AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



Those Amphibia which, like frogs and toads, have four well- 

 developed limbs, but are tailless, constitute the order ANURA, 

 and this includes a number of families, of which one, Eanidae, 

 comprehends frogs with long hind limbs, webbed hind feet, teeth 

 in the upper jaw, and tongue fixed to the front of the mouth- 

 floor. The most important genus in this family is Rana, to which 

 the common and edible frogs belong, and which is characterized 

 by the rudimentary nature of the thumb, by the presence of teeth 

 on the roof of the mouth, and the deeply forked tongue. Some 

 forty " kinds " or species of frog are included in the genus Rana, 

 and this is a convenient place to explain the principle upon which 

 scientific names are given to animals. On the binominal system , 

 which is universally adopted, each animal receives two names, 

 that placed first being the generic, while the other is the specific 

 name. The common or Grass Frog is known as Rana temporaria, 

 and the Edible Frog as Rana esculenta. The species temporaries is 

 marked by a number of distinctive features, among which may 

 be placed the presence of a dark blotch on each side of the head, 

 and the absence of croaking sacs in the male. It is scarcely 

 possible to say what is actually meant by the term " species," and 

 there is much difference of opinion in certain cases. The follow- 

 ing definition, given by De Candolle, will serve as well as any : 

 " A species is a collection of all the individuals which resemble 

 each other more than they resemble anything else, which can by 

 mutual fecundation produce fertile individuals, and which re- 

 produce themselves by generation, in such a manner that we may 

 from analogy suppose them all to have sprung from one single 

 individual." It is generally found impossible to obtain crosses 

 between different species, or if such crosses (hybrids) are pro- 

 duced these are, as a rule, infertile. 



The Frog is not a very good illustration of varieties, to ex- 

 plain which other animals may be taken. The Clouded Yellow 

 Butterfly (Colias edusa), for instance, is usually orange and black, 

 but a small proportion of yellow and black individuals are found, 

 and these constitute a variety* of the species edusa. Another good 

 example is the Field Snail (Helix hortensis), in which the colour 

 and striping of the shell vary in the most remarkable way, so that 

 the species hortensis has been split up into a large number of 

 varieties. In such a case, however, the varieties are perfectly 



* Hyale. 



