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. The question is therefore by no means simple. Yet it is generally conceded by scientists that pigment is a protective device of nature. The negro's skin is comparatively insensitive to a sun heat that blisters a white man. Livingstone found the bodies of albino negroes in Bechuana Land always blistered on exposure to the sun,[55] and a like effect has been observed among albino Polynesians, and Melanesians of Fiji.[56] Paul Ehrenreich finds that the degree of coloration depends less upon annual temperature than upon the direct effect of the sun's rays; and that therefore a people dwelling in a cool, dry climate, but exposed to the sun may be darker than another in a hot, moist climate but living in a dense forest. The forest-dwelling Botokudos of the upper San Francisco River in Brazil are fairer than the kindred Kayapo tribe, who inhabit the open campos; and the Arawak of the Purus River forests are lighter than their fellows in the central Matto Grosso.[57] Sea-faring coast folk, who are constantly exposed to the sun, especially in the Tropics, show a deeper pigmentation than their kindred of the wooded interior.[58] The coast Moros of western Mindanao are darker than the Subanos, their Malay brethren of the back country, the lightness of whose color can be explained by their forest life.[59] So the Duallas of the Kamerun coast of Africa are darker than the Bakwiri inhabiting the forested mountains just behind them, though both tribes belong to the Bantu group of people.[60] Here light, in contradistinction to heat, appears the dominant factor in pigmentation. A recent theory, advanced by von Schmaedel in 1895, rests upon the chemical power of light. It holds that the black pigment renders the negro skin insensitive to the luminous or actinic effects of solar radiation, which are far more destructive to living protoplasm than the merely calorific effects.[61] [Sidenote: Pigmentation and altitude]