PHYSIOLOGICAL RHYTHMS. 297 



conditions play a large role, and the long summer days 

 as well as the long winter nights of the high north tend 

 to prolonged activity followed by prolonged rest. Some 

 happily constructed individuals can sleep for short 

 periods, and thus refreshed take up their work, being 

 fortunately independent of the place or hour. We 

 recognise this capacity as especially developed in the 

 animals whose principal sense is that of smell or hearing, 

 and to whom sleep is an ever-present resource, quite 

 independent of the time of day ; while the birds domi- 

 nated by vision are more strictly controlled by light, 

 and, with the exception of the nocturnal or crepuscular 

 species, in which the conditions are reversed, have their 

 repose determined by the darkness. Yet even in birds 

 during periods of excitement, as when breeding or 

 migrating, the influence of surrounding conditions may 

 be defied. On the other hand, birds mistake the dark- 

 ness of an eclipse for the evening, and return to roost. 

 Thus, among different groups of animals are to be 

 found the same wide variations in the rhythm of rest 

 and activity as among men ; at the same time the fact 

 is not without interest that the animals depending mainly 

 on the intermittent olfactory stimuli exhibit less marked 

 daily rhythm than do those controlled by the visual 

 stimuli that so perfectly dominate the birds. 



Like all rhythms, those of the nervous system demand 

 for their production variations in the stimuli or in the 

 condition of the system itself, or both. The least serious 

 changes are those caused by modifying the incoming 

 stimuli, whereas modifications of the physiological con- 

 dition of the nerve centres are more fundamental. 

 Normally, changes of the latter sort are brought about 

 by variations of the blood supply due to organic reflexes, 

 but to effect them drugs also are often used. It makes no 

 difference whether excitants or depressants are employed, 



