30 Gulick — Divergent Evolution and the Darwinian Theory. 



species that are competing with each other.* In chapter 

 XVI, of his " Variation under Domestication " several causes 

 that interfere with the free crossing of varieties are enumer- 

 ated ; but they are nowhere recognized as essential factors in 

 the evolution of divergent varieties and species, without which 

 diversity of natural selection would be of no avail, and with 

 which divergence will take place though there is no change in 

 the environment. They are looked upon as characteristics in 

 which many varieties more or less resemble species ; but they 

 are regarded as the results rather than the causes of divergent 

 evolution. 



Conclusion. 



We, therefore, find that though Darwin has not recognized 

 segregation, which is the independent propagation of different 

 variations, as a necessary condition for the production of 

 divergent races and species, he has pointed out one process by 

 which segregation is produced in nature. This one process is 

 geographical or local separation under different environments. 

 It may be the result of migration, or of geological and other 

 changes in the environment ; but, in either case, there is the 

 preservation of different variations through diversity of natural 

 selection due to the difference in the environments, and the 

 independent propagation of the same variations due to their 

 geographical or local separation. We have in this process an 

 important cause of segregation resulting in divergent evolu- 

 tion ; but no one can maintain that this is the only cause 

 producing segregation and divergence, unless he ignores the 

 fact that, in some cases, the isolated portions of a species, while 

 exposed to the same environment, acquire divergent habits in 

 the use of the environment, producing diversity of natural 

 selection ; and that, in other cases, without exposure to differ- 

 ent environments, the very process producing the isolation, 

 brings together those of one kind preventing them from crossing 

 with those of other kinds, as when individuals of a special color 

 prefer to pair together. In the former cases, indiscriminate sepa- 

 ration is transformed into Segregation ; and, in the latter cases, 

 the isolation is segregative from the first ; while, in both classes 

 of cases, the divergence is without exposure to different en- 

 vironments. 



Osaka, Japan. 



*Io "Nature," vol. xxxiv, page 407, Mr. Francis Darwin states that in his 

 copy of Belt's "'Naturalist in Nicaragua" the words " No, No," are penciled in 

 his father's handwriting on the margin, opposite the sentence: "All the indi- 

 viduals might vary in some one direction, but they could not split up into dis- 

 tinct species whilst they occupied the same area and interbred without difficulty." 

 This seems to give a decisive answer concerning Darwin's opinion on this subject. 



