FLIGHT OF BIRDS. 289 



therefore necessary that their pursuers should be such perfect 

 flyers as to be constantly on the wing, either following or 

 seeking them. They also require a prodigious strength of wing 

 either to brave the storm or to soar to such an elevation as to 

 rise above its fury, like those Alpine travellers who from the 

 serene height of a giant mountain look down upon the thunder- 

 clouds below. Had not the swallows been endowed with a 

 lightning-like rapidity,- they would never have been able to 

 catch a sufficient quantity of the insects, winged like themselves, 

 on which they feed; and the tiny hummingbird, constantly 

 hovering from flower to flower, must have fallen an easy prey 

 to every more powerful carnivorous bird, if he had not been 

 able to dart along with the swiftness of a meteor. Besides the 

 expansive wings, the small head, the pointed beak, the long and 

 pliant neck, the gently-swelling shoulder, the tapering tail 

 (acting like the rudder of a ship, and enabling the bird to 

 rise or fall, or remain in a horizontal position), are all wisely 

 calculated to assist and accelerate motion through the yielding 

 air. 



The internal structure of birds is no less beautifully adapted 

 to the same purposes : all the bones are thin, and frequently 

 hollow ; and all the muscles, except those which are appropriated 

 to the purpose of moving the wings, are extremely delicate 

 and light ; the lungs are placed close to the backbone and ribs ; 

 the air, entering into them by a communication from the wind- 

 pipe, passes through and is conveyed into a number of membran- 

 aceous cells, which occupy a considerable space of the breast and 

 abdomen, and which can be voluntarily distended with air, like the 

 bladder which enables the fishes to rise in their native element. 

 The feathers with which the birds are invested are so appropriate 

 to their mode of life, that anything more perfect cannot possibly 

 be imagined. Not only are they peculiarly fitted by their 

 lightness as a raiment for creatures destined to hover through 

 the air, but from their being very bad conductors of heat they 

 afford the birds the best protection even against the extremes of 

 cold. Under the cover of its dense white plumage, the snow- 

 goose braves the terrific winters of the North, and the Antarctic 

 petrel endures the freezing winds of its inhospitable seas. 



Even under the equatorial sun, the condor soars high above 



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