Causes of Co/or.—Physiologists have sought to explain the greater deposition of coloring-matter in one individual than in another by the difference in the relative activity of the great secretory organs, the lungs and the liver. Where the lungs are fully developed and in high func tional activity, the venous blood is thoroughly arterialized in passing through the pulmonic circulation ; that is, the carbon it contains passes off as carbonic oxide into the air. Where the pulmonary action is slug gish, the liver acts the part of a compensatory organ, the secretion of bile is increased, but the carbon is by no means so completely abstracted from the blood, and, passing into the fine capillaries lining the papillae of the true skin, is deposited on their surface, thus forming the pigmentary coat above described.
This theory is supported by several observed facts. The arterial and venous bloods of natives of tropical climates, and of Europeans long resi dent in them, do not present that marked contrast of color visible in tem perate latitudes, thus indicating deficient arterialization. Furthermore, it has been shown by extensive dissections that the lungs of Negroes are smaller in proportion to the height and weight of the individual than in the white race. That they are also much less efficient organs is shown by the greater liability of the black race to pulmonary diseases in temperate and cold climates.
Correlated to Exemption from has also been argued that the coloration of the skin and this physiological difference in the activity of the lungs and heart are "correlated," as it is termed, with a greater resistance to the effects of prolonged heat and with an immunity from the action of malarial and similar poisons. The Negroes of the West Coast of Africa live in health where it would be death to any European to spend a night. Nor do they feel the prostrating power of the heat in anything like the same degree. Sunstroke, though not absolutely unknown in the black race, is exceedingly rare. The blacks are usually proof against yellow fever, and even the half-breed mulattoes can generally expose themselves to it without danger.
If we suppose that the causes which bring about a black skin also confer these exemptions from disease, we can readily see that by the pro cess of "the extinction of the unfit" the lighter-colored members of a tribe exposed to a tropical climate would perish more rapidly than the darker, until an entirely dark tribe came to be established. This correlation has not, however, been proved, and there are various reasons rather to believe that the immunity from these diseases is merely one of acclimation with out reference to color. African travellers—Dr. Nachtigall, for instance— state that when the Negroes are exposed to the malaria of a different dis trict from that to which they have been accustomed they frequently are attacked by it ; and, on the other hand, the Spanish Creoles of Cuba, of pure white descent, are no more liable to the yellow fever than the Negroes, and they can support the ardors of a tropical sun with equal indifference.
Distribution of Colored as a general rule, subject to numerous exceptions, and not as a physiological law, can it be said that color darkens as we approach the equator and becomes lighter toward the poles. The natives of the Arctic regions, the Laplanders, the Tchuktchis, the Eskimos, though belonging to different races, are all dark, quite as brown as the majority of residents within the tropics. The inhabitant of the bleak and damp climate of Tierra del Fuego has just as deep a hue as a Botocudo, whose home is in the tropical forests of Brazil ; while the Abipone, who lives about midway between them, is several shades lighter than either. Directly under the equator, on the eastern slopes of the Andes, are the Yurucare Indians, whose name means " White People," given to them by their neighbors, the much darker Quichuas, because of their remarkably fair complexions. The natives of Central Africa are by no means all equally dark. They vary in different tribes from an ebony-black to a chestnut-brown.
It is further significant, as showing that a deep coloration of the skin is probably an acquired rather than an original character of the race, that the new-born infants of all the dark races are several shades lighter in hue than the adults. Several years are required for their skin to become as dark as that of their parents.
Variations of Color in the Same variation of color within the limits of all the races is equally marked. The most completely white communities are found among the Slavonic populations of Southern and Central Russia. Their hair is colorless, and their complexion so near a " dead white " that one anthropologist (Theodor Posche) has selected the vast Rokitno swamps as the original home of the white race, which be thinks arose by an endemic albinism ! The Scandinavians still retain the traits of the "blue-eyed, yellow haired, fair-skinned, large-limbed warriors" who alone of all men checked the advance of Rome in the plenitude of her power, and excited the admi ration of the Latin poets. The Romans themselves, like the Greeks and Iberians, and like their descendants to-day, were much darker than the Teutonic tribes ; the Semites of Northern Africa, Phoenicia, and Arabia present a still deeper hue ; until among the Copts, Berbers, and Abyssin ians, all ranked with the white race, the color shades by imperceptible degrees into a decided brown not distinguishable from the lightest of the pure negro hue. Yet among the Spaniards and Italians examples of pure blondes with light hair occasionally occur. In Spain this is con sidered a sign of Gothic, in Italy of Etruscan, descent ; but it is prob ably a reversion to the ancient type of the race.