VARIATION AND ADAPTATION 221 



vessel, or when blind, and in the same way they turn brown 

 when on rough surfaces such as bark ; he concludes that in 

 frogs the chromatic function depends chiefly on sensory im- 

 pressions received by the skin, while that of fishes depends on 

 the eye. Gadow found, however, that in many cases the frog 

 assimilates its colour to many other tints besides green and 

 brown ; one of his specimens rested during the daytime in a 

 corner of a window-frame near putty and discoloured white 

 paint and it became of a mottled leaden colour. 



In the majority of Amphibia markings are not very definite 

 or conspicuous ; the green of tree-frogs is fairly uniform and in 

 other cases there is nothing but an irregular mottling. A few 

 exceptions occur in species which exhibit what is called warn- 

 ing coloration, that is conspicuous colours and markings 

 associated with some poisonous or dangerous quality, and bold, 

 fearless habits. One of the best examples of these character- 

 istics is afforded by the spotted or fire salamander which is 

 black with large irregular patches on the back and limbs of 

 bright yellow or orange, the colours we see in a wasp ; the 

 lower surface is bluish-black. The animal is poisonous, pro- 

 ducing a poisonous secretion from the glands in a thickened 

 patch of skin behind the tympanic membrane on each side ; 

 there are also glands along each side of the back and on 

 the flanks. The secretion is a milky white liquid which if 

 it accidentally touches the human eye causes burning pain 

 and imflammation ; a few drops of the poison in the stomach 

 or injected into the blood of a small animal are fatal. The 

 parotid glandular patch is present in most of the Anura but 

 the development and the activity of the poison vary greatly : 

 the toad is much more poisonous than the frog, the latter being 

 eaten by many other animals, the former being usually avoided, 

 and if a dog bites a toad the poison causes great pain from 

 its effect on the mucous membrane of the mouth. Among 

 the Anura Bombinator zgneus, called the fire-bellied toad, has 

 the same association of warning colours and poisonous qualities 

 as the spotted salamander. (Plate XIX., B.) In Bombinator 

 the upper side is dark grey or nearly black and the warning 

 colour is on the lower side which is black with large red or 

 orange-red patches. It has often been stated that these toads 

 when disturbed on land turn over on their backs, but this is not 



