io8 THE STRUCTURE AND LIFE OF BIRDS chap. 



(8) The bones of birds that are highly pneumatic 

 are, relatively to their length, larger in girth than those 

 of birds in which the aeration is but slight. 



(9) All young birds have solid bones. As they grow 

 to maturity, if pneumaticity is characteristic of the 

 species, the marrow dries up and the bronchial mem- 

 brane extends into the hollow. 



These facts look tangled and perplexing, but I 

 believe it is possible to some extent to unravel them. 

 I will begin by considering the case of the diving birds. 

 Much aeration of the bones would be an inconvenience 

 to them ; as it is, they can regulate the amount of the 

 body that appears above the water, sometimes sinking 

 till no more than the head is visible. Very often it is 

 impossible to see a Red-throated Diver swimming in a 

 mountain tarn. Only his neck stands out above the 

 water, and you cannot distinguish it among the reeds. 

 The Cormorant uses the same device, but he is not 

 equal to the Red-throated Diver in making himself 

 heavy or cork-like at pleasure. This power to vary 

 their specific gravity resides, no doubt, in the air-sacks, 

 which they can at will empty or innate. Sometimes 

 they help diving birds, it is thought, in another way : 

 those which lie under the skin about the neck 

 and breast of the Gannet may serve as air-cushions to 

 break his fall when he dashes into the sea from a 

 height of over 100 feet. 1 Aerated bones, on the other 

 hand, would be a hindrance and not a help to a diver, 

 for they would make it harder for him to swim under 

 water. Probably, too, the marrow in the bones serves 



1 See on this point a paper by Mr. F. A. Lucas in Natural 

 Science, January, 1894. 



