POWER OF FLYING. 377 



ment of mechanical conformation which can be attained by 

 the development of a vertcbratcd structure. 



The power of flying is derived altogether from the resist- 

 ance which the air opposes to bodies moving throuo-h it or 

 acting upon it by mechanical impulse. In the ordinary 

 movements of our own bodies, this resistance is scarcely 

 sensible, 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, secondly, be sufficient mus- 

 cular 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 possess them in the same perfection. No quadruped, 

 except the bat, has sufficient muscular power in its .limbs, 

 however aided by an expansion of surface, to strike the air 

 with the force requisite for flight. No refinement of me- 

 chanic ingenuity has ever placed the Daedalian art of flying 

 within the reach of human power; for even if the lightest 

 possible wings could be so artificially adapted to the body as 

 to receive the full force of the actions of the limbs, however 

 these actions might be combined, they would fall very far 

 short of the exertion necessary for raising the body from 

 the ground. 



Examples, however, occur in every one of the classes of 

 vertebrated animals, where an approach is made to this fa- 

 culty. In the Exocastus, or flying-fish, the pectoral fins have 

 been enormously expanded, evidently for the purpose of en- 

 abling the animal to leap out of the water, and support itself 

 for a short interval in the air: but its utmost efforts are inade- 

 quate to sustain it beyond a few moments in that element, 

 Vol. I. 48 



