vi FORM AND FUNCTION 109 



a very important physiological purpose. 1 Divers are 

 frequently exposed to great cold when in the water. 

 They are protected against this by a peculiarly thick 

 coat of feathers, and by a deep layer of fat beneath 

 the skin ; and I cannot help thinking that the marrow 

 also helps to maintain their warmth. In other animals 

 it is held to be the birthplace of a large proportion of 

 the red blood-corpuscles, and unless they are very thick 

 in the blood, a high temperature cannot be maintained. 

 But if the marrow is a factory of red corpuscles, what 

 substitute for this have birds whose chief bones have 

 only a thin lining of marrow from which the output 

 must be small ? Though, as a rule, exposed to less 

 cold than diving-birds, they show in severe weather a 

 very great power of generating heat. Birds as a class 

 have more red corpuscles than any other animal. Is 

 the spleen, which in emergencies (e.g. when much 

 blood is lost) is a great red corpuscle factory, more 

 developed in birds which have little or no marrow ? 

 The vital organs are sometimes strangely versatile. 

 When an animal's spleen is removed, its work is done 

 somewhere else in the body and no ill effects are felt. 

 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 in the amount of 

 aeration, and why such a poor flyer as the Hornbill 

 is, in respect of bones, so well equipped for aerial navi- 

 gation. To put physiology aside, is to assume that if 



1 Physiology is the science which aims at explaining the work 

 done by the different organs of the body. It deals with all the 

 processes which maintain life. 



