ice, but by the relative absence of snow in winter and the
consequent rapidity with which the ground becomes warm in the
spring. Hence the Canadian plains from the United States boundary
northward to latitude 57 degrees contain a prosperous
agricultural population of over a million people, while the far
larger forested areas in the same latitude support only a few
thousand.

The question is often asked why, in a state of nature, trees are
so scarce on the prairies--in Iowa, for instance--although they
thrive when planted. In answer we are often told that up to the
middle of the nineteenth century such vast herds of buffaloes
roamed the prairies that seedling trees could never get a chance
to grow. It is also said that prairie fires sweeping across the
plains destroyed the little trees whenever they sprouted.
Doubtless the buffaloes and the fires helped to prevent forest
growth, but another factor appears to be still more important.
All the States between the Mississippi River and the Rocky
Mountains receive much more rain in summer than in winter. But as
the soil is comparatively dry in the spring when the trees begin
their growth, they are handicapped. They could grow if nothing
else interfered with them, just as peas will grow in a garden if
the weeds are kept out. If peas, however, are left uncared for,
the weeds gain the upper hand and there are no peas the second
year. If the weeds are left to contend with grass, the grass in
the end prevails. In the eastern forest region, if the grass be
left to itself, small trees soon spring up in its midst. In half
a century a field of grass goes back to forest because trees are
especially favored by the climate. In the same way in the
prairies, grass is especially favored, for it is not weakened by