QUADBUPEDS. 
327 
their habits of life and the medium in which they reside; 
yet these animals have far greater and 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 creatuies 
of this class are distinguished from all others. They suckle 
tlieir young , which are in all cases brought forth alive (»’. «• 
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. In 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 
