en. n.] BHYTIIM. 305 



have taken place since that event. Nevertheless, though we 

 cannot determine the amounts and durations of the move- 

 ments which have occurred during the geologic history of 

 the earth, we can still securely assert that these movements 

 have been rhythmical iD character. Though the verdict is 

 rendered with less precision, its purport is still the same. 

 In the alternating periods of elevation and depression which 

 have succeeded each other at different places ever since 

 the earth's crust began to be solidified, are exemplified the 

 chief geologic rhythms, due to the slow deflection of the 

 lines of least resistance along which the pressure of the 

 earth's nucleus reveals itself by causing upward motion. 

 But these immensely long rhythms are complicated by minor 

 rhythmical changes of surface, due to continual shifting of 

 river-beds and consequent variations in the areas of denu- 

 dation and in the deposit of sedimentary strata. And these 

 rhythms are still further complicated 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 sea-shore 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 illus- 

 trated, in biology. The commonest physiological act, such as 

 eating, is dependent upon a periodically occurring sensation 

 of hunger, due to a periodic excess of waste over repair. 

 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 



VOL. I. X 



