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THE AMERICAN NATURALIST [Vol. XLVIII 



trinsic modifications, (2) neutral variations in segregated 

 regions, ( 3 ) incipient generic modifications. Generic modi- 

 fications may be (1) intrinsic modifications, or (2) neutral 

 varations, given generic value by a longer period of 

 segregation. 



9. Since different types of a group of animals are 

 usually found associated with different environmental con- 

 ditions or different ecologic niches, and since it is common 

 for animals, if unhindered, not only to extend their range 

 continually into new territory, but also to occupy new 

 ecologic niches, and since these tendencies lead to specific 

 and generic differentiations, respectively, any given area 

 will have a differentiation of species proportionate to its 

 variety of environmental conditions, and of genera pro- 

 portionate to its variety of suitable ecologic niches. 



10. Since, as the area of distribution is extended, the 

 chance of new conditions of climate and environment being 

 represented remains approximately the same, the increase 

 in number of species is nearly proportional to the increase 

 in the area of distribution, but since the chance of new 

 ecologic niches being represented in most cases constantly 

 decreases, the increase in genera proceeds at an ever- 

 diminishing rate. This, going hand in hand with the 

 nearly constant increase in species or subspecies, results 

 in a constantly increasing index of modification. 



LITERATURE CITED 



Cook, O. F. 



1908. Evolution without Isolation. Am. Nat., 4t, 727-731. 

 Cope, E. D. 



1889. The Batrachia of North America. Smithsonian Inst. Nation. Mua 

 Bull., S4, 1-525, 86 pis., 119 figs, in text. 



1898. The Crocodilians, Lizards and Snakes of North America. Smith- 

 sonian Inst. Nation. Mus. Rep., 1898, 155-1270, 36 pis., 347 figs. 



1859.. The Origin of Species, 2d ed., 1869. New York, D. Appleton & Co., 

 . xi, 440, 1 pi. 

 Dewar, D., and Finn, F. 



1909. The Making of Species. London, John Lane Co., xix, 400, 15 pis. 



