114 A COURSE ON ZOOLOGY. 



posit its eggs, impelled by that instinct of necessity that 

 makes known to all animals the conditions most favor- 

 able for the preservation of their young, and consequently 

 of their species. 



Frogs live on the borders of water and feed on in- 

 sects and worms, and during cold weather they bury 

 themselves in the mud of marshes. The green frog is 

 common in ponds, and is sought after as food in many 

 countries. Brown frogs and toads prefer gardens, where 

 they render great service by destroying snails and de- 

 structive insects. The little green tree-frogs have suck- 

 ers in the ends of their digits, by the aid of which they 

 can climb trees and crawl from branch to branch in search 

 of the insects on which they live. 



In conclusion, we must lay stress on the importance in 

 classification of the metamorphosis of the frog. The 

 organization of the tadpole, absolutely that adapted to 

 aquatic life, relates the amphibians to the fishes, while by 

 the organization of the adult animals they are related to 

 other terrestrial animals. The amphibians thus form a 

 natural passage from the study of terrestrial animals to 

 that of fishes. 



CHAPTER XI. 

 The Carp. 



THE carp belongs to the class of fishes, and we will study 

 it as the type of this category of animals. An exami- 

 nation of its external character shows us at once that 

 there are great modifications that distinguish it from 

 all that we have so far studied. Its organization, like 



