problems which these differences present. To begin with, a 
collection of breast-bones of different species 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 investigate 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 regulators 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 
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