546 THE MECHANICAL FUNCTIONS. 



extensive plan, and the most artificial combina- 

 tion of means. The foundations for these pecu- 

 liar forms of mechanism are laid in the primeval 

 constitution of the embryo ; and a long and curious 

 series of preparatory changes must take place 

 before the completion of the finished structures. 

 Of this we have already had a remarkable ex- 

 ample 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, present 

 us with the highest refinement of mechanical 

 conformation which can be attained by the de- 

 velopement of a vertebrated 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 sen- 

 sible, and hardly ever attracts notice : but it 

 increases in proportion 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, se- 



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