3 p. m., while it was still so dark, I put it back in the window with white surroundings; at 3.05 it was considerably lighter brown, at 3.10 much lighter, and at 3.15 it had become cinnamon-colored—a very marked change thus occurring in fifteen minutes.
These experiments were repeated a number of times with several different individuals, and similar results were obtained.
The common green frog (Rana clamata) has the power of changing its color to a considerable extent. Specimens kept for some time amid light surroundings became of a very light green color—even lighter than apple green—while if placed amid a black environment they become very dark. The leopard frog, or spotted frog (R. virescens), is not able to change its appearance so completely, the permanent color markings preventing; but the green ground color varies somewhat. The few observations I have been able to make on the bullfrog (R. catesbiana) indicate that its ability in this direction is very similar to that of the green frog.
The power of color change is also present to a decided extent in our common toad (Bufo lentiginosus). A very large specimen of this species was found in wet grass June 1st, at 11 p. m. It was then of a light wax-yellow color. It was brought to the laboratory and put in a glass jar on a black shelf. Twenty-four hours later it was very much darker, being tawny olive brown. Three days later it had become still darker, being almost clove brown. A similar power has been observed in the European toad.
It is conceivable that these color changes might occur in either of two ways: First, by the direct action of the light reflected from the surroundings upon the pigment cells of the skins, and second, by an indirect action through the eye of the animal. The second method is the one involved. Experiments have shown that, when blinded, a frog does not change its color to agree with the environment. Mr. Poulton describes the process of change by saying that "certain kinds of light act as specific stimuli to the eye of the animal, and differing nervous impulses pass from this organ along the optic nerve to the brain. The brain being thus indirectly stimulated in a peculiar manner by various kinds of reflected light, originates different impulses, which pass from it along the nerves distributed to the skin, and cause varying states of concentration of the pigment in the cells. . . . The pigment cells in the skin are often of various colors, and are arranged in layers, so that very different effects may be produced by concentration in certain cells, leading to the appearance of those of another color, or to a combined effect due to the colors of two or more kinds of cells."
- Loc. cit., p. 85.