jAN.,i894. AIR-SACS AND HOLLOW-BONES OF BIRDS. 37 



or rapidly descends from a great elevation in the air, as do some of the 

 vultures. Soaring birds, such as the vultures, screamers, and some 

 storks, certainly have very pneumatic bones; but I do not pretend 

 that the above theory accounts for the fact, it is merely a suggestion 

 as to one use of the character, and while the theory works very well 

 when applied to such birds as the Gannet and Vulture, it fails in the 

 case of the hornbills. Here, perhaps, we may fall back on the very 

 convenient and much-abused theory of an inheritance from some active 

 ancestor, but this again is merely a suggestion. 



It is somewhat curious that of two nearly related birds one may 

 have certain bones pneumatic, and the other non-pneumatic, a point 

 well illustrated by the titmice. It is, too, a general rule that animals 

 of rapid movements have light bones, and sluggish or aquatic animals 

 non-pneumatic skeletons ; and aside from the gain in size without 

 increase of weight, which is rendered possible by a hollow bone, may 

 there not be an actual oxygenation of the blood in the bones ? 



Frederic A. Lucas. 



