TEE COLOR CEANGES OF FROGS. 493 



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 sev- 

 eral 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 (E. virescens), is not able to change its appearance so com- 

 pletely, the permanent color markings preventing ; but the green 

 ground color varies somewhat. The few observations I have 

 been able to make on the bullfrog (E. 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 labora- 

 tory 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 pig- 

 ment cells in the skin are often of various colors, and are ar- 

 ranged 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. 80. 

 tol. XLni. — 35 



