256 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1952 



twofold (Flint, 1947) . The first factor was a conspicuous world-wide 

 elevation of the lands in general and of many mountain ranges in 

 particular, during the epoch immediately preceeding the Pleistocene 

 and continuing into the Pleistocene epoch itself. This elevation 

 in itself reduced surface temperatures in several ways, though alone 

 it cannot explain repeated temperature fluctuation. The second fac- 

 tor is an assumed fluctuation in the rate at which radiant energy is 

 emitted by the sun. Small present-day fluctuations are currently 

 observed, but the larger fluctuations necessary to form glaciers on 

 the highlands must be assumed. These two factors constitute a rea- 

 sonable and, it seems, probable explanation of the glacial and inter- 

 glacial ages, though the second factor is not at present capable of proof. 



EFFECT OF GLACIATION ON LIFE 



The present-day flora of the North American Arctic includes few 

 endemic forms. There can be little doubt that glacial-age conditions 

 in the glaciated regions almost wholly extinguished the plant cover, 

 which was renewed after each glacial age by immigration from the 

 nonglaciated territory. Probably the principal arctic refuge within 

 which plants with sufficient hardihood survived the glacial ages lay 

 in nonglaciated areas in central Alaska and the adjacent parts of 

 the Yukon. In addition, repopulation of the arctic flora must have 

 taken place to a considerable extent from the belt of country lying 

 south of the southern limits of glaciation. 



If, during the glacial times, there was a conspicuous belt of tundra 

 at the southern margin of the ice sheet, there is little evidence of its 

 former presence. Most of the comparatively few exposures of plant- 

 bearing deposits immediately overlying the drift sheets yield floras, 

 dominated by spruce and fir, such as characterize the subarctic forest 

 of the present day. It seems likely, therefore, that the subarctic 

 forest generally reached close to the margin of the ice sheet and that 

 the intervening belt of tundra was narrow. Along the arctic coast be- 

 tween Point Barrow and the Mackenzie Kiver, as already noted, there 

 are suggestions of tree growth, presumably during some interglacial 

 time (unless the wood in question is merely driftwood). 



Evidence of the effect of the glacial ages on animal life in the far 

 north is very slight, chiefly because there has been little systematic 

 search for fossils. We can be quite sure that in the glacier-covered 

 areas animal life was completely extinguished. The change was grad- 

 ual, and took place through slow migration, generally toward the 

 south, as each glacial age developed. During the interglacial ages 

 the glaciated tracts were repopulated with at least some of their former 

 inhabitants. In the arctic regions, as elsewhere, the Pleistocene record 

 is one of repeated wholesale shifts of f aunal assemblages rather than 



