156 Periodic Rhythms 



But it is plain that the word should not be used to 

 describe consistency or ethical conduct or a methodi- 

 cal way of living. It is very doubtful whether it is 

 profitably used in reference to a pathological state of 

 affairs when an artificial "urge" or craving has been 

 established in the body. The system calls out for the 

 physiological satisfaction afforded by the frequent 

 repetition of a stimulus or a sedative, such as a glass 

 or a pipe or an injection. But this is not in the strict 

 sense a habit. 



When the tide is low hermit-crabs persistently 

 avoid the light ; when it is high they make for it. This 

 periodic alternation has been engrained in the consti- 

 tution of the creature, for it will be exhibited for 

 about three weeks in an aquarium where there is no 

 tide. Similarly the well-known green worm, Con- 

 voluta, common on the Roscoff beach, comes up when 

 the tide goes down, and retires into the sand when the 

 splash of the incoming tide is near; and it will con- 

 tinue for some days exhibiting this alternation in a 

 tideless aquarium. A physiological periodicity has 

 been established in the body, which lasts even when 

 the accustomed stimuli are absent. This is verging 

 on the habitual, but some term like "periodic rhythm" 

 is probably more useful. It is well known that a man 

 or a domesticated animal may be miserable because of 

 some interruption of the accustomed periodicity of 

 three meals a day. Yet many a man, with a different 

 periodicity, is happy with two meals, and many a 

 well-trained dog with only one. 



In an ordinary act in any of the higher animals 



