( 90 ) 



ill the struggle for life the dangers threatening them in the daj-tinie, 

 have led certain species of animals to shorten the day and adequately 

 to lengthen the night, in the course of which process qualities were 

 slowly developed, enabling them to carry on with more surety 

 that struggle at night, whilst the want of sleep was satisfied during 

 day-time. 



As regards the phenomena of winter-sleep and summer-sleep, both 

 may be considered as a state of torpor, being no real sleep, and in 

 all probability originated again in the struggle for life by certain 

 animals digging tliemselves into the earth, after their having been 

 driven away by stronger species to regions, either too cold or too 

 hot. Only the strongest indi\'iduals survived, and after the lapse of 

 a long period, their progenj' may have gradually attained to the 

 power of remaimng alive, for a definite space of time, almost entirely' 

 without functions, as an hereditary quality, no longer dependent on 

 the influences of heat and cold. 



As impossible as it would be for modern man to be kept from 

 sleeping for a somewhat longer period by means of artificial light, 

 as impossible it would be to keep a winter- or summersleeper out 

 of its state of torpor by means of heat or cold, once the season for 

 that state having returned. 



It is not known to us whether amongst animals living under- 

 ground or in the deep of the sea, there exist any species capable of 

 living without sleep. 



Until a period not so very long ago, sleep for the greater part 

 of humanity was wholly determined by the sun. During summer man 

 slept little, during winter much, and even in our modern times the 

 peasant does not consult science about the term of duration of his 

 sleep, as his period of sleeping is determined by the sun. The stimuli 

 that keep him awake (issuing from his soil, his cattle and his machinery), 

 all cease to oi)erate with the setting of the sun, consequently he 

 goes to sleep and is awakened again by the stimulus of the sunlight, 

 either directly or indirectly by intermediary of animals. 



In modern times the way iji which by far the greater majority 

 of men are living, gives rise to the question whether the want 

 of sleep in man may not perhaps wholly or partially disappear 

 in the course of the struggle for life, because we know that inherited 

 qualities tend to disa])pear, when they are no longer of use in that 

 struggle. 



Partly at least this want of sleep has ali-eady been conquered in 

 many instances: numerous men are night-animals, sleeping only for 

 a short period in the day-time, others continue to enjoy unimpaired 



