160 Hev. J. T. Gulick on Divergent Evolution 



how can he say on the next page that forms isolated in a small 

 area, being exposed to uniform conditions, would be modified 

 by natural selection in a uniform manner? He evidently 

 does not intend to be understood as teaching that in these 

 cases mentioned in the second paragraph there is a cause of 

 divergent evolution which produces separate varieties and 

 species in spite of the unifying influence of natural selection 

 resulting from uniform conditions. 



Darwin's Theory of Natural Selection through the Advantage 

 of Divergence of Character. 



There is, however, one passage in the ' Origin of Species ' 

 which may be interpreted as assigning a cause for divergence 

 of character in representatives of the same species that are 

 surrounded by the same environment. These are the words: — 

 " Only those variations which are in some way profitable will 

 be preserved or naturally selected. And here the importance 

 of the principle of benefit being derived from divergence of 

 character comes in ; for this will generally lead to the most 

 different or divergent variations being preserved and accumu- 

 lated by natural selection." (' Origin of Species,' Chap. IV. 

 first page of the section on the " Probable Results of the 

 Action of Natural Selection, through Divergence of Character 

 and Extinction, on the Descendants of a Common Ancestor." 

 In the sixth edition this passage occurs on pp. 90-91.) The 

 connexion in which this passage stands seems to indicate that 

 " the benefit derived from the divergence of character " is con- 

 sidered the cause of " the most different or divergent variations 

 being preserved and accumulated by natural selection," even in 

 the case of the representatives of the same species that are com- 

 peting with each other on the same area, and are in no way 

 prevented from intercrossing. It is therefore necessary to 

 show the difficulties that beset such a theory, especially if we 

 adhere to the more general theory that diversity in the kinds 

 of natural selection affecting a species must be due to differ- 

 ences in the environments by which it is surrounded. 



In the first place natural selection, which is the superior 

 propagation of those best adapted to the environment, prevents 

 the interbreeding of the adapted forms that propagate with 

 the unadapted that fail of propagating ; but it can never pre- 

 vent the interbreeding of those forms which, through different 

 kinds of adaptation to the environment, survive and propa- 

 gate, and therefore it can have no influence in producing 

 accumulated divergence, unless it is supplemented by some 

 segregative principle that prevents the different kinds of adap- 



