THE LAW OF PERIODICITY OR RHYTHM IN NATURE. 37 
that their cessation would arouse the attention of the least 
instructed ; 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 muscular movements dependent on regular waves of 
excitation reaching them along the nerves from the central cells 
which regularly discharge their forces 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 wnconsciously 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 a single 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 suffices to furnish facts which show 
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 
any law of recurrence, when sufficiently studied are found to 
