THE FROG 115 



other species that none other can be regarded as closely allied to it, 

 and in such cases the genus is composed of that species alone. From 

 these two categories the name of the animal is derived, and a double 

 name results, so that this method of naming is known as the system 

 of binomial nomenclature. This system was introduced by the 

 Swedish naturalist, Linne, or Linnaeus, in the eighteenth century. 

 It was a great advance on the previous haphazard way of naming, 

 and it is so useful that it is now universally adopted, and, indeed, 

 it is hard to see how the enormous number of species we now know 

 could have been coped with but for its use. The two names are 

 usually of Greek or Latin derivation, sometimes curious mixtures of 

 both, and in some measure correspond to a Christian and a surname, 

 save that in the animal the order is reversed, and the generic precedes 

 the specific name. 



The name of the common English frog is, as we have seen, Rana 

 temporaria, indicating in the first place that it belongs to the genus 

 Rana. As pointed out in the opening chapter, another frog, Rana 

 esculenta, is common on the continent, and yet another frog, Rana 

 tigrina, is common in Northern India. If there is no doubt as to 

 the animal we intend, it is often customary to reduce the generic 

 name to its initial letter, thus our frog becomes R. temporaria. 

 These frogs just mentioned are alike in their essential features, and 

 so we find them, with a number of others from various parts of the 

 world, included in the one genus Rana. They differ much among 

 themselves in colour, markings and size : the English frog is the 

 smallest, the edible frog a little larger, and the Indian frog con- 

 siderably larger. It is, therefore, necessary to separate them from 

 one another into separate species, each with its own specific name. 



This is a step on the way to classification, but if we stopped here 

 we should still have a large number of genera closely related, or 

 widely separated, from one another, but not in any sort of order. 

 To obviate this we associate allied genera together into Families ; 

 these into larger groups termed Sub-classes or Orders ; these into 

 still larger ones termed Classes. The classes that show a certain 

 common fundamental unity of structure, and, therefore, descent 

 from a common, if remote, ancestor, together form a Phylum, one 

 of the major divisions of the animal kingdom. 



To take the case of the frog : 



Phylum "... ... .. .. Chordata. 



Class ... . , :\: . . . . Amphibia. 



Sub-class or Order . . . . Anura. 



Family . . . . . . . . Ranidae. 



Genus . . . . . . .. Rana. 



Species . . . . '. . .V* Temporaria. 



