106 ANIMAL INTELLIGENCE 



never fail to notice them at considerable intervals, 

 say once in five to nine minutes, even when hibernating 

 most profoundly. The respirations were peculiar. 

 Sometimes one deep inspiration, preceded by a sudden 

 relaxation of the enlarged chest, would be succeeded by 

 a long pause ; again there would be a series of very 

 slight respiratory movements. It was always possible 

 by the respirations alone to predict when the animal 

 was approaching the waking state. 



The awakening was never sudden but gradual, often 

 extending over hours. I have seen something like this, 

 though less remarkable, in the bat. This is no doubt 

 protective to the vital machinery, for when Hall main- 

 tained that bats, suddenly awakened from the hiber- 

 nating condition, died speedily, he was, in the main, if 

 not entirely, correct. 



A brief consideration of natural daily sleep will throw 

 light on winter sleep, etc. 



Sleep is favoured by moderate exhaustion, a good 

 condition of nutrition, and the absence of all sorts of 

 peripheral stimuli. 



The case of the youth having but one good eye and 

 one hearing ear, who could be put to sleep by closing 

 these up, is very instructive. This lad did not, however, 

 continue to sleep indefinitely, but awoke after a number 

 of hours, showing that though there are certain 

 conditions in the environment that favour sleep, the 

 latter is essentially a condition of the central nervous 

 system, and dependent on laws governing the latter. 

 This view makes it clear that sleep is naturally a 

 nocturnal condition for most animals, owing, no doubt, 

 to the evolution of life in relation always to the 

 environment. The fact is, we cannot conceive of life 

 except in, and by reason of, in a sense, some environ- 

 ment. The change of the seasons, day and night, and 

 all the periodicities of the inorganic world have, as a 



