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, one physical and the other social-economic.

The Ladaki have reached an interesting resolution of these two forces by the institution of polyandry, which keeps population practically stationary.
[Sidenote: Pigmentation and climate.] The relation of pigmentation to climate has long interested geographers as a question of environment; but their speculations on the subject have been barren, because the preliminary investigations of the physiologist, physicist and chemist are still incomplete.

The general fact of increasing nigrescence from temperate towards equatorial regions is conspicuous enough, despite some irregularity of the shading.[53] This fact points strongly to some direct relation between climate and pigmentation, but gives no hint how the pigmental processes are affected.

The physiologist finds that in the case of the negro, the dark skin is associated with a dense cuticle, diminished perspiration, smaller chests and less respiratory power, a lower temperature and more rapid pulse,[54] all which variations may enter into the problem of the negroes coloring.