of the thin-skinned, hairless human race with its adaptation to a
highly variable climate with temperatures ranging from freezing
to eighty degrees. Man could not stay there forever. He was bound
to spread to new regions, partly because of his innate migratory
tendency and partly because of Nature's stern urgency. Geologists
are rapidly becoming convinced that the mammals spread from their
central Asian point of origin largely because of great variations
in climate.* Such variations have taken place on an enormous
scale during geological times. They seem, indeed, to be one of
the most important factors in evolution. Since early man lived
through the successive epochs of the glacial period, he must have
been subject to the urgency of vast climatic changes. During the
half million years more or less of his existence, cold, stormy,
glacial epochs lasting tens of thousands of years have again and
again been succeeded by warm, dry, interglacial epochs of equal
duration.

* W. D. Matthew. "Climate and Evolution," N. Y. Acad. Sci., 1915.


During the glacial epochs the interior of Asia was well watered
and full of game which supplied the primitive human hunters. With
the advent of each interglacial epoch the rains diminished, grass
and trees disappeared, and the desert spread over enormous
tracts. Both men and animals must have been driven to sore
straits for lack of food. Migration to better regions was the
only recourse. Thus for hundreds of thousands of years there
appears to have been a constantly recurring outward push from
the center of the world's greatest land mass. That push, with the
consequent overcrowding of other regions, seems to have been one