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38 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 be borne in mind that the time standard is for an irregularity that seems large, as in the instance just referred to, becomes small when considered in relation to the millions of years of geological time; while in the case of music a trifling irregularity, 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 several, it follows that they may seem, until 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 become very greatly compounded, doubtless often beyond recognition.

Among the best examples of rhythm in animals are daily sleep and winter 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 manner 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 deposition, 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.

Finally, and of special interest to the medical student, are