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. [Illustration: _Jays_] 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 of bird's flight, will find that comparisons made between birds, bats, butterflies and beetles when on the wing, are immensely interesting, and help to bring out the peculiarities of each. During the twilight hours of a still summer evening one may compare, with advantage, the rushing swoop of the screaming swift, borne with lightning speed upon long, ribbon-like pinions, with the curiously erratic flight of the woolly bat with beaded eyes, who has ventured abroad for his evening meal. One cannot but feel astonishment at the marvellous dexterity with which he twists and turns, now shooting up into the sky, now darting downward. What bird can beat him, or even match him, in the art of doubling back on his tracks? And one can put his skill at lightning turns to the test if one attempts to catch him in a butterfly net. Often indeed have I attempted this feat, but never yet with success.