However far we go back, the first form of life in common seems to be the horde, an unstable, unorganised aggregate, without recognised kinships, drawn together instinctively in view of utility and defence. But the true social unit, which arose at an early period in various parts of the globe, is the clan (and analogous institutions), a fixed, stable, coherent, closed aggregate, founded on religious or other affiliation (but not on descent), independent of family conditions: a man cannot belong to two clans at once, and in most cases each of these groups is in a hostile attitude towards the rest. How has this social molecule been able to aggregate to itself others, and this closed organism to break its narrow limits, in order to extend itself by increase and fusion? This is a somewhat obscure question; perhaps by exogamy, _i.e._, the imperative custom which forbade marriage within the group (yet in other groups the rule was endogamy, _i.e._, the prohibition of marriage outside them); more probably the great agent of assimilation and fusion was war, followed by the assimilation of the conquered. This simple comparison shows that the family and the clan are not similar institutions: the first is an autonomous group belonging to a master, and having for its end the enjoyment of property; the second is a group of another nature having for its end the common struggle for existence. “Where the interests defended by the family are less important than those of the clan, the family is influenced by the ideas which regulate the clan organisation; and this fact repeats itself in all primitive societies when defence against an outside enemy is the dominant necessity.”[177] The family group and the social group have each sprung from different tendencies, from distinct needs; each has its special, independent psychological origin, and there is no possible derivation from one to the other. ----- Footnote 177: Starcke, _La famille primitive_, p. 116. ----- III. Life in common, even under the gregarious form, requires certain ways of acting, and habits founded on sympathy and determined by the concerted aim pursued by all. In order that it may become stable and constitute a society, an element of fixity must be added—the more or less clear consciousness of an obligation, of a rule, of what has to be done or avoided. This is the appearance of the _moral sentiment_. All conceptions of morality, coarse or refined, theoretical or purely practical, agree on this point; divergences exist, in practice, only as to the characteristics of the act reputed obligatory; in theory, only as to its origin. All _real_ morality which has lived, _i.e._ governed a human society, large or small, which has existed, not in the academic abstractions of moralists, but in the concrete development of history, and has run its complete course, passes through two principal periods.