INTRODUCTION. 35 



mediate contact with the oxygen of the air. The 

 same is true in a still higher degree respecting 

 birds. But the case is widely different with the 

 two remaining classes. In reptiles the respiratory 

 organs are themselves greatly deficient, compared 

 with the same organs in the two higher classes; 

 and in fishes the difference is still more conspi- 

 cuous, inasmuch as they have no lungs at all, hut 

 gills, which execute their functions through the 

 medium of water, their hlood heing thus acted 

 upon only by the portion of oxygen which is con- 

 tained in that fluid. From this diversity of struc- 

 ture results, in the words of Cuvier, " the four 

 different kinds of motion for which the four classes 

 of vertebrated animals are more particularly de- 

 signed, quadrupeds, birds," &c. 



Many specific characters distinguish these four 

 classes of the vertebratae from each other, and espe- 

 cially the two which constitute the extremes, viz. 

 quadrupeds and fishes. The former are viviparous 

 in their mode of reproduction, and suckle their 

 young; which circumstance being peculiar to this 

 division, they hence derive their ordinary appella- 

 tion of Mammalia; a distinction the surest, as 

 well as the most apparent, of all external charac- 

 ters. The latter again are oviparous, and deposit 

 the countless millions of their spawn in the shel- 

 tered creek or the shallow brook. Quadrupeds 

 are warm-blooded animals, having lungs contained 

 in a regular chest, and are more or less covered 

 with hair or wool to protect them from change of 



