Natural History 481 



world, whereas our temperature varies only by a fraction of a 

 degree as long as we are in good health. Now the Spiny Ant- 

 eater is a hibernator, and this is the clue we need : winter-sleeping 

 mammals are imperfectly warm-blooded. When the cold weather 

 sets in, it becomes difficult for them to adjust the debtor and 

 creditor account as regards heat ; they cannot produce enough to 

 make up for their loss, and they give up the attempt. They sink 

 back into a state of comparative coldness and cold-bloodedness; 

 they relapse into the ancestral reptilian condition. 



But if the imperfectly warm-blooded mammals which we 

 have mentioned were to fall asleep in the open, their body-tem- 

 perature would go down and down, and they would die. What 

 they must do is creep into some sheltered nook or comfortably 

 blanketed hole where the temperature soon becomes much higher 

 than that of the world outside. To this temperature that of the 

 sleeper's body approximates without there being any fatal results. 



Along with the snuggling into a confined space must be 

 taken the great reduction of internal activities, and here hiberna- 

 tion approaches the lethargy of frog and tortoise. Income is nil, 

 so expenditure must be reduced to a minimum. The heart beats 

 feebly, the breathing movements are scarcely perceptible, the 

 excretion or filtering which is the work of the kidneys comes to a 

 standstill. The hibernating body is like a fire well banked up in 

 its own ashes, and in an animal like the Hedgehog we know that 

 subtle changes come about in the recesses of the tissues. 



The gist of the matter is to be found in the three facts: (1) a 

 constitutional imperfection in the temperature-regulating ar- 

 rangements; (2) a creeping into a confined space which gets 

 warmed up a little; and (3) a great reduction of expenditure, for 

 even the internal activities come almost to rest. But there are 

 some contributory influences which must be recognised. After 

 the hard work of summer, there is naturally some fatigue and a 

 bodily bias towards rest. Moreover, summer has often been a 

 time of plenty, and the body has accumulated stores of fat and 



