The land traversed by a migrating horde is not to be pictured as a dead road beneath their feet, but rather as a wide region of transit and transition, potent to influence them by its geography and people, and to modify them in the course of their passage. The route which they follow is a succession of habitats, in which they linger and domicile themselves for a while, though not long enough to lose wholly the habits of life and thought acquired in their previous dwelling place. Although nature in many places, by means of valleys, low plains, mountain passes or oasis lines, points out the way of these race movements, it is safer to think and speak of this way as a transit land, not as a path or road. Even where the district of migration has been the sea, as among the Caribs of the Antilles Islands, the Moros of the Philippines, and the Polynesians of the Pacific, man sends his roots like a water plant down into the restless element beneath, and reflects its influence in all his thought and activities. [Sidenote: War as a form of the historical movement.] Every aggressive historical movement, whether bold migration or forcible extension of the home territory, involves displacement or passive movement of other peoples (except in those rare occupations of vacant lands), who in turn are forced to encroach upon the lands of others. These conditions involve war, which is an important form of the historical movement, contributing to new social contacts and fusion of racial stocks. Raids and piratical descents are often the preliminary of great historical movements. They first expand the geographical horizon, and end in permanent settlements, which involve finally considerable transfers of population, summoned to strengthen the position of the interloper. Such was the history of the Germanic invasions of Britain, the Scandinavian settlements on the shores of Iceland, Britain, and France, and the incursions of Saharan tribes into the Sudanese states. Among pastoral nomads war is the rule; the tribe, a mobilized nation, is always on a war footing with its neighbors. The scant supply of wells and pasturage, inadequate in the dry season, involves rivalry and conflict for their possession as agricultural lands do not. Failure of water or grass is followed by the decline of the herds, and then by marauding expeditions into the river valleys to supply the temporary want of food. When population increases beyond the limits of subsistence in the needy steppes, such raids become the rule and end in the conquest of the more favored lands, with resulting amalgamation of race and culture.[154] [Sidenote: Primitive war.]