CLASS II. AVES.* 



THE air is nearly as much the home of the Bird 

 as the earth is that of the Quadruped. Its whole 

 structure is, therefore, fitted for support and rapid 

 progression in that fluid. In order to this, several 

 modifications must be made in such a model-form 

 as any of those we have been describing. In the 

 first place, the weight of its bulk must be very 

 materially lightened : on taking a dead bird into the 

 hand, we cannot help being struck by its lightness, 

 as compared with a quadruped of the same size. 

 This is accomplished by hollowing out cavities in 

 every part of the body, and even in the bones them- 

 selves, in which the air is constantly circulating, so 

 that the whole of the interior has been said to be 

 " bathed in air." The bones of the limbs are strong 

 but thin tubes, the air occupying the place of mar- 

 row. The peculiar covering of the body is remark- 

 able, among other qualities, for its lightness in 

 comparison with its bulk; the feathers also being 

 penetrated by air. In fact, " the whole organization 

 is filled with air, as a sponge with water." Light- 

 ness alone, however, would not qualify a Bird for 

 flight, but merely for floating in the atmosphere ; 

 without strong muscular power, it would be, like 

 a tuft of thistle down, at the mercy of every blast. 



* Avis, a bird. 



