Into this sub-class of physical effects come all questions of acclimatization.[49] These are important to the anthropo-geographer, just as they are to colonial governments like England or France, because they affect the power of national or racial expansion, and fix the historical fate of tropical lands.

The present populations of the earth represent physical adaptation to their environments.

The intense heat and humidity of most tropical lands prevent any permanent occupation by a native-born population of pure whites.

The catarrhal zone north of the fortieth parallel in America soon exterminates the negroes.[50] The Indians of South America, though all fundamentally of the same ethnic stock, are variously acclimated to the warm, damp, forested plains of the Amazon; to the hot, dry, treeless coasts of Peru; and to the cold, arid heights of the Andes.

The habitat that bred them tends to hold them, by restricting the range of climate which they can endure.