PERIODICITY 129 



seat of traditional and individual memory. Through 

 the long succession of days and nights that have 

 lightened and darkened animal history with their 

 alternate periods of greater activity and less, the 

 nervous system has attained a rhythm which tones 

 the activities of the body. This memory of bygone 

 recurring conditions is preserved for us in the slow 

 swing of the brain, which controls the impulse of the 

 moment by the set of its own impetus, gathered 

 through generations of alternate periods of rhythmic 

 change into a fundamental beat. We awake and 

 sleep not only because the sunrise and sun-setting 

 call us to uprising and downsitting, but because in 

 our life and that of our fathers the rising sun and 

 gathering darkness have impressed on our being day 

 by day and night after night an alternation of activity, 

 rapid heart-beat, daytime breathing, with relaxation, 

 slower pulse, and deeper breaths. At our usual hour 

 for sleep or wakening we tend to either, though dark 

 blinds or bright lights may delay our response to 

 habit. So the flowers that close at night will shut 

 their petals even though we turn their night into 

 day. Our fundamental action, if not, indeed, our 

 entire conduct, is thus no immediate response to the 

 calls that arise within us or that fall upon our senses, 

 but is determined to a variable extent by the tra- 

 ditional and by the periodic response of the nervous 

 system that has been established by habit ; and to 

 this habit it remains true, though for varying terms 

 in different cases, even if the original stimulus that 



