Societies which from generation to generation produce in due abundance individuals who relatively to the requirements are the best physically, morally, and intellectually, must become the predominant societies, and must tend through the quiet process of industrial competition to replace other societies. Consequently, marital relations which favour this result in the highest degree must spread; while the prevailing sentiments and ideas must become so moulded into harmony with them that other relations will be condemned as immoral. The monogamic form of the sexual relations is manifestly the ultimate form; and any changes to be anticipated must be in the direction of completion and extension of it. _II.--Political Organisation_ A society is formed only when, besides juxtaposition there is co-operation. Co-operation is made possible by society and makes society possible. It pre-supposes associative men; and men remain associated only because of the benefits co-operation yields them. But there cannot be concerted actions without agencies by which actions are adjusted in their times, amounts, and kinds; and the actions cannot be of different kinds without the co-operators undertaking different duties. That is to say, the co-operators must become organised, either voluntarily or involuntarily. AGGREGATION The political evolution manifested by increase of mass is political aggregation. One of the laws of evolution at large is that integration results when like units are subject to the same force or the like forces; and from the first stages of political integration to the last this law is illustrated. Likeness in the units forming a social group being one conditioned to their integration, a further condition is their joint reaction against external action: co-operation in war is the chief cause of social integration. The temporary unions of savages for offence and defence show the initiatory steps. When many tribes unite against a common enemy, long continuance of their combined action makes them coherent under some common control. And so it is subsequently with still larger aggregates. DIFFERENTIATION