THE BACKGROUND 



13 



Figure 3. — Glaciation of the Wisconsin age compared with that of the present. Black 

 areas are existing glaciation, hachured outline marks limits of Wisconsin glaciation, 

 isotherms are present July means in degrees F. (From L. S. Dillon, "Wisconsin Climate 

 and Life Zones in North America," Science, vol. 123, fig. 1, 1956.) 



deposits do not well preserve the record of the surroundings in which 

 they lived, and we cannot yet list by their remains the land animals 

 which existed in Alaska during the last great glaciation about 10,000 

 years ago or say whether any then changed their range by migration 

 over the land bridge. 



If, as it appears, new species have not evolved during the Pleistocene, 

 and these animals have passed through the climatic variations of 

 recent ice ages without much evolutionary change in form, then the 

 mammals and birds resident in ice-free arctic America and Siberia 

 during the Wisconsin age could have been like those now resident 

 there. Some of the large mammals have recently become extinct and 

 some have moved in from the south since the ice sheet melted. By 

 that time, however, the land bridge probably no longer existed, and in 



