Tree-frogs turn green as the result of the contact with leaves. {36}Dark frogs will turn green when put into an absolutely dark vessel in which there are leaves. This is reflex action, and blinded specimens do the same. The principal centres of the nerves which control the chromatophores, lie in the corpora bigemina and in the optic thalami of the brain. When these centres are destroyed, the frog no longer changes colour when put upon leaves, but if a nerve, for instance the sciatic, be stimulated, the corresponding portion of the body, in this case the leg, turns green. Rough surfaces cause a sensation which makes the frog turn dark. _Rana_ seems to depend chiefly upon temperature and the amount of moisture in the air, so far as its changes of colour are concerned. Biedermann concludes that the "chromatic function of frogs in general depends chiefly upon the sensory impressions received by the skin, while that of fishes depends upon the eye." All this sounds very well, but the observations and experiments are such as are usual in physiological laboratories, and the frogs, when observed in their native haunts, or even when kept under proper conditions, do not always behave as the physiologist thinks they should. There is no doubt that in many cases the changes of colour are not voluntary, but reflex actions. It is quite conceivable that the sensation of sitting on a rough surface starts a whole train of processes: roughness means bark, bark is brown, change into brown; but one and the same tree-frog does not always assume the colour of the bark when it rests, or even sleeps upon, such a piece. He will, if it suits him, remain grass-green upon a yellow stone, or on a white window-frame. I purposely describe such conditions, changes, coincidences, and discrepancies in various species, notably in _Hyla arborea_, _H. coerulea_, _Rana temporaria_, _Bufo viridis_, to show that in many cases the creature knows what it is about, and that the eye plays a very important part in the decision of what colour is to be produced. The sensory impression received through the skin of the belly is the same, no matter if the board be painted white, black, or green, and how does it then come to pass that the frog adjusts its colour to a nicety to the general hue or tone of its surroundings? Boulenger[16] has given us a summary of the action of the POISON of Amphibia: