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THE AMERICAN NATUEALIST [Vol. LIII 



or any combination of these, the animals also tend to 

 migrate with it and they are thus led about all over the 

 surface of the earth. In this we see the importance of 

 cycles of climatic changes, cycles of crustal movements 

 of the earth, changes in circulation of the sea, and the suc- 

 cession of plant and animal associations. It is to the 

 changes of this character that the student of paleontolog- 



ical evidences and causes of evolution gives much thought, 

 and it is to the present evidences of these changes to 

 which the field ecologist gives much attention. 



In my study of migrations of the fresh-water snail lo, 

 in the drainage of the upper Tennessee Elver system 

 (Adams, '15fc), it was found that there were great cycles 

 of change in the history of the streams, and that there 

 were probably corresponding migrations of the snails. 

 This is shown if we compare the map of ancient hypo- 

 thetical drainage, Fig. 1, with that of the present, Fig. 2, 

 and the supposed migration of the snails. Fig. 3, and 

 compare these with the map of their present distribution, 



