reason that those members of a race whose skins vary in the direction of this type will in each generation have the best chance of surviving and begetting children, and that, by the continued increment of successive variations in the same direction, the skin and the climate will ultimately be brought into accord.
The skin consists of two layers: the inner, dense and fibrous, furnished with blood-vessels and nerves, called the dermis, or true skin; the outer, horny, nerveless, and bloodless, called the epidermis, cuticle, or scarf-skin. The cells which compose the latter originate in the rete Malpighii, its lowest part, are gradually forced outward by new cells, and finally exfoliate. In some of these epidermic cells a pigment is found which varies in different races, but always contains a yellow element. The hue of the skin does not depend on this coloring matter alone, but is a compound effect, resulting from the white of the dermis, the red of the blood in the minute vessels near the surface, the color and quantity of the pigment, and the thickness of the cuticle. Where the cuticle is thick, the color of the pigment will predominate over the other elements on account of the greater depth of pigment-cells. "Where it is thin, and the coloring matter light, the tint of the skin will be much affected by any change in the supply of blood to the capillaries at the surface of the body. This is the reason why the whites alone can turn pale and blush.
Closely related to the pigment of the skin are the coloring matters of the eye and hair. Dark-skinned people usually have black eyes and hair; fair hair and blue eyes are seldom found except in conjunction with a fair skin; and the eyes and hair of albinoes, in whom the pigment of the skin is wanting, are likewise destitute of coloring matter. The pink hue of their eyes is due to minute blood-vessels, whose color is masked in ordinary organs by the pigment of the iris.
It is noteworthy that the coloring matters of the epidermis and iris serve a very important purpose: they protect the tender underlying parts from the injurious effects of too much heat and light. Albinoes everywhere find it necessary to protect their skins and eyes from the effects of the sun's rays. In warm countries they seldom go out except at night. There is this difference between them and other men, that long-continued exposure to the sun, which ordinarily develops a condition of the skin capable of resisting its rays, does not do so in their case. It may here be remarked that the deeper the shade of the pigment, the more rays will it reflect, and the more effective will it be as a protective agency. On the contrary, the lighter the shade, the more light and heat will it permit to enter the body.
As an excretory organ, it is the function of the skin to discharge water, carbonic acid, and urea the first in large, the others in small quantities. Perspiration, or the excreting of water, with some saline matter in solution, is effected in two ways: In the first place, sudoriparous glands, imbedded in the true skin, secrete sweat from the blood.
