RAPIDITY OF BIRDS. 215 



acts we have determined, the bird is that which, after the 

 insect, has given the most rapid movements. 



This rapidity is indispensable to flight. In fact, the wing 

 when lowered, can meet with a sufficient resistance in the 

 air only when it moves with great velocity. The resistance 

 of the air against a plane surface which strikes upon it 

 and repels it, evidently increases in the ratio of the square 

 of the velocity with which this plane is displaced. It 

 would be of no use for the bird to have energetic muscles, 

 capable of effecting considerable work, if they could only 

 give slow movements to the wing ; their force could not 

 be exerted for want of resistance, and no work could be pro- 

 duced. It is otherwise with terrestrial animals which run or 

 creep on the ground, with a speed more or less rapid accord- 

 ing to the nature of their muscles, but which in every case 

 utilize their muscular force by means of the perfect resistance 

 of the ground. The necessity of velocity in the movements 

 of fishes has been already observed, since the water in which 

 they swim resists more or less, according to the rapidity 

 with which their tails or their fins act upon it. Thus the 

 muscular action of fishes is rapid, but much less so than 

 that of birds, which move in a medium far more yielding. 



In order to understand the rapid production of movements 

 in the muscles of the bird, we must remember that these 

 movements are connected with chemical action, produced in 

 the very substance of the muscle, where they give rise, as in 

 machines, to heat and motion. We must therefore admit 

 that these actions are excited and propagated more readily 

 in the muscles of birds than in those of any other species of 

 animals. In the same manner the different kinds of powder 

 used in war differ much from each other in the rapidity of 

 their explosion, and consequently give very different velocities 

 to the projectiles which they impel. 



Lastly, the form of movement presents in different species 

 of birds peculiarities which we have already noticed. We 

 have seen in Chapter VIII. how much the dimensions of the 

 pectoral muscles vary according as the strokes of the wing are 

 requii-ed to have much force or great extent ; therefore we 

 shall not recur again to this subject. 



