Of the three main epochs--savagery, barbarism and civilization--naturally only the first two and the transition to the third required his attention. He subdivided each of these into a lower, middle and higher stage, according to the progress in the production of the means of sustenance. His reason for doing so is that the degree of human supremacy over nature is conditioned on the ability to produce the necessities of life. For of all living beings, man alone has acquired an almost unlimited control over food production. All great epochs of human progress, according to Morgan, coincide more or less directly with times of greater abundance in the means that sustain life. The evolution of the family proceeds in the same measure without, however, offering equally convenient marks for sub-division. I. SAVAGERY. 1. Lower Stage. Infancy of the human race. Human beings still dwelt in their original habitation, in tropical or subtropical forests. They lived at least part of the time in trees, for only in this way they could escape the attacks of large beasts of prey and survive. Fruit, nuts, and roots served as food. The formation of articulated speech is the principal result of this period. Not a single one of all the nations that have become known in historic times dates back to this primeval stage. Although the latter may extend over thousands of years, we have no means of proving its existence by direct evidence. But once the descent of man from the Animal Kingdom is acknowledged, the acceptance of this stage of transition becomes inevitable.