482 THE MECHANK.AL FUNCTIONS. 



Structures. Of this we have already had a remark- 

 able example in the metamorphoses of insects, 

 which exhibit, in their last stage of developement, 

 the highest degree of perfection compatible with 

 the articulate type. Birds, in like manner, pre- 

 sent us with the highest refinement of mechanical 

 conformation, which can be attained by the deve- 

 lopement of a vertebrate structure. 



The power of flying is derived altogether from 

 the resistance which the air opposes to bodies 

 moving through it, or acting upon it by mecha- 

 nical impulse. In the ordinary movements of our 

 own bodies, this resistance is scarcely sensible, and 

 hardly ever attracts notice ; but it increases in pro- 

 portion to the surface which acts upon the air, and 

 still more according to the velocity of the moving 

 body ; for the increase is not merely in the simple 

 ratio of the velocity, but as its square, or perhaps 

 even a higher power. In order that an animal may 

 be able to fly, therefore, two principal conditions 

 are required : there must, first, be a considerable 

 extent of surface in the wings, or instruments which 

 act upon the air ; and there must, secondly, be 

 sufficient muscular power to give these instruments 

 a very great velocity. Both these advantages are 

 found combined in the anterior extremities of birds, 

 and no animals belonging to any other class pos- 

 sess them in the same perfection. No quadruped, 

 except the Bat, has sufficient muscular power in 

 its limbs, however aided by an expansion of sur- 

 face, to strike the air with the force requisite for 

 flight. No refinement of mechanic ingenuity has 

 ever placed the Daedalian art of flying within the 

 reach of human power ; for even if the lightest 



