270 THE RHYTHM OF MOTION. 



thinned beneath and not covered by much sedimentary 

 deposit above, will become areas of least resistance, and will 

 then begin to yield to the upward pressure of the Earth's 

 contents; whence will result, throughout such areas, long 

 continued elevations, ceasing only when the reverse state 

 of things has been brought about. Whether this specula- 

 tion be well or ill founded, does not however affect the gen- 

 eral conclusion. Apart from it we have sufficient evidence 

 that geologic processes are rhythmical. 



85. Perhaps nowhere are the illustrations of rhythm 

 so numerous and so manifest as among the phenomena of 

 life. Plants do- not, indeed, usually show us any decided 

 periodicities, save those determined by day and night and by 

 the seasons. But in animals we have a great variety of 

 movements in which the alternation of opposite extremes 

 goes on with all degrees of rapidity. The swallowing of 

 food is effected by a wave of constriction passing along 

 the oasophagus; its digestion is accompanied by a muscular 

 action of the stomach that is also undulatory; and the peri- 

 staltic motion of the intestines is of like nature. The blood 

 obtained from this food is propelled not in a uniform cur- 

 rent but in pulses ; and it is aerated by lungs that alternately 

 contract and expand. All locomotion results from oscilla- 

 ting movements: even where it is apparently continuous, 

 as in many minute forms, the microscope proves the vibra- 

 tion of cilia to be the agency by which the creature is moved 

 smoothly forwards. 



Primary rhythms of the organic actions are compounded 

 with secondary ones of longer duration. These various 

 modes of activity have their recurring periods of increase 

 and decrease. "We see this in the periodic need for food, 

 and in the periodic need for repose. Each meal induces a 

 more rapid rhythmic action of the digestive organs; the 

 pulsation of the heart is accelerated; and the inspirations 

 become more frequent. During sleep, on the contrary, these 



