78 SEVEN- YEAK SLEEPERS 



nation is. Everybody knows nowadays, I suppose, that 

 there is a very close analogy between an animal and a 

 steam-engine. Food is the fuel that makes the animal 

 engine go ; and this food acts almost exactly as coal does 

 in the artificial machine. But coal alone will not drive an 

 engine ; a free draught of open air is also required in order 

 to produce combustion. Just in like manner the food we 

 eat cannot be utilised to drive our muscles and other organs 

 unless it is supplied with oxygen from the air to burn it 

 slowly inside our bodies. This oxygen is taken into the 

 system, in all higher animals, by means of lungs or gills. 

 Now, when we are working at all hard, we require a great 

 deal of oxygen, as most of us have familiarly discovered 

 (especially if we are somewhat stout) in the act of climbing 

 hills or running to catch a train. But when we are doing 

 very little work indeed, as in our sleeping hours, during 

 which muscular movement is suspended, and only the 

 general, organic life continues, we breathe much more 

 slowly and at longer intervals. However, there is this 

 important difference (generally speaking) between an 

 animal and a steam-engine. You can let the engine run 

 short of coals and come to a dead standstill, without im- 

 pairing its future possibilities of similar motion ; you have 

 only to get fresh coals, after weeks or months of inaction, 

 and light up a fresh fire, when your engine will immediately 

 begin to work again, exactly the same as before. But if an 

 animal organism once fairly runs down, either from want 

 of food or any other cause in short, if it dies it very 

 seldom comes to life again. 



I say ' very seldom ' on purpose, because there are a few 

 cases among the extreme lower animals where a water- 

 haunting creature can be taken out of the water and can 

 be thoroughly dried and desiccated, or even kept for an 

 apparently unlimited period wrapped up in paper or on the 



