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.

The constant heat and excessive moisture of the atmosphere in the torrid zone is apt to produce a feeling of lassitude among the dwellers in such regions, moreover, and great bodily activity is out of question. These conditions seriously affect the lives of the people, and, with few exceptions, tropical peoples are rarely noted for energy or enterprise. Great commercial enterprises are the exception rather than the rule, and they are usually carried on by foreigners who must live a part of the time in cooler localities.

Polar regions are deficient both in the heat and light necessary for food-stuffs. Neither the grasses nor the grains fructify. As a result, but few herbivora can live there, and these are practically restricted to the musk-ox and the reindeer, which subsist on mosses and lichens. The native people are stunted in growth; their food consists mainly of raw blubber, and they are scarcely above savagery.

The temperate zones are the regions of the great industries and activities of human life. The larger part of the land surface of the earth is situated in these zones; moreover, the people who dominate the world also live in them, and their supremacy is due largely to conditions of climate. The alternation of summer and winter causes a struggle for existence that develops the intellectual faculties and results in industrial supremacy.

Effects of Altitude.—There is a decrease of temperature of 1° F. for about every three hundred feet of ascent. But few people live at an altitude of more than six thousand feet above sea-level, and in many cases they depend on other localities for the greater part of their food-stuffs, because very few of such regions produce food-stuffs abundantly.

The chief exceptions to this rule are found in tropical regions. The highlands of Mexico, the plateau-regions of Bolivia and Ecuador, and the highlands of southern Asia are habitable, but they are not densely peopled. Because of their altitude they are relieved of the enervating effects of tropical climate at the sea-level.

Altitude likewise affects the amount of rainfall. Most plateaus are arid. As a rule, they are arid because of their altitude; and because of their aridity they are deficient in their power to produce food-stuffs. They are therefore sparsely peopled.

Effects of Rainfall.—Regions having considerably more than one hundred inches of rain annually are very apt to be forest-covered, and therefore to be deficient in food-producing plants. Such localities have usually a sparse population, in spite of the profusion of vegetation. In some parts of India, lands that have been left idle for a few seasons produce such a dense jungle of wild vegetation that to reclaim them for cultivation is wellnigh impossible.

A deficiency of rainfall is even a greater factor in restricting the density of population than too much rain. With less than fifteen or twenty inches a year few regions produce good crops of grains and grasses, and as a result they are sparsely peopled. Some of the exceptions, however, are important. If the rainfall is not quite enough to produce a normal overflow to the sea, the soil may be very rich, because the nutrition is not leached out and carried away.

Many small areas of this character produce enormous crops when artificially watered, and many of them, such as Persia, parts of Asia Minor, northern Utah, and large areas of Australia and Chile have become regions of considerable commercial importance. The products of such regions are apt to be unique in character and of unusual value. Thus, the wool of Persia and Australia and the fruit of the Iberian peninsula are important articles of commerce.

In Egypt one may see the results of irrigated lands. The area of geographical Egypt is somewhat less than half a million square miles; the habitable part of the country is confined to a narrow strip, which, one or two places excepted, varies from three to six miles in width. In other words, almost the whole population of the country is massed in the flood-plain and delta of the Nile; the remaining part is a desert producing practically nothing.

The water that makes these lands productive falls, not in Egypt, but in the highlands of Abyssinia, 2,000 miles away. The September overflow of the flood-plain is the chief factor in the irrigation of these lands, but the area has been greatly increased by the construction of barrages and dams at Assiut and Assuan.

In the western highland region of the United States considerable areas already have been made productive by irrigation, and it is estimated that about two million acres of barren land can be reclaimed by impounding the waters of the various streams now running to waste.

The distribution of rain with respect to the season in which it falls is quite as important as its distribution with respect to quantity. In tropical regions the ocean winds, and therefore the rainfall, come from the east. The eastern slopes of such regions, therefore, have a season in which rains may be expected daily, and another in which no rain falls for several months. In the temperate zones seasonal rains for a similar reason are on the western coasts.

Thus on the Pacific coast of the United States the rainfall varies from about one hundred inches in southern Alaska to about twelve in San Diego, Cal. Practically all the rain falls between October and the following May; very little or none falls in the interval between May and October. As a result, ordinary turf-grass, which will not withstand long droughts, grows in only a few localities of the Pacific slope. It is replaced by hardier grasses whose roots, instead of forming turf, grow very deep in the soil.

Common clover will not grow in this region unless irrigated; it is replaced by burr-clover, a variety of the plant that will not thrive in moist regions. Now the quality of the merino wool clip of California depends in no slight degree upon the burr-clover and other food-products that thrive in regions of seasonal rains; that is, a great commercial industry exists because of this feature of rainfall, and it could not long survive in spite of it.

The seasonal rainfall also affects other agricultural industries. The sacked wheat-crop may be left in the field without cover or protection until the time is convenient for shipping it. The absence of summer rains makes possible in California what would be out of question in the Mississippi Valley, where a rainstorm may be expected every few days.

The quality of certain fruits depends largely on the season during which the rainfall occurs. Apples, pears, and grapes grown in regions having dry summers have usually a very superior flavor. The raisin-making industry of California also depends on the same condition, because, in order to insure a good quality of the product, the bunches of grapes, after picking, must be dried on the ground. To a certain extent this is also true of other fruits, such as dates, figs, and prunes, which frequently are sun-dried.

The presence of large bodies of water, which both absorb and give out their heat very slowly, tempers the climate of the nearby land and to that extent modifies the commerce of such districts. The grape-growing industry of central New York is a great one and its product is famous. Its existence depends almost wholly upon the lake-tempered climate. Elsewhere in the State the industry is on a precarious basis, and the product is inferior.

Effects of Inclination of the Earth's Axis.—The inclination and self-parallelism of the earth's axis is undoubtedly a very important factor in climate. Practically it more than doubles the width of the belts of ordinary food-stuffs by lengthening the summer day in the temperate zone. Beyond the tropics the obliquity of the sun's rays are more than balanced by the increased length of time in which they fall.

Thus, in the latitude of St. Paul, the longest day is about fifteen and one-half hours long; at Liverpool it is nearly seventeen hours long; a greater number of heat units therefore are received in these latitudes during summer than are received in equatorial regions during the twelve-hour day. Moreover, the summer temperature is higher in these latitudes than in the torrid zone, because the sun is shining upon them for a greater length of time.

The result of these various influences is far-reaching. Because of the long summer days and short nights, wheat can be cultivated to the sixtieth parallel. Corn, which gets scarcely enough warmth and light in the torrid zone to become a prolific crop, attains its greatest yield in the latitude of fourteen-hour days.

These factors, it is evident, carry the grain and meat industries into regions that otherwise would not be habitable. Because the long summer days produce these great food-crops, commerce and its allied industries have reached their maximum development in these regions. Human activities are greatest in the zones bounded by the thirty-fifth and fifty-fifth parallels, the zone that includes the greater parts of the United States, Europe, China, Japan. They are greatest, moreover, because of their geographical position.

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