change their color to agree with the environment. The latter method would, of course, from the frog's point of view, be decidedly the more desirable, saving him much exertion in seeking safe retreats; and recent researches have shown that this is in fact the method adopted.
The ability of the tree frogs, or "tree toads" (Hylidæ), to change their color has long been known, but precise studies of the color changes of the common ground frogs (Ranidæ) have Fig. 1.—Wood Frog. Adult. only been made comparatively recently, and as yet the record is far from complete. A few years ago Dr. Fickert, of Tübingen, experimented with the color adaptability of the common European frog (Rana temporaria): "Three frogs approximately similar in color were placed in three glass vessels, of which the first stood on a black, the second on a green, and the third on a white surface, being surrounded up to a height of some five centimetres with the same color. After about an hour and a half the frog a on the black surface was the darkest, b on the white the lightest, while the frog c surrounded by green was intermediate in color between the two. Hereupon the frog a was transferred to the glass on the white, frog b into the one on the black surface. After three quarters of an hour they were again examined, and a was the lightest, b the darkest. Then c and b were interchanged, and in a quarter of an hour c was the darkest, while b was intermediate in color between c and a. When, finally, b and a were interchanged, a change of coloring appeared immediately; b became light again, and a took the intermediate tint between b and c."
A similar but less complete experiment with the same species of frog was made many years previously by Sir Joseph Lister, who found that "a frog caught in a recess in a black rock was itself almost black, but after it had been kept for about an hour on white flagstones in the sun was found to be dusky yellow, with dark spots here and there. It was then placed again in the hol-
- Eimer, Organic Evolution, Cunningham's translation, p. 148.