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GENERAL BIOLOGY. 39 



any law of recurrfence, when sufficiently studied are found to 

 fall under the same principle. Thus it took some time to learn 

 that volcanic eruptions occurred with a very fair degree of 

 regularity. 



In judging of this and all other rhythmical events it must 

 he home in mind that the time standard is for an irregularity 

 that seems large, as in the instance just referred to, becomes 

 small when considered ia relation to the millions of years of 

 geological lime; while in the case of music a trifling irregu- 

 larity, judged by fractions of a second, can not be tolerated by 

 the musical organization — ^which is equivalent to saying that 

 the interval of departure from exact regularity seems large. 



As most of the rhythms of the universe are compounded of 

 ■Sfevieral, it follows that they may seem, untU closely studied, 

 very far from regular recurrences. This may be observed in 

 the interference in the regularity of the tides themselves, the 

 daily changes of which are subject to an increase and decrease 

 twice in each month, owing to the influence of the sun and 

 moon being then either coincident or antagonistic. 



In the functions of plants and animals, rhythms must be- 

 come very greatly compounded, doubtless often beyond recog- 

 nition. 



AlHong the best examples of rhythm in animals are daily 

 sleep and wint^ sleep, or hibernation ; yet, amid sleep, dreams 

 or recurrences of cerebral activity are common — that is, one 

 rhythm (of activity) overlies another (of repose). In like man- 

 ner many hibernating animals do not remain constantly in their 

 dormant condition throughout the winter months, but have 

 periods of wakefulness ; the active life recurs amid the life of 

 functional repose. 



To return to the world of inanimate matter, we find that the 

 crust of the earth itself is made of layers or strata the result of 

 periods of elevation and depression, of denudation and deposi- 

 tion, in recurring order. 



The same law is illustrated by the facts of the economic and 

 other conditions of the social state of civilized men. Periods 

 of depression alternate with periods of revival in commercial 

 life. 



There are periods when many more marriages occur and 

 many more children are born, corresponding with changes in 

 the material conditions which influence men as well as other 

 animals. 



