No. 624] MIGRATION A FACTOR IN EVOLUTION 



Fig. 4. The shells of these snails are shown in Fig. 5. 

 The presence of these snails in the headwaters of streams 

 appears to be due to the ordinary creeping movements of 

 the snails taken in connection with the up-stream migra- 

 tion or growth of the stream habitat, because, on the 

 other hand, the current tends only to favor a down-stream 

 dispersal. Such animals, therefore, appear to be led 

 about hy the migration of their habitat. This sort of 

 migration is comparable to those land migrations w^hich 



have clearly taken place during climatic migrations, as 

 during the ice age, and during similar changes in humid- 

 ity, and with base-leveling changes (Woodw^orth, '94; 

 Adams, '01). 



The competition among different kinds of animals has 

 long been recognized as an important factor in animal 

 migrations. Overcrowding produces a condition of 

 stress, and as a result of this stimulus, animals tend to 

 migrate and become diffused from the region of pressure 

 in all possible directions. Thus new conditions are en- 

 countered wiiich necessitate changes on the part of the 

 animal, and thus this process continues indefinitely. 



