MAN FROM THE FARTHEST PAST 
an increase of cold alone is not enough; there has to be 
at the same time a fairly heavy annual fall of snow. 
Northern Siberia, at least in part, lay under a vast station- 
ary field of ice; and some of the great Asiatic mountain 
masses formed important centers of glaciation. But, speak- 
ing generally, in Asia the Glacial Period appears to have 
been far more local in character than it was either in 
Europe or in North America. The same, too, may be 
said of South Africa, of Australia, and of New Zealand. 
Geologists have found further that the Glacial Period 
consisted not of a single intensely cold epoch, but of 
several, alternating with epochs of mild or even decidedly 
warm climate. In most of western Europe the great ice 
sheets underwent no less than three or four successive 
epochs of expansion and retreat, known as glacial and 
interglacial stages respectively, even the shortest of which 
lasted for many thousands of years. There 1s, moreover, 
reason to believe that the interglacial epochs were much 
longer than the glacial ones. 
As a glacial stage approached, the winters must have 
grown imperceptibly more and more severe, through not 
merely hundreds but many thousands of years. The 
glaciers far in the north crept further south, those in high 
mountain masses further down the valleys, until they over- 
spread the northern portions of Europe and North America 
with enormous ice sheets, in some places thousands of feet 
thick. Conditions then must have resembled those of 
today in Greenland and in the Antarctic Continent. 
Further, the glaciers transformed the covering of the 
neighboring ice-free regions from forest and meadow- 
land and swamp into tundras—treeless plains of black 
mucky soil, with a permanently frozen subsoil overgrown 
with moss, lichens, and dwarf shrubs, as in northern 
Alaska and Siberia today. The animals living on them 
were quite typical, and included forms like the hairy 
mammoth, the woolly rhinoceros, and the reindeer. The 
terrible storms of winter often killed multitudes of these 
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