This is conveyed to the air by minute ducts passing through the epidermis. It is obvious that, the blacker the pigment, the less light and heat will be transmitted to excite these glands into activity. In the second place, there is a continual transudation of sweat from the minute vessels of the surface of the body through the epidermis at every point. The thicker or the more oily the scarf-skin, the less will the amount of this transudation be. If it be both thick and oily, as in many dark races, the quantity transuded will be reduced to a minimum; if it be thin and not oily, as in the fairest members of the white race, transudation will be copious.
The amount of transuded sweat depends, however, not merely on the thinness of the cuticle, but also on the degree to which the air in contact with the body is saturated with moisture; for there is a limit to the quantity of vapor which the air can absorb. This limit varies with the temperature, warm air absorbing more than cold. Such being the nature of the skin, I now proceed to inquire what kind of it will best suit particular regions. For this purpose climates may be classified as—
| I. | Arctic. |
| II. | Temperate humid. |
| III. | Temperate dry. |
| IV. | Tropical humid. |
| V. | Tropical dry. |
I. When the skin is exposed to great cold, perspiration by transudation is accelerated. The frosty air, raised many degrees in temperature by contact with the body, becomes very dry, and greedily drinks in its moisture. At the same time the body loses, not only the heat which the air carries off, but also that which is rendered latent by the evaporation of the sweat. As a protection against the injury which a too rapid loss of perspiration and heat may inflict in an arctic climate, a thick integument is desirable. On account of the obliquity of the sun's rays a dark pigment will be a disadvantage, because it will prevent the passage of light and heat. Some pigment will, however, be needed, as not even in northern regions can albinoes expose themselves to sunlight with comfort. The coloring matter, then, will be light; but, owing to the thickness of the cuticle, the general effect will be yellow.
II. By a humid temperate climate I mean one occurring in a temperate zone, in which the air constantly contains a large amount of moisture. Humidity does not to any considerable extent depend on the amount of the annual rainfall. The annual rainfall of London is twenty and one half inches, that of Toronto thirty inches; yet the air of the former place is incomparably more humid. Countries in which this climate is found are distinguished from others in the same latitude by the limited range of the thermometer. This is due partly to the fact that water can not be so rapidly heated as air, and partly to the
