In still another way man's physical organism gives rise to a cycle in society, namely, through the length of time required for an individual to come to maturity and then to live out his term of life. It has often been noted that a particular movement in a people lasts for about a generation. This is as much as to say that it has a term of life, a cycle through which it lives. In politics the cycle finds its most distinct beginning in the popular acceptance of a set of radical ideas; it develops next into a period of reform and ends in a period of reaction. In military history it begins in a period of peace with the shifting of the balance of power between nations or the growth of some strained relation; active preparation of armaments follows, the cycle culminates in the war itself; the treaty of peace marks the end of one cycle and the beginning of another. A striking example of this, familiar to the student of history, is the interval of a generation which came between the great Crusades of the twelfth century. In business there is the familiar cycle which may be said to begin with the opening of new forms of wealth-getting and to progress through the stages of slow expansion of credit, speculation, panic, and depression. In art, science, philosophy, religion, education, and other lines of higher culture there is the launching of a new idea, its more or less general acceptance, its application in practice or to other lines of thought, ending in the recognition of its shortcomings and the reaction against it.

The synchronizing of the beginning and ending of these movements with the coming and passing of a generation, though a commonplace idea, has never, so far as the writer is aware, been worked out inductively; it is therefore impossible to make positive statements in regard to it. By deductive analysis the following inferences would seem legitimate. A generation of young men come to maturity, eager to be up with the times, impatient of the old things and ready to give attention to the new; the social mind in which they grow up is therefore that of the more radical thinkers of the time. When these young men get into active life they fight the campaign for the new ideas, in time winning some measure of success. The new ideas prove useful in some respects and disappointing in some. The movement as a whole finally works itself out and off as the men who brought it on die or grow old. Then there is another generation of young men ready to take hold, and the ground is clear for a new movement of some kind. This cycle of a generation coincides roughly in duration with the longer cycles of rainfall which Moore makes the basis of the greater economic crises.