The natural world in which we live, and of which we humans constitute a part, works on the basis of cycle. It is a commonplace of physics that every process tends to run down and stop because the energy back of it becomes exhausted in overcoming resistance, and changes to some other form. Whatever may seem to be a continuing process is really a moving equilibrium which continues by rhythm or cycle; one force dominates for a time, only to yield the dominance sooner or later to some other force. Every process goes by ebb and flow like the tides of the sea. A stream has a winding course because the current impinges against the banks, first on one side, and then on the other. Day alternates with night, winter alternates with summer. There are also longer cycles: for a series of years there is abundant rainfall, then for some years succeeding these the rainfall is scanty; sun-spots have their cycles, and electrical storms; and probably also earthquakes and glacial epochs; the precession of the equinoxes makes a cycle of 26,000 years. But nature makes no more use of regular curves that form exact circles than she does of straight lines. Her curves are segments of ellipses or hyperbolas; the elliptical curves, however, when carefully followed, usually do not return to themselves to make true ellipses but run into cycloids. Nevertheless, rhythm in the physical world is everywhere present in some form or other.

As from antagonist physical forces, as from antagonist emotions in each man, so from the antagonist social tendencies men's emotions create, there always results, not a medium state, but a rhythm between opposite states. The one force or tendency is not continuously counterbalanced by the other force or tendency, but now the one greatly predominates, and presently by reaction there comes a predominance of the other. . . - Spencer, The Study of Sociology, p. 164.

The approach of the second glaciation is indicated along the southeast coast of Great Britain by the subsidence of the land and the rise of the sea, accompanied by a fresh arctic current, bringing with it an invasion of arctic mollusks which were deposited in a layer of marine beds directly over those which contain the rich warm fauna and flora of the "Forest Bed of Cromer," Norfolk. It also appears probable that a cold northern current swept along the western coasts of Europe, and Geikie estimates that a lowering of temperature occurred of not less than 20 degrees Fahr., a change as great as is now experienced in passing from the south of England to the North Cape.

. . . The largest of the present glaciers of the Pyrenees is only 2 miles in length and terminates at a height of 7200 feet above the sea. During the greatest glaciation the snow appears to have descended 4265 feet below its present level. From the Pyrenees . . . into Spain there flowed a glacier 38 miles in length, while to the north the glacier of the Garonne flowed for a distance of 45 miles. . . . Even in its lower reaches this glacier was over half a mile in thickness. To the east was a glacier 38 miles in length. . . .

. . . The climate immediately following the retreat of the glaciers was cool and moist in the glaciated regions, but this was followed by such a prolonged period of heat and dryness that the glaciers on the Alps withdrew to a point far above their present limits. - Osborn, Men of the Old Stone Age, pp. 86-00.

With living things the rhythmical process underlying all others is the one which the physiologist calls metabolism. One phase of it is anabolism; the taking of nutrition and the assimilation of it into the cells which compose the various tissues of the body. The other phase is katabolism: the using up of the stored material in producing activity of one kind or another. Since this second phase involves oxidation, we may assist our conception of it by likening it to the burning of fuel under the boiler of an engine. These two phases tend to alternate with each other, though neither ever ceases entirely; strictly speaking, it is the periods of stress that alternate. Stimulus comes to an animal through its nervous system; the animal plays, runs from enemies, hunts food, or builds a home, according to the nature of the stimulus and the conditions of its organs. This activity exhausts the tissues of the organs used, thus causing hunger and weariness; the animal eats and sleeps and then eats again; the anabolic process becomes predominant. This alternation of anabolism and katabolism is sometimes very brief, as in the movements of the heart or those of the sense organs. The eye can gaze at an object without interruption for only a few seconds, then it gets rest by being covered with the eyelid and turning away to another object; but in a few seconds it has a new supply of energy and is ready to gaze again.

. . . The best test is a man's daily work, the thing to which he devotes most of his time and energy. Accordingly, I have taken the records of over five hundred factory operatives in the cities of New Haven, New Britain, and Bridgeport, in Connecticut, three or four thousand operatives in southern cities from Virginia to Florida, and over seventeen hundred students at the United States Naval Academy at Annapolis, and the Military Academy at West Point. In most cases each person's record covers an entire year, or at least the academic year. All the records have been compared with the various conditions of the weather. The results are surprising. Changes in the barometer seem to have little effect. Humidity possesses a considerable degree of importance, but the most important element is clearly temperature. The people here considered are physically most active when the average temperature is from 60 to 65 degrees, that is, when the noon temperature rises to 70 degrees or even more. This is higher than many of us would expect. Mental activity reaches a maximum when the outside temperature averages about 38 degrees, that is, when there are mild frosts at night. Another highly important climatic condition is the change of temperature from one day to the next. People do not work well when the temperature remains constant. Great changes are also unfavorable. The ideal conditions are moderate changes, especially a cooling of the air at frequent intervals.

Man is not the only organism that is benefited by changes of temperature. Numerous experiments have shown that plants are subject to a similar influence. If a plant is subjected to unduly low or high temperature, its growth is retarded. As the temperature approaches the optimum, the rate of growth increases. When the optimum is maintained steadily, however, not only does the increase cease, but retrogression sets in, and the rate of growth declines. A moderate change of temperature away from the optimum and then back again after a few hours checks this decline, and keeps the plant at a maximum degree of activity. Thus conditions where the thermometer swings back and forth on either side of the optimum are distinctly better than where the optimum is maintained steadily. Thus it seems to be a law of organic life that variable temperature is better than uniformity.

... It is universally recognized that one of the most important of the bodily functions is the circulation of the blood. . . . Changes of temperature are a powerful agent to this end Witness the effect of a bath, either cold or very hot. ... - Huntington, Civilization and Climate, pp. 8, 120, 121.