350 



NATURAL SCIENCE, 



Nov., 



very great power of generating heat. Birds, as a class, have more 

 red corpuscles than any other animals. Is the spleen, the other 

 great red corpuscle factory, more developed in birds which have little 

 or no marrow ? These physiological problems are of the greatest 

 interest, and I only wish that I were in possession of facts that would 

 throw light upon them. 



Putting physiology out of sight, I am going now to consider why 

 it is that, among birds of powerful flight, we find differences so great 



.-.<S6& 



Humerus of Pomatorhine Skua (a), Vociferous Sea Eagle (b), and Rhinoceros 

 Hornbill (c). 



in the amount of aeration, and why such a poor flyer as the hornbill, 

 is, in respect of bones, so well equipped for aerial navigation. To 

 put physiology aside, is to assume that if hollow bones are advan- 

 tageous to a bird, Natural Selection can bring it about that they 

 become hollow, and that the bird is able to dispense with the 

 marrow. This would indeed be a bold assumption, did we not 

 know that it is an accompUshed fact. The bones are hollow and the 

 processes of life continue. 



