218 THE FROG CHAP. 



The general method of classification employed by 

 zoologists may be illustrated by reference to the 

 different kinds of frogs already referred to in explanation 

 of the terms genus and species. 



The common frog (Rana tempo? aria) is distinguished 

 from the " edible frog" (R. esculenta) by its smaller size 

 and brown colour, by the large dark patch in the tym- 

 panic region, and by the rudimentary character of a 

 pair of inflatable vocal sacs at the sides of the head in the 

 male, which are very large and highly distensible in 

 R. esculenta. On the other hand, these two frogs 

 agree with one another and with all the other 

 species of the genus Rana, in having teeth on the 

 upper jaw, and in not having the transverse 

 processes of the sacral vertebra dilated. Compar- 

 ing all these frogs with our English toads, of which 

 there are two species, the common toad and the rarer 

 "natterjack," we find that in them the skin is 

 comparatively dry and covered with glandular warts, 

 the hind-limbs are proportionally shorter, there are 

 no teeth, and the transverse processes of the sacral 

 vertebra are more or less dilated. These differences 

 are so great as not only to necessitate placing the toads 

 in another genus (Bufo), but also to relegate the frogs 

 just mentioned to one family the Ranidce (including 

 many genera), and the toads to another the Bufonides. 

 All frogs and toads, however, agree with one another in 

 having no tail in the adult, while the trunk is relatively 

 short and broad, and the hind-limbs are longer than the 

 fore-limbs. They therefore differ fundamentally from 

 such animals as the common English newts and 

 the salamanders, which retain the tail throughout life, 

 and in which the fore- and hind-limbs are of approxi- 

 mately equal size. The differences here are obviously 

 far greater than those between the families mentioned 



