146 ANIMAL COLORATION. 



faculty of colour change may here have a twofold value ; it 

 may be both defensive and aggressive. 



The European tree frog has not, perhaps, so many enemies 

 as those of some tropical countries which occasionally fall 

 victims to tree-frequenting snakes. There are green tree 

 frogs in many parts of the world, just as there are here and 

 there green tree snakes. In Guatemala there is a green tree 

 viper which, according to an illustration accompanying a paper 

 "by Mr. Salvin * on some of the reptiles of this region, preys 

 upon green tree frogs ; and there are non-poisonous green 

 and greenish-brown tree snakes, which are probably <juite as 

 destructive to the tree frogs ; so also, no doubt, are many other 

 snakes. It cannot, of course, be urged against the advantage of 

 this coloration to the species, that there are tree frogs in which 

 the colour is not so thoroughly protective : in Ibjla ('\cndea, 

 for example a large Australian species the green colour is 

 just as bright as in the European Ifyla, but its value for 

 defensive or aggressive purposes is to some extent interfered 

 with by a number of white spots and patches along the side. 

 At the moment of writing there are several of these frogs in 

 the reptile house at the Zoological Gardens. 



Assuming that snakes are the principal enemies of tree 

 frogs, it is necessary to make observations upon the habits 

 of the snakes before admitting the defensive value of the 

 coloration in the frogs. Generally speaking, snakes only 

 strike at a moving object ; if this is the case with the tree 

 species, no amount of protective coloration will avail the 

 frogs.f 



As to the aggressive value of the colour, it does not seem 

 at all clear how far insects are affected by the circumstance ; 



* Proc. Zool. Snc., 18GO. 



f Snakes will, however, Dr. Stradling informs me, eat pieces of meat 

 n captivity. 



