diminished in size until now its diameter is from two hundred to
four hundred miles less than formerly. During the process of
contraction the crust has collapsed in four main areas, roughly
triangular in shape. Between these stand the six ridges which we
have called the bones. Each of the four depressed areas forms a
side of our tetrahedron and is occupied by an ocean. The ridges
and the areas immediately flanking the oceans form the
continents. The side which we may think of as the base contains
the Arctic Ocean. The ridges surrounding it are broad and flat.
Large parts of them stand above sea-level and form the northern
portions of North America, Europe, and Asia. A second side is the
Pacific Ocean with the great ridge of the two Americas on one
hand and Asia and Australia on the other. Next comes the side
containing the Indian Ocean in the hollow and the ridges of
Africa and Australia on either hand. The last of the four sides
contains the Atlantic Ocean and is bounded by Africa and Europe
on one hand and North and South America on the other. Finally the
tip of the pyramid projects above the surrounding waters, and
forms the continent of Antarctica.

It may seem a mere accident that this tip lies near the South
Pole, while the center of the opposite face lies near the North
Pole. Yet this has been of almost infinite importance in the
evolution not only of plants and animals but of men. The reason
is that this arrangement gives rise to a vast and almost
continuous land mass in comparatively high latitudes. Only in
such places does evolution appear to make rapid progress.*

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