problems which these differences present. To begin with, a 

 collection of breast-bones of different sj)ecies of birds with 

 their attached shoulder-girdles should be made, and these 

 should be studied together with careful observations of the 

 flight of the living bird. So far only a few comparisons of 

 this kind have been made. 



It must not be supposed that the whole secret of flight in 

 birds is concentrated in the skeleton of the breast-bone and 

 its shoulder-girdle, and the muscles attached thereto. But 

 those who would inv^tigate the modifications of the rest of 

 the body which have taken place in harmony with the require- 

 ments of flight, must turn to more learned treatises. There 

 is, however, one point which demands notice here. And this 

 is the popular belief that birds have the power of materially 

 reducing their weight when on the wing by drawing air into 

 their lungs, and storing it in large air-chambers enclosed 

 within the body. These chambers are indeed concerned with 

 the needs of flight. But the precise part they play is yet to 

 be discovered. They certainly have no effect of rendering 

 the body lighter. So far as our knowledge goes it would 

 seem that they act as regvdators of the temperature and as 

 reservoirs of breathing air, during the strenuous efforts of 

 flight. 



It is a mistake to suppose that it is unnecessary to con- 

 sider other kinds of flight when studying that of birds, » Even 



6 



