305. =Fur, Hair, and Feathers.=--Animals that live in cold climates are provided with suitable coverings for their protection. Quadrupeds, for example, are covered with fur, and birds have an abundance of downy feathers. These coverings have no warmth in themselves, though in common language we speak of them as being warm. They are simply non-conductors, and so prevent the heat which is made in the body of the animal from escaping as fast as it otherwise would. But why are they non-conductors? It is not because the substance of which they are made is a non-conductor, but because among their numberless fibres is partially confined that great non-conductor, air. Let the fur or down be condensed into a thin hard plate upon the animal, and it would prove of little service as a protection against cold. Down is much more abundant on the birds of cold climates than on those in warmer regions, because more air can be confined among the fibres of down than among those of common feathers. Quadrupeds that are natives of warm climates generally have hair instead of fur. When therefore the horse is taken to a cold climate he requires in winter the defense of a blanket; and the ox needs under the same circumstances to be better housed than he ordinarily is. As the elephant is a native of a climate positively hot, his hairs are scanty and coarse. Formerly there were elephants in the cold regions of Siberia, as has been ascertained by remains found there. But the elephant of Siberia had under its hair, close to the skin, a fine wool to protect it against the cold. Animals that live in cold climates have their coverings become finer in fibre in the cold season of the year, to give them the additional protection which they then need. And animals with a furry covering, if they are carried into a warm climate, have their fur become coarse, and approximate the condition of hair.