THE EARTH'S GARMENT OF VEGETATION The Importance of Plants and Animals.—Thus far we have de voted our attention chiefly to the direct responses of man to the five great elements of physical environment. We have passed from lo cation, land forms, water bodies, soil and minerals, and climate directly to man's activities. Only here and there have we touched on the plants and animals which form the second column in the geo graphic diagram of Fig. 1. Now that we have studied climate, however, we are ready to consider how plants and animals influence man's activities. They exert their influence chiefly through agricul ture, the great industry which furnishes most of our food and raw materials.
How important plants and animals are may be judged from the fact that in the United States about 40 per cent of the population depend directly upon agriculture. Someone has well said that previous to 1900 the chief manufactured product of the United States was 5,740,000 farms with an area of 840,000,000 acres. The number of farms is still increasing, although not so rapidly as formerly, because the greater part of the good land has already been occupied. In 1920 the farms of the United States, including buildings, equip ment, and animals, as well as the soil where the crops are grown, were worth about one hundred billion dollars. This is more than twice as much as the capital invested in all kinds of manufacturing enterprises in this country. The number of people who live on the farms is also twice as large as the number who depend on manu facturing.
Even in a country like England, where manufacturing is predomi nant and home production of food does not begin to supply the de mand, agriculture employs more people than all the railroads, steam ships, and other means of communication, and more than the metal industries which have made British cutlery and other hardware famous all over the world. Elsewhere agriculture is still more im portant. In Russia three-fourths of the people are peasants, while in India and China the proportion is even larger. Thus plants and animals determine the mode of life and the prosperity of far more than half the world's inhabitants.
How the Nature of the Vegetation Determines the Character of Agriculture.—Although the farmer uses both plants and animals, plants are much the more important because animals as well as men depend upon them. The full importance of plants, however, does
not appear until we also realize that the differences in agriculture from region to region depend largely on the different kinds of plants which the climate and soil permit. The man who clears the tropical jungle cannot possibly raise the same crops as the one who lives in the far north where a growing season of only three months permits little save barley to be raised. Nor can he plant and reap his crops in the same way, or use the same variety of animals. So, too, the man who lives in the fertile grasslands of the prairie raises corn, wheat, horses, and cows, while the one who inhabits a hot desert oasis raises millet, dates, camels, and goats. What the chestnut and olive are to the Spanish peasant, the bread-fruit tree is to the scantily clad inhabitant of the tropical Marquesas Islands in the South Pacific. Even in the same latitude the parts of Yucatan that favor the growth of sisal give rise to a kind of farming different from that which prevails in the wetter regions where rubber trees and cacao thrive. The Lapp who raises reindeer does so because the vegetation that will grow in his cold northern region will support no other kind of animal, and will not furnish crops that man can eat. In all these cases vegetation is the chief factor in determining how the people get a living.
The Three Great Types of Vegetation.—Since plants are the most important factor in the lives of such a vast number of people, we must understand the variations of natural vegetation and the causes of their distribution. The ordinary plants that form the earth's garment of vegetation may be divided into three great groups: (1) trees; (2) bushes, scrub, and woody perennials; and (3) grasses and other her baceous forms. Without this varied garment of vegetation the lands of the earth would be as barren as the moon with its wastes of desola tion. While soil and relief have much to do with the local distribu tion of these three groups, their general distribution over the world as a whole depends chiefly upon two climatic factors: (a) the length of the season warm enough for growth; and (b) the proportion of that season during which there is moisture enough to promote growth.