36 AMPHIBIA CHAP. 



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. Eough surfaces cause a sensation which 

 makes the frog turn dark. Eana 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 experi- 

 ments 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 

 Hylci arborea, If. coerulea, Rana temporaries, Bufo mridis, 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 * has given us a summary of the action of the 

 poison of Amphibia : 



1 Nat. 8d. i. 1892, p. 185. 



