adapted to these conditions, to the greatest abundance where conditions are most favorable.
Man is influenced by much the same controls as those which affect plants and the lower animals. From the highest latitudes he is excluded by cold. The highest altitudes are hostile both because of cold and of diminished pressure. The deserts of sand are uninhabited, or thinly populated, by reason of aridity. Forests, where rainfall is abundant, are unfavorable to a dense population. The trees must be cleared away before settlement is easy. The waves of civilization, as one writer has expressed it, beat up against the forest, but only with difficulty do they break through it. The equatorial forests of Africa; the densely wooded Amazonian provinces of Peru; the forests of northern Sumatra; the eastern forested slopes of Central America, left longest to the native tribes, while the western, more open, and drier slopes were first settled by white men, and are best developed—these are examples of the repelling effect of dense tree-growth where the advance of civilized man is concerned. Even the earlier American civilizations, the Aztec and the Inca, halted before forested areas. The Incas were almost as much hemmed in by the forests on the east as by the Pacific on the west. Travel through dense forests is difficult. Narrow paths, along which travelers move in Indian file, are the natural, and in fact the only, ways of communication, unless travel can be by boat. It requires no wide stretch of the imagination to see a connection between the method of carrying goods in the African forests, on the backs or heads of negro porters, and the slave trade, which sells the man who carried the goods as well as the goods. Many of the natives who secure the rubber from the Amazonian forests, or from those of the Congo, are to-day subjected to hardships which equal those of slavery.
Man is widely distributed over the earth's surface. The coldest place in the world in January is a large Siberian city, Verkhoyansk, while one of the hottest places in the world is Massowa, on the Red Sea, the capital of the Italian colony of Eritrea. But the life of man is harder here and easier there, according to climatic conditions, and the scarcity or abundance of plant and animal life.
Man is distributed in great belts around the world, corresponding roughly to the broad zones of vegetation, desert, steppe and forest, the limits of which are set by temperature and rainfall, but man is far more dependent on rainfall than on temperature. There are certain common conditions of life which affect the people who live in the same zone in the same broad, general way. This, as Ratzel first pointed out, means that there is a climatic factor at work to maintain differences between the people of different zones, in spite of the great movements which are constantly tending to produce uniformity. All the regions of sparse population are gradually being encroached upon by an invasion from their borders. Forests are being cleared, and replaced by
