482 The Outline of Science 



other reserves, which may also incline the creature to somnolence. 

 And once the quiescence has begun, it will tend to continue, for 

 the closeness of the retreat must be soporific, and the cessation of 

 the kidney functions will tend to keep the sleepers sleepy. Just 

 as drowsiness sometimes sets in when man's kidneys are not work- 

 ing rightly, so in the hibernating mammal there may be a poison- 

 ing of the body with its own waste-products a sort of 

 ' 'auto-intoxication. ' ' 



Yet this is not all. We must not think of hibernation as an 

 individual reaction merely; it expresses a racial rhythm. In the 

 course of thousands of generations a certain periodicity has been 

 established, like that of our sleepiness at night and wakefulness 

 in the morning, and with the enregistered bodily rhythm there is 

 associated an instinct which prompts the hibernator to seek out a 

 comfortable corner when the weariness or sleepiness sets in. For 

 ages, it must be remembered, our hedgehogs have not known any 

 winter. They have slept through them all, just as the migratory 

 birds have circumvented them all. It must be remembered, too, 

 that the winter-sleep or hibernation of an animal like the Hedge- 

 hog cannot be distinguished from the summer-sleep or aestivation 

 of the Tenrec of Madagascar. 



Only a few mammals are hibernators, and some of these, like 

 the Dormouse, are "light sleepers," while others, like the Hedge- 

 hog, are "deep sleepers." In all cases there is some imperfection 

 in the warm-bloodedness, and what has been wrought out is what 

 we might call a rather neat way of making a strength out of 

 weakness. There is a relapse to a reptilian condition, but this 

 handicap is counteracted. For it is not merely that the difficulties 

 of the winter scarcity, cold, and storms are circumvented ; the 

 hibernation gives an opportunity for a long rest, which even the 

 food-canal may be the better for. There may be an opportunity 

 for processes of recuperation or rejuvenescence to stave off the 

 processes of senescence or ageing. Why, then, are there not more 

 hibernators? The answer must be that hibernation is the "answer- 



