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University of California Publications in Zoology [Vol. 12 



tion according to which there may be segregation of sections of the 

 same species in the same place, and so divergent evolution, through 

 preferential mating? 



It will be apparent immediately that the facts here adduced do not 

 harmonize with the requirements of any of the hypotheses above 

 mentioned; for on any one of them we ought, at least occasionally, 

 to find species arranged in nature as illustrated in the accompanying 

 diagram (fig. 0). According to this diagram each group is repre- 

 sented to have undergone divergence in the same place, until its mem- 

 bers occupy different eeologic niches, the horizontal lines representing 

 physiological, the heavy vertical lines geographical, barriers. What we 

 do find in nature is that species are arranged quite otherwise (fig. P). 



Geographic Areas 



Fig. P. Diagram illustrative of species arrangement as we actually do 

 find it in nature. 



Each group, speaking now in general terms, for there are some excep- 

 tions, occupies the same eeologic niche in different places, rather than 

 different eeologic niches in the same place. The two propositions are 

 theoretically well balanced, and on a priori grounds one would per- 

 haps be unable to discover any argument in favor of the one arrange- 



