THE FLIGHT OF BIRDS 297 



(rectrices). Wing-quills and tail-quills are respectively covered 

 to some extent by wing-coverts and tail-coverts. 



The structure of feathers has already been dealt with (see 

 vol. i, p. 142), and it need only be repeated here that they com- 

 bine lightness with strength in a remarkable way, and that the 

 little branches which make up the thin expansion on either side 

 of the central shaft are hooked together in an elaborate manner. 

 We have no direct evidence regarding the evolution of feathers, 

 but they may be supposed to have originated by the fraying out 

 of the scales no doubt possessed by ancestral forms. And it is 

 interesting to note that the legs of birds are still partly clothed 

 by overlapping scales. 



The body of a Bird is light in proportion to its bulk, for 

 although the bones have a dense external layer, they are other- 

 wise of spongy texture, and commonly more or less traversed 

 by air-spaces, especially in forms which fly well. The lungs 

 communicate with large air-sacs, which take up a good deal of 

 space within the body, and communicate with some of the air- 

 sacs in the bones. Not only is an increase of lightness gained 

 in this manner, but also a larger external surface for the attach- 

 ment of muscles than would otherwise be the case. Too much 

 stress must not be laid upon the lightness of the body, and it 

 would be entirely erroneous to compare a bird with a balloon. 

 It would be much more correct to draw a parallel between it 

 and a flying-machine, for active flight is a matter of sheer mus- 

 cular effort. Even here, however, the principles of the kite and 

 parachute are involved, and this is still more the case in those 

 varieties of movement through the air known as passive flight. 

 The heavier parts of the body are placed towards the under 

 surface, and stability is thus secured. 



The immense amount of energy displayed by flying birds is 

 associated with great activity of the circulatory and breathing 

 organs, as to which they stand pre-eminent among backboned 

 animals (see vol. ii, p. 426). The chief significance of air-sacs 

 and air-spaces is probably to be found in the fact that they 

 increase the efficiency of the lungs. This activity is of course 

 associated with a rapid wasting of the substance of the body, 

 which has to be made good by a correspondingly large amount 

 of food. And it is therefore not surprising to find that birds 

 are possessed of large appetites and great digestive powers. 



Vol. III. 82 



