MAMMALIA. 5 



Fishes, which respire least of all, require to be sup- 

 ported in a fluid whose specific gravity is nearly as 

 great as their own. The last three of these classes 

 are marked, moreover, by producing young enclosed 

 in eggs, while the first bring forth living offspring. 



CLASS I. MAMMALIA.* 



It has always been usual to begin the great chain 

 of Nature with this class ; being that in which the 

 sensations and faculties are found in the highest state 

 of developement, and which in its highest link, MAN, 

 takes hold of a superior form of being, joining the 

 merely animal existence with that which is purely in- 

 tellectual and spiritual. 



It is not necessary, in a work like the present, to 

 enter into the details of its comparative anatomy fur- 

 ther than may suffice to give a clear notion of the 

 distinctive characters of the animals contained in it. 



Though, as the quantity of their respiration is but 

 moderate, they are generally designed for vigorous 

 motion on a solid support, a few can, by a modifica- 

 tion of the extremities, and an extension of certain 

 membranes, perform real and continued flight ; while 

 others, by a very different structure, are adapted foi 

 an aquatic life alone. Both the Bats and the Whales, 

 however, are true Mammalia. 



The upper jaw is always fixed to the skull ; the 

 lower, formed of but two pieces, is attached by a sort 

 of hinge to a fixed temporal bone ; the neck consists 



* Mamma, the female breast. 



