gh the Hindu Kush, Caucasus, and Asia
Minor ranges to southern Europe and the Alps. Then it passes on into
Spain and ends in the volcanoes of the Canary Islands. The American
cordillera swings eastward in Mexico and continues as the isolated
ranges of the West Indies until it ends in the volcanoes of Martinique.
Central America appears at first sight to be a continuation of the
great cordillera, but really it is something quite different--a mass
of volcanic material poured out in the gap where the main chain of
mountains breaks down for a space. In neither hemisphere, however, is
the main southward sweep of the mountains really lost. In the Old World
the cordillera revives in the mountains of Syria and southern Arabia and
then runs southward along the whole length of eastern Africa. In America
it likewise revives in the mighty Andes, which take their rise fifteen
hundred miles east of the broken end of the northern cordillera in
Mexico. In the Andes even more distinctly than in Africa the cordillera
forms a mighty wall running north and south. It expands into the plateau
of Peru and Bolivia, just as its African compeer expands into that of
Abyssinia, but this is a mere incident. The main bone, so to speak,
keeps on in each case till it disappears in the great southern ocean.
Even there, however, it is not wholly lost, for it revives in the cold,
lofty continent of Antarctica, where it 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 a