352 NATURAL SCIENCE. Nov., 1893. 



be no great reduction of weight, since the exterior shell of the bones 

 forms a great part of their bulk. In the case of a larger bird, with 

 bones many times multiplied in size, but the thickness of the walls 

 increased very little, the removal of the 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 content eight times that of the 

 smaller cube. This will be true of other figures, so that if the 

 average diameter of one bone be double that of another, and if the 

 length also be double, its cubic content will be approximately eight 

 times as great. And as the walls thicken very little with increased 

 girth nearly all the enlarged interior can be filled with air. Clearly, 

 therefore, a large bird has much more to gain by dispensing with 

 marrow than a small one. 



The eagle, then, has gained in lightness. It must also have 

 gained in strength, for increased length of wing means an altogether 

 disproportionate increase of work. The longer the wing, the greater 

 the pace at which its extremity will move, and the resistance of the 

 air increases as the square of the velocity. 



The hornbills are a puzzle. The extreme shortness of the hand 

 bones, a ridiculous anticlimax following upon so grand an ulna and 

 so portentous a humerus, might suggest that they were once better 

 flyers, and that the wing is slowly undergoing reduction. But the 

 mountainous beak seems to show that colossal bones are an ancient 

 heritage of the family, and that even feeble flight might have 

 been difficult had they not become hollow. In either case they have 

 been very great gainers by aeration. 



Anyone who wishes to see clearly the relation in birds' bones of 

 slimness to solidity, and of large girth to aeration, should inspect 

 collections such as those at the Royal College of Surgeons, or at the 

 Natural History Museum at South Kensington, where a large number, 

 representing different families, may be seen side by side. It is easy, 

 then, to see that big, long-winged birds have wing-bones thicker in 

 proportion to their length in order to bear the far greater strain upon 

 them : the aeration of the bones has obviated the natural increase of 

 weight, which would have been a serious hindrance. But there 

 remains the perplexing physiological problem : what organ of the body 

 does the work that, in mammals, and presumably in birds with solid 

 bones, is done by the marrow ? 



F. W. Head ley. 



