In writing this book the author has aimed first to present in readable form the main facts about the geographical environment of American history. Many important facts have been omitted or have been touched upon only lightly because they are generally familiar. On the other hand, special stress has been laid on certain broad phases of geography which are comparatively unfamiliar.
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.