other things being equal, richest in coloring-matter. The backs of wild animals are usually and with few exceptions (as among nocturnal and burrowing animals) more strongly colored than their bellies. Another class of exceptions may be seen among fishes of certain families which lie on their sides instead of on their bellies, and expose, not their backs, but one of their sides to the light. In these fishes the upper side is colored, while the under side, next to the ground and the darkness, is not. Articulates also have their upper sides most strongly colored, although what in them answers most nearly to the dorsal column is next to the ground. The parts of the shells of mollusks which are in contact with the ground are uncolored, while the parts exposed to the light shine with varied tints; and this, whatever may be the peculiar positions assumed by particular shells.
For individuals of the same race, the abundance of the coloring matter is generally proportioned to the intensity of the light to which they are exposed. This fact is generally understood, though exact observations bearing upon it are not as numerous as it is desirable they should be. It is well known that the skin is tanned by light, that people from the north are browned by living in the south, and that ruddiness and freckles appear under the action of the sunlight. Some peoples of the white race, like the Hindoos and the Moors, that live in southern climates, are frequently darker-skinned than the negroes themselves. Still, we can not affirm that light is the only cause of these changes.
Mr. Gould has observed that birds are more strongly colored when they live in countries having a clear sky than on islands or the seashore. Berchstein says that the colors of the plumage of cage-birds are affected by the shade in which they are kept. Mr. Allen has shown that the color of several species in the United States changes as we go from north to south.
On account of their close relations with one another, it is hard to distinguish the effects of heat on color from those of light. External temperature can not have much effect upon the skin of warm-blooded animals whose bodies are kept by the internal heat at a uniform degree; but with the fur it is different, and it is possible that cold may induce an abstraction of coloring-matter from the hairs, and that the white color of animals of the polar zone may be partly owing to this fact. According to Pallas, the horse and the cow in Siberia become paler during the winter. The ermine seldom becomes as white during winter in England as in Norway. Its summer color persists till late in the season, when the extreme cold comes on, and then changes in a few days. The isatis fox, which in the polar regions becomes white in winter from brownish-gray, changes but little when taken to Europe. The Alpine hare does not put on its white dress at a fixed period, but at a time that depends on the greater or less earliness of the beginning of winter.
