216 THE FROG CHAP. 



persons or places with Latinised terminations are em- 

 ployed. In giving the name of an animal, the generic 

 name is always placed first, the specific name following 

 it, and being written as a rule with a small letter. Thus 

 the common English frog is called Rana temporaria, and 

 the continental form referred to above, often spoken of 

 as the edible frog, Rana esculenta. 



You will probably have noticed certain differences in 

 colour and markings in the different individual frogs 

 you have examined, and it is matter of common observa- 

 tion that no two individuals of a species are exactly 

 alike. In the case of human beings and many of the 

 more familiar animals this is very apparent to everyone : 

 in other cases a more careful examination of the indi- 

 viduals is necessary in order to tell them apart ; thus, for 

 instance, the individuals in a flock of sheep appear all 

 alike to the casual observer, but the shepherd can easily 

 distinguish them from one another. These differences 

 we designate individual variations, and it is often difficult 

 to decide whether two kinds of animals should be con- 

 sidered as distinct species, or as varieties of a single 

 species, and no universal rule can be given for determin- 

 ing this point. Among the higher animals, mutual fer- 

 tility is a fair practical test, the varieties of a species 

 (e.g., common pigeon, fowl, dog, horse) usually breeding 

 freely with one another and producing fertile offspring, 

 while distinct species usually do not breed together, or 

 else produce infertile hybrids or mules. Compare for 

 instance the fertile mongrels produced by the union of 

 the various breeds of domestic dog with the infertile mule 

 produced by the union of the horse and ass. But this 

 rule is not without exception, and in the case of wild 

 animals is, more often than not, impossible of applica- 

 tion ; failing it, the only criterion of a " good species " 

 is usually the presence of constant differences from 



