In no character is natural selection so evidently at work as in the amount of pigmentation of the outer epithelial structures, skin and hair. Color has long been used as a means of classifying man as it is so evident. As soon as we discovered that it was an advantage we began searching for that benefit. At first it was thought to be an assistance in radiating heat in the tropics and conserving it in cold countries by the physical law that dark, hot bodies radiate more quickly to cooler bodies than do bright, hot ones. The water in a black tea kettle will cool off far sooner than that in the bright teapot. Hence, blondness enabled men in the North to save heat and to keep warm, but was a disadvantage in the tropics, as it kept men too warm, even feverish, and many an observer has given this as the reason why white men in the tropics may have a temperature above normal. As this injures nerve tissue it is only a question of time when exhaustion and collapse occurs. There is much truth in this theory, and we do know that the negro in the North has much greater difficulty in keeping warm than the white man. Not only does he require more clothing, and lives in houses which are stifling to us, but he avoids outdoor labor as much as possible in winter, inclining to house labor for warmth. When he does go out he is more easily overcome by the cold and suffers dreadfully from frost bite. This law of radiation is undoubtedly one reason for the evolution of blackness in tropical animals and whiteness in the North, for nearly all black animals are in the tropics and white ones in cold places.
* Journal of Tropical Medicine, January 15, 1906.
Radiation must not be confounded with absorption. Cool, dark bodies likewise absorb heat from hotter sources, much easier than cool, light bodies. The tea kettle heats up on the stove more quickly than a bright teapot. Likewise black clothing is warmest in the sun. But this law is of little effect in evolution because the dark tropical animals are mostly nocturnal and, therefore, are at no disadvantage.
A great deal of extra heat in our bodies in the tropics is no doubt due to the greater difficulty of disposing of it. We radiate but little because surrounding objects are nearly as warm as we are. At home, evaporation of perspiration carries off immense quantities by reason of the fact that to change a pound of water to vapor requires 540 times as much heat as to raise a pound of water one degree C. In the dry Western plains this evaporation entirely prevents the classical sunstroke. In the tropics evaporation is at a minimum when the air is saturated and the perspiration pours off. This is the reason for the tendency to bathe so often. It is a curious thing that a cold bath checks heat conduction to the surface, because it drives the blood from the surface. Hence, the English in India find that tepid baths, which dilate the superficial capillaries, are really more cooling. Indeed, they are preferable for other reasons, as cold baths are too stimulating to the jaded nervous system which needs rest and recuperation.
Sexual selection may modify man's coloration as it certainly does in case of birds as first noted by Darwin, though his explanation is not wholly accepted. Where the healthiest and best are of a certain complexion, that color will be most attractive to each sex, and selection be along that line intensifying it. This may be the reason for the intensifications and unifications we see in long settled places, as in Africa and Asia, but it is only a modifying element after all. I have observed this sexual selection among Maryland negroes who ridicule the half-breeds as something unnatural, and pride themselves on purity of blood. Hence, in all races there is a tendency to marry their kind.
Haytian negroes have long shown this intense hatred for half-breeds whom they have excluded from offices and discriminated against in many ways. So that the Island is nearly pure blooded negro already. Hindus, Japanese, Chinese - indeed, all races have an antipathy for their half-breeds, and in Japan and India they are practically excluded from native society.
Von Schmaedel was the first to describe the real reason for the evolution of skin pigmentation. In a paper read before the Munich Anthropological Society, about 1894, he showed that it exists because it excludes the dangerous actinic rays of light - those of short wave length, the violet, indigo, blue and ultra violet rays. The non-actinic rays at the other end of the spectrum, green, yellow, orange and red, are more or less harmless. Skin color is then an opaque armor. Dr. R. W. Felkin was the first to call attention to this fact in English journals.* A full technical proof of Von Schmaedel's theory can be found in the book which the author devoted to this topic * It may be consulted for the details of the proof but it is sufficient here to state that in every part of the world - hot or cold - the native is pigmented in accordance with the maximum intensity of the light to which he is exposed. The Eskimo is very dark for protection from the snow glare and the negro is black for protection from the tropic sun. Mediterraneans are brown, but the people of Northwestern Europe, where the cloudiness is at a maximum, are the only real blonds in the world. It has been proved that white ants are killed if they are exposed to excessive light, and the same result follows in the case of white men. This one fact alone will fully account for the evolution of the blond Aryan of Europe and his gradual disappearance whenever he migrated far to the South.
* Journal of Tropical Medicine, September, 1900.