MAN'S CHANGING SURROUNDINGS.
Geographic Constants and Variables.—Among the physical features of man's surroundings three, namely, location, land forms, and water bodies may be regarded as constants. The location of a place in relation to the poles and the equator or in relatiOn to the lands and the oceans never changes, or at least changes so slowly that man is not conscious of it. Land forms are almost equally constant. Although the mountains may be worn down a little by erosion in the course of hundreds of thousands of years or raised a little higher by earth movements, they have not changed appreciably during the period covered by human history. With water bodies, the third element, the changes are equally unimportant except where variations of climate cause a desert lake, for instance, to dwindle in size, or a river like the Hwang Ho to become China's sorrow, or where man himself has built reservoirs, enlarged harbors, and reclaimed land from swamps or from the sea as in Holland.
Soils and minerals, the fourth great feature of man's physical surroundings, are more variable than the first three. Their changes, however, are either extremely slow or are due to man's own actions. Slow changes consist of the weathering of new soil, the accumulation of humus, and the formation of new mineral deposits by water that percolates through the rocks. These changes, however, are almost as slow as the changes in relief. The changes caused by man are more important. By cultivating the soil he robs it of its wealth. In China thousands of square miles have entirely lost the valuable soil cover because it has been washed away after the cutting down of the forests. In long-cultivated countries like Greece the soil has suffered much from constant cultivation without the addition of proper fer tilizers. In Italy and other countries such exhaustion of the soil probably helped to cause the fall of the Roman Empire. Mineral deposits are likewise exhausted by man. In any mining country one can find towns like Virginia City, Nevada, that once were pros perous, but now have fallen to ruins because the earth has been robbed of its mineral deposits.
Climate, the fifth great feature of physical environment, is far more variable than any of the others. A cool wet summer may cause an Adirondack resort to be almost deserted, and thus bankrupt the hotel keepers, cause the guides to go elsewhere for a living, and make the boats and railroads run at a loss. A drought of a few months may cause famines like those we have discussed in India.
Often the climate grows wetter and colder for a few years and then becomes drier and warmer until it returns to the original condition, only again to enter upon a new cycle of the same sort. There are irregular climatic cycles of every grade from those of about three years, through those of 11, 33, 100, and so on up to the great cycles known as glacial periods. Thus, climate is the one great physical condition which varies appreciably. The other four—location, land forms, water bodies, and soil and minerals—are relatively constant except when long periods are considered.
Though most of the physical features of man's environment change but little, the plants and animals upon which man depends so largely are subject to many variations. These usually take the form of migrations, blights, and diseases. Let us begin with some of the variations in plants and animals and then pass to those due directly to climate.
Examples of Geographic Variables. Animal Migrations: Lo custs.—The migrations of animals illustrate the effect of geographic variables. Those of insects are, on the whole, much more important than those of larger animals. One of the best known migratory in sects is the locust. Its movements depend largely upon climate. In years when the eggs are able to hatch in large numbers vast swarms of the insects infest states like Kansas. Having eaten every green thing where they were hatched they begin to migrate, and move across the country by the million, all headed in the same direction, although no one knows why. They leave behind them a desert peopled by poverty stricken and discouraged farmers.