2 6o THE POPULAR SCIENCE MONTHLY 



adapted to these conditions, to the greatest abundance where condi- 

 tions 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 ex- 

 cluded 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 north- 

 ern Sumatra; the eastern forested slopes of Central America, left long- 

 est 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 re- 

 quires 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 se- 

 cure 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 Eed 

 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 Eatzel 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 in- 

 vasion from their borders. Forests are being cleared, and replaced by 



