Page:Popular Science Monthly Volume 43.djvu/649

Rh as soon as a certain part of the skin receives no more blood, the color-cells receive less oxygen, the black cells contract, and the animal assumes a lighter color. But the effects of light are even more interesting. Pouchet had shown that those fishes which usually adapt their color to their dark or light surroundings cease to do so when they have lost sight; they remain dark even in light surroundings. The indirect effects of light through the intermediary of the visual organs are thus certain. But Steinach has proved that light acts in a direct way as well—perhaps, we may add, in the same way as it acts upon the chlorophyll grains of the leaves. He glued strips of black paper to the skin of frogs which were kept in the dark, and when these animals were exposed to light, only the open parts of their skin returned to a lighter color, while the covered parts remained dark. To avoid all doubts, the experiments were repeated on skin separated from the body, and photograms of letters and flowers, cut out of black paper and glued to the skin, were reproduced upon it. Besides, blind tree-frogs do not darken as the fishes do, and Biedermann has proved that the chief agency of their changes of color is not in the sensations derived from the eye, but in those derived from the skin. Frogs, whether blind or not, become dark green, or black, if they are kept in a dark vessel in a sparingly lighted room. But when a larger branch with green leaves is introduced into the vessel, they all recover their bright-green color, whether blind or not. In some way unknown, the reflected green light acts either upon the nerves of the skin, or, what seems more probable, if Steinach's experiments are taken into account, directly upon the pigment-cells. Moreover, the sensations derived from the toes have also an influence upon the changes of color. When the bottom of the vessel is covered with felt, or with a thin wire net, the frogs also become black, recovering their green color when a green branch is introduced in the vessel.

We have here temporary changes of color produced by the surroundings; but various gradations may be traced between the temporary and the permanent changes. Thus Lode provoked local contractions of the pigment-cells in fishes by electrical irritations applied locally. And Franz Werner's researches upon the coloring of snakes, recently embodied in a separate work, show that the temporary and irregular spots which appear in fishes and frogs under the influence of artificial irritations are of the same