208 



EEV. J. T. GULICK ON DIVERGENT EVOLUTION 



Independent Greneration o£ these forms ; and, if we can discover 

 these causes and trace tliem to general principles, they will, in 

 connection with the laws of Variation and Selection, explain 

 divergent evolution, that is the transformation of one form into 

 many forms, of one species into many species. As community 

 of evolution arises where there is community of breeding between 

 those that through superior fitness have opportunity to propa- 

 gate, so I believe it will be found that divergent evolution arises 

 where there is separate breeding of the different classes of the 

 successful. In other words. Exclusive Breeding of other than 

 average forms causes Monotypic Evolution, and Segregate 

 Breeding causes Divergent or Polytypic Evolution. 



The facts of geographical distribution seem to me to justify 

 the following statements : — 



(1) A species exposed to different conditions in the different 

 parts of the area over which it is distributed is not represented 

 by divergent forms when free interbreeding exists between the 

 inhabitants of the different districts. In other words, Diversity 

 of Natural Selection without Separation does not produce diver- 

 gent evolution. 



(2) We find many cases in which areas, corresponding in the 

 character of the environment, but separated from each other by 

 important barriers, are the homes of divergent forms of the same 

 or allied species. 



(3) In cases where the separation has been long continued, 

 and the external conditions are the most diverse in points that 

 involve diversity of adaptation, there we find the most decided 

 divergences in the organic forms. That is, where Separation 

 and Divergent Selection have long acted, the results are found 

 to be the greatest. The 1st and 3rd of these propositions will 

 probably be disputed by few if by any. The proof of the 2nd 

 is found wherever a set of closely allied organisms is so distri- 

 buted over a territory that each species and variety occupies its 

 own narrow district, within which it, is shut by barriers that 

 restrain its distribution, while each species of the environing 

 t}'pes is distributed over the whole territory. The distribution 

 of terrestrial mollusks on the Sandwich Islands presents a great 

 body of facts of this kind. 



