62 CHAS. C. ADAMS. 



It also appears that some of the mountain biota of the Cana- 

 dian Rockies were driven north into unglaciated Alaska as the 

 ice spread from the Cordilleran center of ice accumulation. 



Third Wave. 



4. Southeastern Center of Dispersal. The region occupied by 

 the southeastern type of biota was largely south of the territory 

 invaded by the ice, and its biota has therefore been relatively 

 stable in its geographic position when compared with the exten- 

 sive migrations of the first and second waves. As today, dur- 

 ing Glacial times, this biota was bounded on the west by the arid 

 plains. This is a region of low plains and plateaus, the higher 

 mountains within this area still retaining the second wave types 

 as Glacial relicts. It is probable that the first wave type never 

 reached in abundance so far south. The climate of this south- 

 eastern center is equable and there is abundant rainfall. The 

 dense deciduous forests furnish favorable conditions for animal 

 life. This region has not only been important as a region of 

 preservation, but also as a center of origin. Here there is the 

 best development of the deciduous forest and the most charac- 

 teristic features of the land and fresh water shell life of North 

 America. This has also been the center of distribution of sev- 

 eral vertebrate types and also for certain plants. But as this 

 center has been discussed elsewhere (Adams, '02, a) only brief 

 mention will be made here of its characteristic features. 



Migration and Dispersal Routes. With the retreat of the ice 

 this biota formed the eastern element of the third wave. It 

 moved north and northwest behind the coniferous forest zone. 

 But as this biota w r as relatively stable its center of dispersal can 

 be definitely defined as occupying southeastern United States, 

 east of the Great Plains. This stability therefore makes the dis- 

 persal routes of more importance than the migrations of the 

 biota as a whole, as the spread of this biota has apparently been 

 influenced more by the normal increase of a populated area than 

 by a great change in the physical conditions which was such a 

 dominant factor farther north. 



The primary routes fort he land forms were the Coastal Plain 

 and its valleys, the Appalachian plateaus, and the Mississippi 



