Page:Popular Science Monthly Volume 43.djvu/648

630 confirmed by direct experiment, namely, by Westhoff, that several fresh-water and marine fishes change their color from white to dark as soon as they have been transferred from a medium with a light-colored bottom to another medium the bottom of which is dark. Fishermen, we are told by Mr. Poulton, even keep their bait in white-colored vessels in order to make it assume a lighter color. The common frog also can change its color to some extent in harmony with its surroundings, while the green tree-frog of southern Europe was long since known for this capacity. It is bright green among green leaves, and dark green when seated on the earth or among brown leaves. Like changes are also known in the chameleon and in some South American lizards. The causes of these changes have already been investigated by Pouchet in 18-48 and Brücke in 1852, but now we have a more elaborate research by Biedermann upon the same subject. He has discovered three different layers of cells which contribute to give the frog its varying colors. There is first, deeply seated in the skin, a layer of pigment-cells which contain black pigment both in their interior and in their ramified processes, spreading within the skin. These cells are covered by a second layer of "interference-cells" containing bright yellow granules as well as granules of a pigment which sometimes appear blue or purple, and sometimes gray—the whole being covered with a transparent outer skin. The normal green color of the frog is produced by a combination of blue and yellow interference-cells appearing on a black background; but if the black pigment of the deepest layer is protruded into its ramifications, the color of the animal becomes darker; and if it retires deeper, the yellow granules of the middle layer become more apparent, and the frog assumes its lemon-yellow color. Finally, when the yellow pigment gathers into round drops between the bluish interference-cells—not above them—the skin acquires a whitish-gray tint. The same arrangements exist in other reptiles and amphibia.

Now, how is it that the cells change their position in various lights? Is it some reflex action in the nervous system, as it appears in fishes, which cease to change their color when they become blind? Or have we to deal with some direct action of light? Facts are in favor of the second explanation. The slightest change of temperature affects the mutual disposition of the pigment-cells, and consequently the color of the frog; it is enough to keep the animal in the hand to provoke a contraction of its black cells. The amount of blood-supply also has a definite effect;