VI FORM AND FUNCTION 113 
marrow will be a great advantage. This will be clear 
if we take two cubes, a side of one of which is twice 
the length of a side of the other. 
Then the face is four times as large and the cubic 
contents eight times as large. This will be true of other 
figures besides cubes, so that if the average girth of one 
bone be double that of another, and if the length also be 
double, its cubic contents will be approximately eight 
/ 
a 
Fic. 29.—Cubes. 
times as great; and as the walls do not thicken in pro- 
portion to the increased girth, nearly all the enlarged 
interior can be filled with air. Clearly, then, a large 
bird has much more to gain by dispensing with marrow 
than a small one. 
Thus the Eagle has gained in point of lightness. It 
must also have gained in point of strength, for in- 
creased length of wing means an altogether dispro- 
portionate increase of work. The longer the wing, 
I 
