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.

Evolution is especially stimulated by two conditions. The first is that
there shall be marked changes in the environment so that the process
of natural selection has full opportunity to do its work. The second is
that numerous new forms or mutants, as the biologists call them, shall
be produced. Both of these conditions are most fully met in large
continents in the temperate zone, for in such places climatic variations
are most extreme. Such variations may take the form of extreme changes
either from day to night, from season to season, or from one century to
another. In any case, as Darwin long ago pointed out, they cause some
forms of life to perish while others survive. Thus climatic variations
are among the most powerful factors in causing natural selection and
hence in stimulating evolution. Moreover it has lately been shown
that variations in temperature are one of the chief causes of organic
variation. Morgan and Plough, * for example, have discovered that when a
certain fly, called the drosophila, is subjected to extremes of heat or
cold, the offspring show an unusually strong tendency to differ from
the parents. Hence the climatic variability of the interior of large
continents in temperate latitudes provides new forms of life and then
selects some of them