88 ANIMAL PHYSIOLOGY. 
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 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 
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 be- 
come very greatly compounded, doubtless often beyond recog- 
nition. 
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 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, 
Finally, and of special interest to the medical student, are 
