VI FORM AND FUNCTION 109 
avery important physiological purpose.! 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 (eg. 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, 1 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. 
