498 THE WONDER OF LIFE 



into a normal animal the serum, or, better, the cerebro- 

 spinal fluid of an animal exhausted by loss of sleep. In 

 about half an hour there is induced an imperative need for 

 sleep. 



' The animal so injected is benumbed little by little, its 

 eyelids blink, its limbs relax, its eyes close, it loses all 

 attention, and it responds but feebly to strong stimulation. 

 Its brain presents the characteristic lesions of insomnia. 

 The injections, under the same conditions, of liquids from 

 a normal animal have no effect at all'. 



It seems evident, then, that in some form or other, the 

 injection from the exhausted animals acts like a sleeping 

 draught. 



It seems to us very probable that there are many cases 

 where this general biological theory of sleep is quite suffi- 

 cient. The successful long-lived animals are those that 

 can take rests. There has been an age-long selection of 

 the methodical, who work when they work, and rest when 

 they rest. An established rhythm of alternate working 

 and resting pays best, and it has become conveniently 

 hitched on to the great external periodicity of day and 

 night. The works have to be slowed down to permit of 

 re-stoking and thorough cleaning, and these functions are 

 effected most readily when their recurrence is rhythmic. 



It is very interesting, however, to find Legendre, physiolo- 

 gist as he is, declaring that although physiology has im- 

 portant and fundamental contributions to offer towards 

 a theory of sleep, ' physiology alone cannot dream of solving 

 the problem '. We understand that his view has particular 

 reference to Man and the higher vertebrates, where it seems 

 that psychological factors must also be taken into account. 



