I02 DARWINISM AND THE PROBLEMS OF LIFE 



and so lessen its specific gravity. The latter point is 

 undoubtedly correct, and it is also true that a thin 

 atmosphere offers less resistance to flight than a denser 

 one. However, the chief use of the air-sacs, which 

 extend in the form of hollow outgrowths of the lungs 

 into the body-cavity and the bones, and between the 

 muscles, is now thought to be that they spare the bird 

 the trouble of respiratory movements during flight. In 

 particular, the reservoirs of air between the flying-muscles 

 are pressed together like bellows by the movement of 

 the wings, and thus to some extent automatically renew 

 the air in the lungs. 



Besides this adaptation for flying there are many 

 others. The bird is, as we say, a mass of adaptations. 

 There is on its breast-bone a strong comb to which 

 the flying - muscles are attached ; the whole of the 

 shoulder-blade is most beautifully constructed ; and the 

 rigid pelvis, formed by the fusion of a number of 

 vertebrae, enables it to keep upright while sitting. 

 In harmony with the laws of leverage all the heavy 

 parts are placed in the centre of the frame ; the crop 

 supplies the place of teeth ; the muscles of the leg are 

 developed well up in the body, so that the limbs 

 themselves are thin and light ; and a particularly good 

 digestion enables it to disburden itself at one effort. 

 In a word, we could show the adaptiveness of every 

 single organ, to say nothing of the feathers. Every 

 stroke of the wings lifts the bird up ; and as the 

 wings are lifted the air rushes between the feathers, 

 so that they meet no resistance. A wind coming 

 from the front fills the wings, and enables the creature 



