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 requirements 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 consider 
other kinds of flight when studying that of birds. Even those 
who are not interested in the abstruse problems of the mechanism 
