56 CHAS. C. ADAMS. 



American elements, a much greater degree of definiteness may 

 be secured. From our knowledge of the distribution and con- 

 ditions of life of the present biota, it seems that while the 

 northern part of the continent lay deeply buried under the mantle 

 of the Wisconsin ice sheet there existed, in all probability, south 

 of the ice margin three distinct belts of life (Adams, '02, />). At 

 or near the ice margin, and perhaps only forming a narrow trans- 

 continental belt, was the tundral or barren ground biota. Below 

 this first came stunted trees and shrubs, and farther back conif- 

 erous forests forming a transcontinental belt, but composed of 

 two distinct types, an eastern and a western one. Below this, in 

 turn, came a third belt of still less homogeneity ; in the east it 

 was composed of deciduous forests and their associated fauna, 

 while in the west it was made up of plains and desert types of 

 life. 



With these conditions in mind let us now turn to a more de- 

 tailed consideration of the various elements which go to make up 

 these belts of life, and attempt to follow some of the dynamic 

 phases which this biota has shown since Glacial times. The 

 fundamental idea in following these dynamic changes is that we 

 have belts of physical conditions migrating to the north. Thus 

 there has been given a definite trend to the environment. This 

 fact cannot receive too much emphasis. Just as when studying 

 the littoral fauna of a pond or lake it becomes necessary to bear 

 in mind the dynamic tendency of this littoral zone that is for 

 it to move inward toward the deeper water so in a similar man- 

 ner, to understand the dynamic changes in life areas or zones 

 we must bear in mind the dynamic tendencies in such areas 

 (Adams, '02, a, p. 126). Nor is this limited to climatic and topo- 

 graphic influences alone ; it includes organic factors as well. It 

 is also necessary to keep such dynamic tendencies in mind when 

 attempting to follow the relations and movements of these three 

 belts in their Postglacial migrations. 



The three belts or waves just mentioned were composed of 

 such distinct elements and have had such varied histories that 

 they demand separate treatment. The wanderings of these dif- 

 ferent types, since Glacial times, is likely to make the application 

 of geographic names confusing (Fig. i). The members of the 



