=Use of color.=--The prevalence of color and the oftentimes striking and intricate coloration patterns of animals demand some explanation. As naturalists are accustomed to find the frequently bizarre and seemingly inexplicable shapes and general structure of animals readily explained by the principle of adaptation, that is, special modification of body-structure to fit special conditions of life, so they look to use as the chief explanation of color and markings. Some uses are obvious; bright colors and striking patterns may serve to attract mates or to avail as recognition marks by which individuals of a kind may readily recognize each other. The white color of arctic animals probably serves to help keep them warm by preventing radiation of heat from the body; on the other hand dark color may also help to keep animals warm by absorbing heat. "But by far the most widespread use of color is for another purpose, that of assisting the animal in escaping from its enemies or in capturing its prey." It is common knowledge that the young and old, too, of many kinds of ground-inhabiting animals, when startled by an enemy will not run, but crouching close to the ground remain immovable, trusting to remain unperceived. But a blue or crimson rabbit, however still it might keep, would be easily seen by its enemy and killed. Rabbits, however, which are good examples of animals having this habit of lying close, are neither blue nor green nor red, but are colored very much like the ground on which they crouch. This harmonious coloration is as necessary to the success of this habit as is the keeping still. A grasshopper flying or leaping in the air is conspicuous; when it alights how inconspicuous it is! Unless one has followed it closely in its flight and has kept the eye fixed on it after alighting it is usually impossible to distinguish it from its surroundings. And this is greatly to the advantage of the grasshopper in its efforts to escape its enemies, that is, in its struggle for existence. On the other hand a green katydid would be very conspicuous in a dusty road. But dusty roads are precisely where katydids do not rest. They alight among the green leaves of a tree or shrub. The animals that live in deserts are almost all obscurely mottled with gray and brownish and sand-color so as to harmonize in color with their habitual environment. The arctic hares and foxes and grouse which live in regions of perpetual snow are pure white instead of red or brown or gray like their cousins of temperate and warm regions.