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 for preservation. The fossils found in
the rocks of the earth's crust support this view. They indicate
that most of the great families of higher animals originated in
the central part of the great land mass of Europe and Asia. A
second but much smaller area of evolution was situated in the
similar part of North America. From these two centers new forms
of life spread outward to other continents. Their movements were
helped by the fact that the tetrahedral form of the earth causes
almost all the continents to be united by bridges of land.