RHYTHM 



changes of surface, due to continual shifting of 

 river-beds and consequent variations in the areas 

 of denudation and in the deposit of sedimentary- 

 strata. And these rhythms are still further com- 

 plicated by rhythmic variations in the operation 

 of climatic agencies, entailing periodic changes 

 in the amount and distribution of rainfall, in the 

 size and movements of icebergs and glaciers, 

 and in the activity of frost. On the seashore 

 we may witness the compound rhythm of the 

 tides, in " which the daily rise and fall undergo 

 a fortnightly increase and decrease, due to the 

 alternating coincidence and antagonism of the 

 solar and lunar attractions; " a source from which 

 arise the most minute geologic rhythms, as those 

 which arise from the secular cooling of the 

 earth, and from its ever varying position in 

 space, are the most vast. 



But the subject of complex rhythms is still 

 better illustrated in biology. The commonest 

 physiological act, such as eating, is dependent 

 upon a periodically occurring sensation of hun- 

 ger, due to a periodic excess of waste over re- 

 pair. The taking of nutriment is accomplished, 

 in all animals, by a series of rhythmical motions, 

 — either the motions of cilia, or of sphincter 

 muscles, or of jaws, or indeed of all three at 

 once. Mr. Spencer adds that " the swallowing 

 of food is effected by a wave of constriction 

 passing along the oesophagus ; its digestion is 

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