QUADRUPEDS. 



327 



their habits of life and the medium in which they reside; 

 yet these animals have far greater f?nd more important 

 affinities with Quadrupeds than with Fishes, and must 

 therefore be grouped with the former rather than with the 

 latter. 



The term " Mammalia," derived from mamma (the fe- 

 male breast), suggests a character of great physiological 

 value and of invariable application, by which the creatures 

 of this class are distinguished from all others. They suckle 

 their young, which are in all cases brought forth alive (». e. 

 not enclosed in eggs), and are nourished for a time with 

 milk, a fluid secreted in the body of the female parent. 



As, with, the trivial exception of the Bats, which flutter 

 in the air, the sphere of this Class is the solid earth or the 

 dense water ; the provisions for that energetic respiration 

 and for that high temperature of the blood, which are 

 necessary for Birds, are wanting here. In other respects, 

 however, the respiratory and circulating systems do not 

 essentially differ in the two Classes. 



The jaws are, almost invariably, furnished with teeth, 

 which play an important part in the economy of the 

 animal, being intimately associated with its sustenance. 

 They are solid pieces of bone covered with a much harder 

 substance called enamel, which grow out of sockets in each 

 jaw. They are placed in single series, and vary much in 

 form, according to the nature of the food which sustains 

 the animal, as well as according to their position in the 

 mouth. AJn Man, there are in each half of each jaw two 

 front teeth with a chisel-like edge, called Incisors, or cut- 

 ting teeth • one more pointed, called the Canine, or Dog- 

 tooth, or sometimes Eye-tooth ; two somewhat flattened at 



