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 who dwell in them.
In the torrid zone the temperature varies but little. During the season of rains it rarely falls to 70° F., and in the dry season it is seldom higher than 95° F. As a result, all sorts of plants that are sensitive to low temperatures thrive in the torrid zone. It is not a climate suitable for heat-producing food-plants, and they are not required.