216 THE FROG CHAP. 



frequently modern names of persons or places with latinised 

 terminations are employed. 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 temporaries, 

 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 observation 

 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 every one : in other cases a 

 more careful examination of the individuals 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 considered as distinct species, or as varieties of a 

 single species, and no universal rule can be given for deter- 

 mining 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 dis- 

 tinct species usually do not breed together, or else produce 

 infertile hybrids or mules. Compare, for instance, the fer- 

 tile 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, impos- 

 sible of application ; failing it, the only criterion of a " good 

 species " is usually the presence of constant differences from 



