30 ANIMALIA VERTEBRATA. 



the form of their respiratory organ, and to that of Fishes from the nature 

 of their surrounding element. 



The quantity of respiration in Birds is even superior to that of Qua- 

 drupeds, not only because they have a double circulation and an aerial 

 respiration, but also because they respire by many other cavities besides 

 the lungs, the air penetrating throughout their bodies, and bathing the 

 branches of the aorta, as well as those of the pulmonary artery. 



Hence result the four different kinds of motion for which the four 

 classes of vertebrated animals are more particularly designed : Quadru- 

 peds, in which the quantity of respiration is moderate, are generally 

 formed to walk and run, both motions being characterised by precision 

 and vigour: Birds, which have more of it, possess the muscular strength 

 and lightness requisite for flight ; Reptiles, where it is diminished, are 

 condemned to creep, and many of them pass a portion of their lives 

 in a kind of torpor; Fishes, in fine, to execute their motions, require 

 to be supported in a fluid whose specific gravity is nearly as great as 

 their own. 



All the circumstances of organisation peculiar to each of these four 

 classes, and those especially which regard motion and the external 

 sensations, have a necessary relation with these essential characters. 



The Mammalia, however, have particular characters in their vivi- 

 parous mode of production, in the mammae by which they suckle their 

 young, &c. 



The other classes, on the contrary, are oviparous ; and if we compare 

 them to the first, we shall find such numerous points of resemblance 

 as announce a peculiar system of organization in the great general plan 

 of the Vertebrata. 



CLASS I. 



MAM.MAMA. 



THE Mammalia are placed at the head of the animal kingdom, not 

 only because it is the class to which Man himself belongs, but also 

 because it is that which enjoys the most numerous faculties, the most 

 delicate sensations, the most varied powers of motion, and in which all 

 the different qualities seem combined in order to produce a more 

 perfect degree of intelligence the one most fertile in resources, most 

 susceptible of perfection, and least the slave of instinct. 



