DISPERSAL OF NORTH AMERICAN BIOTA. 63 



and tributary valleys. For the aquatic types the Tennessee and 

 Mississippi Rivers were the leading highways. From the upper 

 end of the Coastal Plain a limited number of land forms pushed 

 up the Hudson and even worked west via the Mohawk valley to 

 the Great Lake region. From the Mississippi numerous tribu- 

 tary valleys were followed, the Ohio, Wabash, Illinois, and Mis- 

 souri, and thus this biota radiated rapidly. It even invaded the 

 Great Plains along eastward flowing streams, especially along the 

 Missouri River. 



The second wave types reach their most southern extension 

 along- mountains while this third wave reaches its most northern 



o 



extension along valleys, not only to the north but also upon the 

 arid plains of the northwest. 



5. The Soutliwestern Center of Dispersal. The area occupied 

 by the southwestern biota was largely far beyond the ice margin 

 and, like that of the southeastern, was relatively stable in its geo- 

 graphic position. At present this type is represented by the life 

 of the arid southwest, including the Great Plains, the Great 

 Basin, the central valley of California and the Mexican Plateau. 

 It is a vast region of arid plains, desert plateaus, and mountains, 

 subject to great climatic extremes. In spite of the severity of 

 the conditions of life the biota is quite varied, and many kinds 

 are abundant. Attention has already been called (Adams, '02, b, 

 p. 12 1) to the importance of this center and too much emphasis 

 cannot be placed upon its importance, not only as a center of 

 distribution, but also as the region of origin of the arid North 

 American biota. It seems equally evident that before a reliable 

 estimate can be made of this biota it must be carefully compared 

 with that of the and regions of South America and of Asia. 

 The life of the first and second waves in the Postglacial migra- 

 tions contained many forms not peculiarly American, but the 

 southeastern and southwestern elements of the third wave show 

 much more individuality. The southeastern center has certain 

 endemic elements in its flora and fauna, yet several other types 

 have their affinities duplicated in eastern Asia, and thus its indi- 

 viduality is somewhat lessened. On the other hand the south- 

 western center, although it shows some Asiatic duplication, does 

 not appear to be so marked. So far as known to me no one has 



