This second of three volumes sets out a general account of the structure and evolution of human societies. The author argues first that societies are to be defined as sets of roles whose incumbents are competitors for access to, or control of, the means of production, persuasion and coercion; and second, that the process by which societies evolve is one of competitive selection of the practices by which roles are defined analagous, but not reducible, to natural selection. He illustrates and tests these theses with evidence drawn from the whole range of societies documented in the historical and ethnographic record. The result is an original, powerful and far-reaching reformulation of evolutionary sociological theory which will make it possible to do for the classification and analysis of societies what Darwin and his successors have done for the classification and analysis of species.