14 



NATURAL HISTORY OF VERTEBRATE ANIMALS 



lowed at the present time. Beginning with any small group of 

 animals, such as a species of frog or grasshopper, one can follow 

 it into larger and larger groupings until the Phylum is reached, and 

 finally to the Animal Kingdom, which is coordinate with the other 

 great group of living things, the Plant Kingdom {cf. Fig. 1 17 and 

 p. 241). Conversely, if one should begin with the Animal King- 

 dom, one might conceivably follow it to everj^ subdivision until 

 all the species were reached, and thus pass in review all the varied 

 forms of animal life that are known to exist. Our present know- 

 ledge of classification is the result of an enormous amount of study, 

 by virtue of which hundreds of thousands of species have become 



Fig. 6. — A footless amphibian, Ccecilia, one of the Order fHyinnophiona or 



Apoda. a, anus. 



described and arranged in the manner indicated. This group- 

 ing of species is necessary as a means of cataloguing the multitu- 

 dinous organisms that make up the Animal Kingdom ; but classi- 

 fication is further significajit because it attempts to follow the 

 lines of evolutionary relationship and hence becomes equivalent 

 to a " family tree " of living beings. 



Distribution. — With few exceptions, the Amphibia are con- 

 fined to water and its immediate vicinity, or to a moist atmos- 

 phere. It is this characteristic that has given the name amphibian, 

 which means " leading two lives," to this class of the Vertebrata. 

 Many of the salamanders pass their entire lives in water. Most 

 species of frogs are amphibious. The common toad, Bufo ameri- 

 canus, passes the greater part of its life away from the water, but 

 is quite sensitive to atmospheric moisture. The reader may have 

 noticed that toads are seldom seen in exposed localities in times of 

 drought but will appear after a heavy rain. During the dry inter- 



