2i8 THE FROG chap. 



The general method of classification employed by zoolo- 

 gists 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 temporarid) is distinguished 

 from the "edible frog" {R. esculentd) by its smaller size and 

 brown colour, by the large black patch in the tympanic 

 region, and by the rudimentary character of a pair of infla- 

 table 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. Comparing all these 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, the 

 genus Bufo, but also to relegate the frogs just mentioned 

 to one family — the Ranida, and the toads to another — the 

 BufonidcE. 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 approximately equal 

 size. The differences here are obviously far greater than 

 those between the families mentioned above, and are 

 emphasised by placing the frogs and toads in the crder 



