is already secured without divergent adaptation, and there 

 can be no further advantage of that kind. 



Again, it is not difficult to show that divergence is in itself 

 no benefit, for multitudes of more divergent forms fail, leaving 

 the field to less divergent ones. This is generally true of mon- 

 strosities, and frequently true of other kinds of variations. 

 Neither can it be claimed that freedom from competition is 

 an advantage unless it results in freer access to unappropriated 

 resources, and this advantage is most frequently gained by 

 migrating into a locality presenting the same environment but 

 not previously occupied by the species. In this last case, the 

 access to unappropriated resources does not depend on new 

 adaptations; and, as any new adaptations that might bring 

 advantage to the representatives of the species in one district 

 would be of equal advantage in the other district, no diver- 

 gence of character could be advantageous. It is this impossi- 

 bility of advantage in divergence of character in portions of a 

 species exposed to the same environment which leads many 

 naturalists to maintain that isolation does not tend to produce 

 divergence unless accompanied by exposure to different en- 

 vironments. But their reasoning is inconclusive inasmuch as 

 they have never shown that divergence depends on its being 

 advantageous. In my study of Sandwich Island molluscs I 

 have found very strong reasons for believing that divergence 

 may arise in the representatives of one species during exposure 

 to the same environment, producing not only non-adaptive, 

 but also adaptive differences. But whether adaptive or non- 

 adaptive, whether due to natural selection or to some other 

 principle, differences that arise under the same environment 

 cannot be advantageous differences, and the divergence through 

 which the differences are reached is not advantageous diver- 

 gence. It seems to me evident that, neither is divergence 

 always advantageous, nor is the advantage of access to unappro- 

 priated resources necessarily dependent on divergence ; that, 

 neither does the accumulation of divergence depend on its 

 being advantageous, nor is advantageous divergence always 

 accumulated. 



