218 LIFE OF THE PLEISTOCENE 



forced every form within the glaciated area to move on, while the fringing 

 zones of depressed temperature encircling each ice-sheet forced plant and 

 animal life, even beyond the ice border, to seek new fields and new relations, 

 both physical and organic. An incidental result of this wholesale migration 

 was an unwonted commingling of plants and animals, for every aggressive 

 form pushed forward in the van of the advancing zone, and hence came into 

 new organic environment, while every laggard form fell behind and was over- 

 taken by the less reluctant migrants. " 



Professor Osborn 2a does not believe that the biota was greatly affected by 

 the ice sheets, at least until toward the latter half of the Glacial Period. He 

 says: "Until the close of Third Interglacial time no traces of northern, much less 

 of Arctic forests and animals, are discovered anywhere, except along the borders 

 of the ice-fields. It would appear as if the animal and plant life of Europe 

 were, in the main, but slightly affected by the first three glaciations. We 

 cannot entertain for a moment the belief that in glacial times all the warm 

 flora and fauna migrated southward and then returned, because there is not a 

 shred of evidence for this theory. It is far more in accord with the known 

 facts to believe that all the southern and eastern forms of life had become very 

 hardy, for we know how readily animals now living in the warm earth belts 

 are acclimatized to northern conditions." 



The facts, in America at least, seem to indicate a mingling of Arctic, sub- 

 aractic, and temperate types of animals south of the border of the ice sheets, 

 and a migration northward during each interglacial interval. In another 

 place (p. 241) Osborn says, "As a result of favorable interglacia' conditions 

 arboreal vegetation flourished to the far north along the Arctic Ocean, and 

 the present tundra regions of Siberia and British America then supported 

 forests which have long since been extirpated, the northern limit of similar 

 living trees now lying far to the south. " There is apparently no reason why 

 this condition may not have prevailed during the first two interglacial inter- 

 vals — th ■ Af tonian and Yarmouth. 



At the maximum extension of the ice sheet, the biota of the upper Mississippi 

 Valley was concentrated along the southern border of the ice, in Kansas, Iowa, 

 Illinois, Kentucky, Ohio, and Pennsylvania. The aquatic biota was massed 

 in the lower Mississippi and Ohio rivers and in tributary streams. There were 

 five areas from which the biota could repeople the wasted territory left bare 

 by the retreating ice sheet (Plate LVI). (1), that part of the United States 

 lying south of Illinois and Ohio, west of the Allegheny Mountains, and of the 

 Missouri River Valley, and east of the Rocky Mountains, including the lower 

 drainage area of the Mississippi Valley and the adjacent prairies and plains; 

 (2), an area south of British Columbia and Assiniboia, including Montana, 

 Idaho, and Washington, and embracin , r the upper drainage areas of the Missouri 



ja Men of the Old Stone Age, p. 108, 1916. 



