EVOLUTION IN NATURE 161 



mode of life. Sharp claws which had served it for 

 climbing might be turned to account for aggression 

 or for burrowing, and it might well take on special 

 food habits without passing the limits of the wider 

 range of diet of the stock from which it sprang. 



Are there such cases in nature.^ Ruthven regards 

 the scutellation of the garter snakes as an indica- 

 tion of probable association with varying environ- 

 mental stimuli as these reptiles spread from their 

 center of origin in Mexico. ^^ Certainly his descrip- 

 tion of the group indicates a close parallel between 

 transition of structure and transition of environ- 

 ment, even though the direct relationship of the 

 two is not brought out. Borodin speaks of the 

 "slow modification in the structure of the organ- 

 isms under the influence of changed life conditions " 

 in discussing the herrings (Clupea) of the Black 

 and Caspian seas and Tscharchal Lake.^^ Accord- 

 ing to his conclusions the Tscharchal Lake form 

 cannot have been more than 20,000 years in de- 

 veloping. Again he fails to establish the association 

 of environmental conditions directly with the char- 

 acters of the fishes; he applies Osborn's idea of 

 speciation to the problem, with naturally vague 

 results. Moore shows that the fauna of Lake 

 Tanganyika has distinct aflSnities with the marine 

 fauna and that the lake was once connected with 



16 Bull. 61, U. S. N. M., 1908. 



" Am. Nat., Vol. LXI, pp. 266-271, 1927. 



