At the basis of all social organizations lie two elementary forces,—the “instinct for association” and the “sexual instinct.” The sexual instinct is primary, because it is obviously an essential condition of life. The instinct for association is secondary in so far as its expression in outward form is concerned. It is as old as the sexual instinct; but, since at the initial stage of human development the sexual instinct is so strong a force, the instinct of association had no observable influence on the actions of men. The forms of social organization which the sexual instinct conditions are those based upon certain kinds of blood relationship. These forms are primary. To establish the priority of the forms thus imposed by blood relationship, we have but to remember that, as the relationship of individuals to one another preceded everything else, so the social forms based upon blood relationship must have preceded all other social forms. We are therefore to regard as the earliest stage of social organization that of groups bound together by blood relationship. But what has been the force differentiating these groups? Obviously not the same sexual instinct that has caused the formation of these primary groups. To explain the factors that have caused this differentiation we must call to our aid two phenomena,—first, that of sexual solidarity; and, secondly, that of the instinct for association. Sexual solidarity has its roots in the nature of man and woman, and is possessed by them in equal intensity. The instinct for association is, however, a specifically masculine trait. It is found among women only in a minimal degree. An important corollary follows from this fact: If women societies are found anywhere, they are to be considered merely as imitations of men’s societies. If women are found as members in a society, this is to be regarded as secondary and purely adventitious. These, and some more specific points to be enumerated later, must be borne in mind continually, as Schurtz makes a far-reaching use of them. The instinct for association, he goes on to say, expresses itself, however, between those of like interests; that is, between those who would most likely be of the same age. It is not likely, for instance, to occur between married and unmarried men. We have here two apparently organically determined classes. In the earliest stages of social development, however, when the norms of social expression conditioned by the sexual instinct were still of paramount importance, insistence was most naturally placed upon the most important stage of man’s physiological development,—the age of puberty. The strong line of demarcation between the period preceding and following sexual maturity was so ever-present a fact to the mind of primitive man, that it found expression in the multitude of initiatory rites. In these initiatory rites we have another of the specific “symptoms” with which we shall have to deal afterward.