SOME ACCESSORIES 159 



body. In other words, he is parting rapidly with 

 caloric, and the result is that his temperature is 

 regulated as effectively as any mammal's, though 

 the system is in the main unlike that which operates 

 in ourselves. All mammals, of course, throw off 

 aqueous vapour from the lungs, but in most of them 

 perspiration plays a very large part. Before passing 

 on to other matters we must note that the air in 

 pneumatic bones can be of little or no service in 

 breathing, since it cannot be expelled at will. 



Repair of the Machine. 

 Very often unfair comparisons are drawn between 

 nature's machines and those which men manufacture. 

 It is forgotten that the former have not only to do 

 their special work, but also to keep themselves in 

 repair ; besides which they must reproduce them- 

 selves, i.e. they must be practically immortal. What 

 a contrast to this is presented by an aeroplane which, 

 without the constant attendance of skilled artificers, 

 is speedily reduced to helplessness ! Of the bird's 

 stoking I have already spoken. The repairing is 

 very wonderfully effected (see pp. 88, 89) without 

 the machine having to go into hospital. The great 

 wing-feathers on which flight depends and those in 

 the tail also are moulted in pairs, so that, though 

 not at his best while the process is going on, the bird 

 is at no time incapable of flight. A few birds, as I 

 have pointed out above, are exceptions, but under 

 their special circumstances Sightlessness , though, 

 no doubt, an inconvenience, does not as a rule bring 

 disaster. But for all warm-blooded beasts there is, 

 of course, a time of helplessness when they sleep and 



