Whether a body is animate or inanimate, if it has a temperature above that of its surroundings, as has been shown, it gives away more heat to them than it can receive from them, and it thus continually grows cooler in proportion to the difference between the two temperatures and to the degree of its exposure. As the temperature of the animal body is, except under very rare conditions, always a great deal higher than that of the surrounding atmosphere, animals are constantly losing heat, and it becomes a matter of vital importance that some non-conducting substance shall be interposed between the warm blood and the cold air.
All warm-blooded animals are endowed by nature with some protection against the constant loss of heat. Thus seals, whales, porpoises, and other warm-blooded animals living much in water are protected by a layer of fat through which water cannot pass, and which resists the passage of heat. Land animals also have this protective fat, though in a less degree. Moreover, the skin, being to a certain extent non-conducting, partially prevents excessive loss of heat, and in this duty it is supplemented in the lower animals by feathers or fur, and in man by clothes. Animal substances, such as hair, fur, wool, and feathers, are non-conductors of heat, and the colder the habitat of the creature the thicker the feathers or fur which cover it.
Look, for example, at the thick fur of an Arctic bear, or the feathers of the grebe, which are closely laid over one another to the height of about an inch and a half above the skin. These are nature's pro-
