*"Negroes and other black-skinned peoples inherit dark skin pigment, while brunet whites and members of other races of intermediate skin color inherit the ability to acquire it by seasonal tanning. In regions of high latitude and seasonal cloudiness a white skin is advantageous, at least in winter, for it permits a maximum of Vitamin-D irradiation, which would be partly impeded by more pigment granules. In man, as in other hairless animals, Vitamin D is absorbed directly into the blood stream because it is produced inside the skin instead of on the hair cover.
Negro skin also has a thick superficial horny layer. This resists scratching and impedes the penetration of germs, which need ten times as much U-V to kill them in a hot, damp atmosphere as in dry air. Negroes infect less than white men in the tropics. For all these reasons the distribution of black-skinned peoples around the equatorial belt of the Old World makes sense, as does the concentration of pale-skinned peoples in the cloudy regions of northwest Europe.
The human eye is also concerned with U-V regulation. The lens, while transparent to visible light, in all races completely absorbs atmospheric U-V, thus removing a potential source of blindness. The eye also regulates the amount of light which it lets in by means of an automatic diaphragm, the iris. Except in the eyes of albinos, defective in this sense, the back of the iris is rendered opaque by a layer of melanin comparable to the dull black paint on the inside of the diaphragm of a camera, and serving the same purpose: to keep light from entering the chamber of the eye or camera except through the hole provided for it. In all races this peculiar layer of pigment is present in the eye of the embryo. Like the pigment of a Negro’s skin, the organism produces it unaided by the sun.*
