coalesces once more with the other great tetrahedral ridges of
Africa and Australia.

It is easy to see that these great cordilleras have turned most
of the earth's chief rivers toward the Atlantic and the Arctic
Oceans. That is why these two oceans with an area of only
forty-three million square miles receive the drainage from twenty
million square miles of land, while the far larger Indian and
Pacific Oceans with an area of ninety-one million square miles
receive the rivers of only ten million square miles. The world's
streams of civilization, like the rivers of water, have flowed
from the great cordilleras toward the Atlantic. Half of the
world's people, to be sure, are lodged in the relatively small
areas known as China and India on the Pacific side of the Old
World cordillera. Nevertheless the active streams of civilization
have flowed mainly on the other side--the side where man
apparently originated. From the earliest times the mountains have
served to determine man's chief migrations. Their rugged
fastnesses hinder human movements and thereby give rise to a
strong tendency to move parallel to their bases. During the days
of primitive man the trend of the mountains apparently directed
his migrations northeastward to Bering Strait and then
southeastward and southward from one end of America to the other.
In the same way the migrations to Europe and Africa which
ultimately reached America moved mainly parallel to the
mountains.

From end to end of America the great mountains form a sharp
dividing line. The aboriginal tribes on the Pacific slope are
markedly different from those farther east across the mountains.