146 ANIMAL COLOEATION. 



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 quite 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 Hijla Ccerulea, 



for example — a large Australian species — the green colour is 



just as bright as in the European Hyla, 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.t 



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



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



* Proe. Zool. Soc, 1860. 



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

 n captivity. 



