No. 493] ISOLATION AND SELECTION 



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a number of species presenting both forms. If direct 

 crossing is impossible, it follows that the only chance for 

 continued connection between the two forms is through 

 the subsequent mutation of one or both of the forms ; and 

 we know that in some species this occurs in a small per- 

 centage of the offspring of each generation. 



That isolation is not a necessary condition for the pro- 

 duction of this mutation is shown by the fact that it rises 

 suddenly in the midst of an intergenerating group; but 

 the fact that it remains a distinct form, returning to the 

 original form in only a small percentage of the offspring, 

 seems to be due to the fact that by its structure it is pre- 

 vented from crossing with the original stock. In other 

 words, structural isolation, which is one form of auto- 

 nomic isolation, produces racial demarcation and initial 

 segregation, opening the way for intensive segregation, 

 through the introduction of divergent forms of selection. 



That natural selection is not the cause producing this 

 mutation is shown by the fact that the old form and the 

 new form are equally adapted to the same environment. 

 I, however, maintain that the structural isolation pro- 

 duced by the mutation prepares the way for the introduc- 

 tion of permanently divergent forms of selection. 



