Speaking roughly, we may call any persistent social group, other than the family, existing within a larger group, a class. And every society, except possibly the most primitive, is more or less distinctly composed of classes. Even in savage tribes there are, besides families and clans, almost always other associations: of warriors, of magicians and so on; and these continue throughout all phases of development until we reach the intricate group structure of our own time. Individuals never achieve their life in separation, but always in coöperation with a group of other minds, and in proportion as these coöperating groups stand out from one another with some distinctness they constitute social classes. We may say of this differentiation, speaking generally, that it is useful. The various functions of life require special influences and organization, and without some class spirit, some speciality in traditions and standards, nothing is well performed. Thus, if our physicians were not, as regards their professional activities, something of a psychological unit, building up knowledge and sentiment by communication, desiring the approval and dreading the censure of their colleagues, it would be worse not only for them but for the rest of us. There are no doubt class divisions that are useless or harmful, but something of this nature there should be, and I have already tried to show that our own society suffers considerably from a lack of adequate group differentiation in its higher mental activities. Fundamental to all study of classes are the two principles, of inheritance and of competition, according to which their membership is determined. The rule of descent, as in the hereditary nobility of England or Germany, gives a fixed system, the alternative to which is some kind of selection—by election or appointment as in our politics; by purchase, as formerly in the British army and navy; or by the informal action of preference, opportunity and endeavor, as in the case of most trades and professions at the present day. Evidently these two principles are very much intermingled in their working. The hereditary distinctions must have a beginning in some sort of selective struggle, such as the military and commercial competition from which privileged families have emerged in the past, and never become so rigid as not to be modified by similar processes. On the other hand, inherited advantages, even in the freest society, enter powerfully into every kind of competition. Another consideration of much interest is that the strict rule of descent is a biological principle, making the social organization subordinate to physical continuity of life, while selection or competition brings in psychical elements, of the most various qualities to be sure, but capable at the best of forming society on a truly rational method.