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. In
the zone of the Andean slope lying between 4,000 and 6,000 feet of
altitude, which produces the best flavored coffee and which must be
cultivated, the imported Indians from the high plateaus and from the low
Amazon plains alike sicken and die after a short time; so that they take
employment on these coffee plantations for only three or five months,
and then return to their own homes. Labor becomes nomadic on these
slopes, and in the intervals these farm lands of intensive agriculture
show the anomaly of a sparse population only of resident managers.[51]
Similarly in the high, dry Himalayan valley of the upper Indus, over
10,000 feet above sea level, the natives of Ladak are restricted to a
habitat that yields them little margin of food for natural growth of
population but forbids them to emigrate in search of more,--applies at
the same time the lash to drive and the leash to hold, for these
highlanders soon die when they reach the plains.[52] Here are two
antagonistic geographic influences at work from the same environment,