CHAPTER XIII. 



THE FROG (continued) : MEANING OF THE TERM SPECIES 



THE PRINCIPLES OF CLASSIFICATION EVOLUTION 

 ONTOGENY AND PHYLOGENY HEREDITY AND VARIA- 

 TION STRUGGLE FOR EXISTENCE SELECTION ORIGIN 

 OF SPECIES. 



THE frog which you have been studying is the only one 

 commonly found in Great Britain. Another kind, very 

 similar to it, is, like the common frog, abundant in Ger- 

 many and other parts of the European continent, but is rare 

 in this country, only occurring in parts of the eastern counties ; 

 and various other frogs, differing in certain minor respects 

 from these are found in different parts of the world. This 

 fact is expressed in the language of systematic zoology by 

 saying that there are various species of frogs, belonging to 

 the same genus, which are distinguished from one another 

 by certain definite characteristics as regards form, structure, 

 and colour. 



According to the system of binomial nomenclature intro- 

 duced by Linnaeus, each kind of animal receives two names 

 one, the generic name, common to all the species of the 

 genus ; the other the specific name, peculiar to the species in 

 question. Both generic and specific names are Latin in 

 form, and are commonly Latin or Greek in origin, although 



