SOME ACCESSORIES 159 
body. In other words, he is parting rapidly with 
caloric, and the result is that his temperature is 
regulated as efiectively 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 flightlessness, 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 
