38 COMPARATIVE PHYSIOLOGY. 



that their cessation would arouse the attention of the least in- 

 structed ; food is demanded at regular intervals ; the juices of 

 the digestive tract are poured out, not constantly but period- 

 ically ; the movements by which the food is urged along its 

 path are markedly rhythmic ; the chemical processes of the 

 body wax and wane like the fires in a furnace, giving rise to 

 regular augmentations of the temperature of the body at fixed 

 hours of the day, with corresponding periods of greatest bodily 

 activity and the reverse. 



This principle finds perfect illustration in the nervous sys- 

 tem. The respiratory act of the higher animals is effected 

 through rauscular movements dependent on regular waves of 

 excitation reaching them along the nerves from the central cells 

 which regularly discharge their foi-ces along these channels.' 

 Were not the movements of the body periodic or rhythmical, 

 instead of that harmony which now prevails, every muscular 

 act would be a convulsion, though even in the movements of 

 the latter there is a highly compounded rhythm, as a noise is 

 made up of a variety of musical notes. The senses are subject 

 to the same law. The eye ceases to see and the ear to hear and 

 the hand to feel if continuously stimulated ; and doubtless in 

 all art this law is unconsciously recognized. That ceases to be 

 art which fails to provide for the alternate repose and 'Excita- 

 tion of the senses. The eye will not tolerate continuously one 

 color, the ear the same sound. Why is a breeze on a warm day 

 so refreshing ? The answer is obvious. 



Looking to the world of animate nature as a whole, it is 

 noticed that plants have their period of sprouting, flowering, 

 seeding, and decline ; animals are born, pass through various 

 stages to maturity, diminish in vigor, and die. These events 

 make epochs in the life-history of each species ; the recurrence 

 of which is so constant that the agricultural and other arrange- 

 ments even of savages are planned accordingly. That the in- 

 dividuals of each animal group have a definite period of dura- 

 tion is another manifestation of the same law. 



Superficial observation sufiices to furnish facts which show «j^ 

 that the same law of periodicity is being constantly exemplified 

 in the world of inanimate things. The regular ebb and flow of 

 the tides ; the rise and subsidence of rivers ; the storm and the' 

 calm ; summer and winter ; day and night — are all recurrent, 

 none constant. 



Events apparently without any regularity, utterly beyond 



H, 



