graphic map of the United States.

[Illustration: MEAN ANNUAL RAINFALL]

CHAPTER IV

CLIMATIC CONTROL OF COMMERCE

In its effect upon life and the various industries of peoples, climate
is a factor even more important than topography. Of the 53,000,000
square miles of the land surface of the earth, scarcely more than
one-half is capable of producing any great amount of food-stuffs, and
only a very small area can support a population of more than one hundred
people to each square mile.

=Climate and Habitability.=--In the main, regions that are inhabited by
human beings produce either food-stuffs or something of value that may
be exchanged for food-stuffs; and inasmuch as food and shelter are the
chief objects of human activity, regions that will not furnish them are
not habitable.

The growth and production of food-stuffs is governed even more by
conditions of climate than by those of topography. Thus the great
Russian plain is too cold to produce any great amount of food-stuffs,
and it is, therefore, sparsely peopled. The northern part of Africa and
the closed basins of North America and Asia lack the rainfall necessary
to insure productivity, and these regions are also unhabitable. The
basin of the Amazon has a rainfall too great for cereals and grasses,
and the larger part of it is unfit for habitation.

All the food-stuffs are exceedingly sensitive to climate. Rice will not
grow where swampy conditions do not prevail at least during part of the
year. Turf-grass will not live where there are repeated droughts of more
than three months' duration, and corn will not ripen in regions having
cool nights. Wheat does not produce a kernel fit for flour anywhere
except in the temperate zone; and the banana will not grow outside the
torrid zone.

The two chief factors of climate are temperature and moisture. No forms
of life can withstand a temperature constantly below the freezing-point
of water, and but few, if any, can endure a constant heat of one hundred
and twenty-five degrees, although most species can exist at temperatures
beyond these limits for a short time.

=Zones of Climate.=--The belt of earth upon which the sun's rays are
nearly or quite vertical is comparatively narrow. But the inclination of
the earth's axis and the fact that it is parallel to itself at all times
of the year create zones of climate. These differ materially in the
character of the life, forms, and the activities of the people