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THE AMEBIC AN NATURALIST [Vol. XLYII1 



tions depends on the duration of geographic segregation, 

 and may therefore be of specific, generic, family, or ordi- 

 nal rank. 



To sum up, specific modifications may be of three kinds : 

 (1) extrinsic modifications, induced by changes of climate 

 and environmental conditions; (2) neutral modifications, 

 due to a different trend of evolution in segregated regions ; 

 (3) incipient generic modifications. On the other hand, 

 generic modification may be either intrinsic modifications, 

 concomitant with changes in mode of life or habits of the 

 animal, or neutral modifications as above, given generic 

 value by a longer period of segregation. 



Having dwelt for some length on these preliminary con- 

 siderations, let us now apply them to the case in hand and 

 see how they affect differentiation into species and genera 

 through extension of range. 



It is a well-known biological fact that different types of 

 a group of animals, at least of higher animals, are found 

 associated with different environments; nearly related 

 species do not, as a rule, live comfortably together in the 

 same environment, and nearly related genera do not 

 occupy the same ecologic niche in a given zoogeographical 

 area. This does not seem to hold true for animals of 

 lower organization, as conclusively shown by Kofoid 

 (1907). It is common for a group of animals, unless hin- 

 dered by an impassable barrier or unfavorable environ- 

 mental conditions, not only to continually extend its range 

 into new territory, but also to attempt to live in as many 

 different niches in nature as possible within a given area. 

 Such attempts to invade new ecologic niches are frequently 

 concomitant with heritable modifications better fitting 

 them to occupy their new situation, though it is difficult to 

 say whether these modifications are causes or results of 

 the change in mode of life. However this may be looked 

 upon, the tendency to occupy new niches in nature is fre- 

 quently accompanied by intrinsic modifications, and there- 

 fore by generic differentiation. 



From this we may safely assume that in a given area 



