98 THE POPULAR SCIENCE MONTHLY. 



other things being equal, richest in coloring-matter. The backs of 

 wild animals are usually and with few exceptions (as among noc- 

 turnal and burrowing animals) more strongly colored than their bel- 

 lies. 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 ne- 

 groes themselves. Still, we can not afiirm 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 sea- 

 shore. 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 accoimt 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 de- 

 gree ; 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 Eu- 

 rope. 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. 



