12 U. S. NATIONAL MUSEUM BULLETIN 217 



hypothesis that the moisture for the latest American ice cap came 

 from the Arctic seas, which they postulate as unfrozen while the 

 maximum glaciation developed. Geological and geophysical methods 

 are rapidly developing evidence of climatic history, which is probably 

 closer to elucidation than is indicated by these contrasting views on 

 the climatic mechanisms of glaciation. 



For the present we must be cautious in postulating what the climate 

 was like on the arctic coast of Alaska adjacent to a sea which might 

 have been frozen or unfrozen but in the interior the climate could not 

 have been warm and was probably cold in the proximity of the great 

 ice cap over the Alaska Range and the extensive glaciation in the 

 Brooks Eange. It appears that, in most of the ice-free northern 

 Am erican area which was connected with Siberia, the climate in the 

 area that served as a "refuge for survival" was at least cold and could 

 suit only animals like those now hardy in the north. 



Climate and the Animal Populations 



The masses of water frozen in polar and arctic ice near the end, and 

 apparent maximum, of Wisconsin glaciation so reduced the oceans 

 as to connect Alaska and Siberia over Bering Strait. Fossils of ex- 

 tinct animals as well as the systematic relations of many animals now 

 found, indicate that some taxonomic groups in America derive from 

 Asiatic predecessors and some in Asia from American predecessors. 

 It is popular to speculate that the intercontinental exchange implied 

 m the relations of the present and fossil animals of America and Asia 

 was effected by traffic over the land bridge. 



On the basis of systematic relations and the geographical and chron- 

 ological sequences in which they are f oimd, it is reasonable to propose 

 that between Asia and America migrations have occurred that have 

 altered the previous geographical distribution of animals. Walters 

 (1955), for example, has shown that the land bridge of the late Wis- 

 consin age could have lead to the present distribution of fresh water 

 and coastal fishes in western arctic America and eastern Siberia. In 

 the aquatic environment of fishes in the north, the temperature is al- 

 ways above freezing, but not much warmer, and suitable climatic con- 

 ditions for fishes can be reasonably inferred for the waters about the 

 recent land bridge. 



Most of the species now living can be traced through fossil records 

 to Pleistocene times but their evolutionary modification has been too 

 slow to produce variation in form which would indicate an exchange 

 between Alaska and Siberia in Recent time. A number of land 

 mammals, many conspicuous by their size, have become extinct since 

 the Pleistocene, some of them only a few thousand years ago (Flint, 

 1947) . Unfortunately the situation of their remains in Alaskan loess 



