Among primitive peoples this movement is simple and monotonous. It involves all members of the tribe, either in pursuit of game, or following the herd over the tribal territory, or in migrations seeking more and better land. Among civilized peoples it assumes various forms, and especially is differentiated for different members of the social group. The civilized state develops specialized frontiersmen, armies, explorers, maritime traders, colonists, and missionaries, who keep a part of the people constantly moving and directing external expansion, while the mass of the population converts the force once expended in the migrant food-quest into internal activity. Here we come upon a paradox. The nation as a whole, with the development of sedentary life, increases its population and therewith its need for external movements; it widens its national area and its circle of contact with other lands, enlarges its geographical horizon, and improves its internal communication over a growing territory; it evolves a greater mobility within and without, which attaches, however, to certain classes of society, not to the entire social group. This mobility becomes the outward expression of a whole complex of economic wants, intellectual needs, and political ambitions. It is embodied in the conquests which build up empires, in the colonization which develops new lands, in the world-wide exchange of commodities and ideas which lifts the level of civilization, till this movement of peoples becomes a fundamental fact of history. [Sidenote: Nature of primitive movements.]