Conflict groups, like sects and parties, and accommodation groups like castes and classes depend for survival upon isolation. Free intercourse of opposing parties is always a menace to their morale. Fraternization between soldiers of contending armies, or between ministers of rival denominations is fraught with peril to the fighting efficiency of the organizations they represent. The solidarity of the group, like the integrity of the individual, implies a measure at least of isolation from other groups and persons as a necessary condition of its existence. The life-history of any group when analyzed is found to incorporate within it elements of isolation as well as of social contact. Membership in a group makes for increasing contacts within the circle of participants, but decreasing contacts with persons without. Isolation is for this reason a factor in the preservation of individuality and unity. The _esprit de corps_ and morale of the group is in large part maintained by the fixation of attention upon certain collective representations to the exclusion of others. The memories and sentiments of the members have their source in common experiences of the past from which non-members are isolated. This natural tendency toward exclusive experiences is often reinforced by conscious emphasis upon secrecy. Primitive and modern secret societies, sororities, and fraternities have been organized around the principle of isolation. Secrecy in a society, like reserve in an individual, protects it from a disintegrating publicity. The family has its "skeleton in the closet," social groups avoid the public "washing of dirty linen"; the community banishes from consciousness, if it can, its slums, and parades its parks and boulevards. Every individual who has any personality at all maintains some region of privacy. A morphological survey of group formation in any society discloses the fact that there are lateral as well as vertical divisions in the social structure. Groups are arranged in strata of relative superiority and inferiority. In a stratified society the separation into castes is rigid and quite unalterable. In a free society competition tends to destroy classes and castes. New devices come into use to keep aspiring and insurgent individuals and groups at the proper social level. If "familiarity breeds contempt" respect may be secured by reserve. In the army the prestige of the officer is largely a matter of "distance." The "divinity that doth hedge the king" is due in large part to the hedge of ceremonial separating him from his subjects. Condescension and pity, while they denote external contact, involve an assumption of spiritual eminence not to be found in consensus and sympathy. As protection against the penetration of the inner precincts of personality and the group individuality, there are the defenses of suspicion and aversion, of reticence and reserve, designed to insure the proper social distance. 3. Classification of the Materials