THE INSURGENCE OF LIFE 161 



spell by flight, a solution which we shall presently discuss 

 in detail. Other creatures, unequal to the long and adven- 

 turous journeys of the birds, retire into winter-quarters, 

 in which they he low, awaiting happier days. Thus the 

 earthworms burrow more deeply than ever below the reach 

 of the frost ; the lemmings tunnel their winding ways be- 

 neath the icy crust of the Tundra ; all manner of insects 

 in their pupa-stages lie inert within cocoons or other pro- 

 tective envelopes in sheltered corners ; the frogs bury 

 themselves deeply in the mud of the pond, and lie there 

 mouth shut, nose shut, eyes shut, with the heart beating 

 feebly, and breathing through their skin ; and the slow- 

 worms coil themselves up together in the penetralia of their 

 retreats all trying to get below the deadly grip of the 

 frost's fingers, and usually succeeding. Let us take, as a 

 diagrammatic detail, Professor Arnold Lang's observa- 

 tion, that the heart of the snail beats more and more slowly 

 as the temperature falls and the animal sinks more deeply 

 into hibernation. A heart that can beat fifty times a 

 minute in summer may only beat 2-36 times a minute at 

 a temperature of 2-65C. in February. 



Very effective, too, is the deep hibernation of such mam- 

 mals as hedgehog, hamster, and marmot. The normal 

 power of ' warmbloodedness ', that is, of keeping an 

 approximately constant body-temperature, is in abeyance 

 for a time ; the body cools to a degree which in ordinary 

 life would be fatal ; the fat accumulated in days of plenty 

 is slowly burnt away ; irritability wanes to a minimum 

 and the ordinary reflexes are faint ; the heart beats feebly ; 

 the breathing movements may be scarcely perceptible ; 

 the creature steadily loses weight. But it keeps alive ! 



Others, again, such as the Arctic fox, the mountain hare, 



M 



