That the history of the race is an unbroken continuum goes without saying. What this means in the way of transmission of the arts, the sciences, the religion, the ideas, the customs of one generation to the next, we shall presently see. Cultural continuity is made possible by the more fundamental fact of the actual biological continuity of the race. This biological continuity extends back, as far as we can infer from the scientific evidence, unbrokenly through the half million years since man has left traces of his presence on earth. The continuity of life itself goes back to that still more remote time when man and ape were indistinguishable, indeed to that postulated epoch when life as it existed on earth was no more complex than it is as it now appears in the one-celled animal. Evolution has taught us that life, however it started, has been one long continuous process which has increased in complexity from the unicellular animals to man. The continuity of the human race is a contrivance of nature rather than of man. It is, as it were, a by-product of the sex instinct. Man is endowed natively with a powerful desire for sex gratification, and though offspring are the chief utility of this instinct, desire for reproduction is not normally its primary stimulus. But while the production of offspring may thus be said to be an incidental result of the sex instinct, human reproduction may be subjected to rational consideration and control, according as offspring are or are not considered desirable. The sense of the desirability of offspring may, in the first place, be determined by social rather than individual considerations. To the group or the state a large birth-rate, a steady increase of the number of births over the number of deaths, may be made desirable by the need of a large population for agriculture, herding, or war. In primitive tribes, superiority in numbers must have been, under conditions of competitive warfare, a pronounced asset. In any imperialistic regime, where military conquest is highly regarded, the maintenance and replenishment of large armies is a factor that has entered into reflection on the question of population. In cases where a small ruling class is benefited by the labor of a slave or serf class, there is, at least for the ruling classes, a marked utility in the increase in population. It means just so much opportunity for increase of wealth on the part of landowning and slaveholding or serf-controlling classes. In any country, increase in the labor supply means just so much more human energy for the control of natural resources, so many more units of energy for the production of national wealth.