70 CLASSIFICATION AND CREATION. 



of Reptiles will show that they too are true to 

 this structural plan. These are the typical char- 

 acters of the whole branch, and exist in all its 

 representatives. 



What now are the different modes of express- 

 ing this structural plan that lead us to associate 

 certain Vertebrates together in distinct classes ? 

 Beginning with the lowest class, the Fishes are 

 cold-blooded, they breathe through gills, and they 

 are egg-laying ; in other words, though they have 

 the same general structure as the other Verte- 

 brates, they have a special mode of circulation, 

 respiration, and reproduction. The Reptiles are 

 also cold-blooded, though their system of circula- 

 tion is somewhat more complicated than that of 

 the Fishes ; they breathe through lungs, though 

 part of them retain their gills through life ; and 

 they lay eggs, but larger and fewer ones than the 

 Fishes, diminishing in number in proportion to 

 their own higher or lower position in their class. 

 They also bestow greater care upon their offspring 

 than most of the Fishes. The Birds are warm- 

 blooded and air-breathing, having a double cir- 

 culation ; they are egg-laying, like the two other 

 classes, but their eggs are comparatively few in 

 number, and the young are hatched by the moth- 

 er and fed by the parent birds till they can pro- 

 vide for themselves. 



The Mammalia are also warm-blooded and 



