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Not every human being fits neatly into one of those three categories, but most of them do. The system is at least workable, all the more so because the physical disparities in man are not limited to the color of his skin. The so-called Mongolian race, for example, can also be distinguished by the epicanthic fold that gives some Asian peoples, among them the Japanese and the Chinese, a slant-eyed look. Evolutionary hypothesis has traced this feature to its probable source. The predominant theory is that it developed from a mutation—a random change in the elaborate chemistry of human chromosomes, which govern man's biological evolution. For arctic and desert-dwelling people, subjected to blinding blizzards of snow or sand, the eye fold had definite survival value: it increased the eyes' protection against such hazards. Thus the trait endured.
The dark skin that usually, though not invariably, characterizes members of the Negroid race may also be a protective device. If man was first born in tropical Africa, as some anthropologists now suggest, then it is possible that his skin, whatever color it may have been to begin with, took on added pigment—again, starting with chance mutation—as a screen against harmful radiation from the sun. It is a fact that Negroes seldom have skin cancer, though its incidence is rising noticeably in the white population of the U.S. The same pigment, by filtering solar radiation, impedes synthesis of vitamin D, which prevents rickets and is manufactured from the sun's rays by the body. As early man migrated out of the tropical sun—into the green jungle, north to less torrid zones—light skin thereupon conferred an advantage by admitting more vitamin D-produc-ing sunlight. And the lottery of evolution, patiently awaiting the appropriate mutation, then fixed this advantage into place. Thus, over the centuries, environmental factors were producing genetic changes.
Man's extended tropical sojourn appears to have generated other useful or once useful adaptations more frequently found in dark-skinned peoples. A hereditary blood condition known as the sickle-cell trait, which grants resistance to certain types of malaria, is only now beginning to wane among U.S. Negroes, who no longer have any need of it. The Negro's woolly black hair once provided insulation against the heat of the blazing tropical sun; his thick lips, by exposing more mucous membrane, may have increased the body's evaporative cooling powers in torrid climates; his characteristically long legs and lean frame were once distinctly helpful to some prehistoric race of hunters.
The list of apparently Negroid characteristics can be extended, since dark-skinned persons come in so many shapes and sizes, from the storklike Watutsi, to the Pygmies of Central Africa. Generally, Negro skull capacity—affecting the size of the brain—runs about 50 cc. below that of whites.