re'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 of the chief forces impelling people to migrate and cover the
earth.

Among the primitive men who were pushed outward from the Asian deserts
during a period of aridity, one group migrated northeastward toward
the Kamchatkan corner of Asia. Whether they reached Bering Sea and the
Kamchatkan shore before the next epoch of glaciation we do not know.
Doubtless they moved slowly, perhaps averaging only a few score or
a hundred miles per generation, for that is generally the way with
migrations of primitive people advancing into unoccupied territory. Yet
sometimes they may have moved with comparative rapidity. I have seen a
tribe of herdsmen in central Asia abandon its ancestral home and start
on a zigzag march of a thousand miles because of a great drought. The
grass was so scanty that there was not enough to support the animals.
The tribe left a trail of blood, for wherever it moved it infringed upon
the rights of others and so with conflict was driven onward. In some
such way the primitive wanderers were kept in movement until at
last they reached the blea