78 THE FLIGHT OF BIRDS 
other big, long-winged birds have wing-bones of 
larger girth (of a gi#@h, that is, disproportionate to 
their superiority in length of wing) in order that 
they may be able to bear the far greater strain put 
upon them. A proportionate increase in weight 
would, no doubt, have caused difficulty, and this 
has been obviated by the aeration of the bones. 
Pic,’ 22. 
Humerus of (1) Skua; (2) Rhinoceros Hornbill; (3) Sea Eagle 
—actual length 6} inches. Drawn toscale. F: Foramen, where 
the bronchial membrane enters, on the upper surface. 
To bring out this point more clearly I have taken 
the humerus of the Skua as the standard, and have 
calculated what would have been the length of the 
same bone in the Sea Eagle if it had been built on 
the same lines. 7 
After all this I have to admit that there remains 
a very puzzling case on which I cannot throw any 
light. The Hornbills are slow, heavy flyers, they 
are not very big, and they are the most pneumatic 
