Occasional Papers of the Museum of Zoology 9 



these, upon the retreat of the ice and the estabhshment of the 

 present systems of drainage, the waters of the glaciated area 

 were re-peopled. 



The result of all these changes in the earth's surface is 

 shown in our present fauna by the fact that we have: ist. 

 East of the Appalachian Mountains a very distinct fauna which 

 extends from Florida to New England, and thence northwest- 

 €-rly toward Hudson Bay. 2nd. Between the Appalachians 

 and the arid regions of the West, occupying the Mississippi 

 Valley and all its tributary streams and drainage systems, the 

 characteristic American fauna, known as the Mississippian. 

 3rd. In the Alabama River and its tributaries, a very distinct 

 .subfauna, evidently derived from the ancient fauna of the 

 Tennessee, but still sufficiently differentiated to be recognized 

 as a distinct faunal element. 4th. West of the Rocky Moun- 

 tains the Unione fauna is very meagre and is evidently de- 

 rived from migration from Siberia. It has none of the dis- 

 tinctly American types characteristic of the eastern areas. 



In these great faunal areas our modern species have been 

 -evolved. Separated from each other, the diverse local in- 

 fluences of climate, food supply and chemical elements con- 

 tained in the water have all played their part in bringing about 

 the evolution into the numerous species that now exist. This 

 evolution has in some instances gone so far that new genera 

 have been formed, but just when in geologic time these had 

 their beginning, is not known, because the evidence afforded 

 by the fossils of the Tertiary Period is as yet too imperfect 

 to give us the connecting links. We can only imagine when 

 and where they originated. Why they were evolved, like so 

 many similar questions in other branches of the animal king- 

 dom, is almost wholl}- a matter of speculation. 



