The task of tracing out the history of social organizations of whatever grade can now be defined in precise terms: in simple words, it is to learn how the activities of the component biological units making up any association really differ from the vital performances of biological units existing by themselves. What is it that distinguishes a savage of antiquity from an American of to-day? The modern example is just as much an animal as the earlier type, and his physiology is essentially the same. It is something added to the common biological qualities of all men, some relation which does not appear as such in the life of rude tribes, that makes the distinction. And it is just this superadded relation that requires explanation, as regards its exact biological value and its historic development as well. In undertaking this difficult task, it seems best to begin with the very simplest organisms that biology knows, working upwards through the scale to man. By this course the most basic elements of organization can be discovered without having to look for them among the intricate details of our own vital situation, where secondary and adventitious elements stand out in undue prominence, and where the impersonal view is well-nigh impossible. Step by step we will then work up the scale of social morphology, approaching in the natural evolutionary order that part of the subject which interests us most deeply. Just as the construction of an edifice must begin with the fashioning of the individual brick and bolt and girder, so the evolution of a biological association begins with the unitary organisms consisting of single cells, like _Amoeba_. We have had occasion to discuss this animal many times in our previous studies of one or another aspect of evolution, and once again we must return to it in order to reestablish certain points that are of fundamental importance for our present purposes. Within the limits of its simple body, _Amoeba_ performs the several tasks which nature demands a living thing shall do; it feeds and respires and moves, continually utilizing matter and energy obtained from the environment for the reconstruction of its substance and replenishment of its vital powers; it cooerdinates the activities of its simple body, and by its reflex responses to environmental influences it maintains its adjustment to the external conditions of life. The animal does all of these things with a purely individual benefit, namely, the prolongation of its own life. While it is performing these individual tasks, it does not concern itself with anything else but its own welfare; the interests of other living things are not involved in any way, excepting in the case of other organisms that may serve the animal as food. _Amoeba_, like every other living thing, if it is to exist, must unconsciously obey the first great commandment of nature,--"_Preserve thyself_."