THE ORGANISM AND ITS ENVIRONMENT 351 



region. The lungs are large to provide for a rapid exchange of 

 gases. The body temperature is high, which makes for more rapid 

 oxidative metabohsm with a consequently more rapid transforma- 

 tion of energy. The rate of energy transformation is high in pro- 

 portion to weight, a relation necessary for the flight of a heavier- 

 than-air object. The lungs are somewhat different from those of 

 other air breathers, in that the bronchioles communicate with each 

 other. Thus when air is drawn in, it circulates through the various 

 passages in a more or less continuous stream; the fine tubules do 

 not end in blind alveoli as in' other air-breathing vertebrates (p. 

 171). The bird in flight breathes by the action of the large muscles 

 of flight, their contraction and relaxation operating the lungs some- 

 what in the fashion of a bellows. By this arrangement energy is 

 conserved, for the effort put forth in flying also serves to ventilate 

 the lungs; the muscular act of flying includes the muscular act of 

 breathing. 



An examination of the structure of the wing (Figs. 212 and 85) 

 shows that by a reduction of the bony framework of the vertebrate 

 forelimb and by a marked increase in the size of the feathers, an 

 effective propelling structure, consisting of a broad surface with a 

 light framework, has been derived from the same essential features 

 which in other vertebrates develop into fins, forelegs, or forearms. 

 The flight of a self-propelled, heavier-than-air object requires great 

 power in proportion to weight, broad surfaces, a proper distribution 

 of weight with respect to such surfaces, and the reduction of wind 

 resistance to a minimum by stream-lining. The body of the bird 

 represents just this sort of structure, apparently modified from 

 ancestral land types whose characteristics were quite otherwise. The 

 bird body constitutes a most striking example of the modification 

 of the vertebrate body plan to adapt for a special type of locomo- 

 tion. This unified adaptation is impressive because it confers powers 

 that the human body does not possess. But from the standpoint of 

 the general subject of adaptations, all animals are similarly adapted 



