Every country whose history we examine proves the recipient of successive streams of humanity. Even sea-girt England has received various intruding peoples from the Roman occupation to the recent influx of Russian Jews. In prehistoric times it combined several elements in its population, as the discovery of the "long barrow" men and "round barrow" men by archaeologists, and the identification of a surviving Iberian or Mediterranean strain by ethnologists go to prove.[126] Egypt, Mesopotamia, and India tell the same story, whether in their recorded or unrecorded history. Tropical Africa lacks a history; but all that has been pieced together by ethnologists and anthropologists, in an effort to reconstruct its past, shows incessant movement,--growth, expansion and short-lived conquest, followed by shrinkage, expulsion or absorption by another invader.[127] To this constant shifting of races and peoples the name of historical movement has been given, because it underlies most of written history, and constitutes the major part of unwritten history, especially that of savage and nomadic tribes. Two things are vital in the history of every people, its ethnic composition and the wars it wages in defense or extension of its boundaries. Both rest upon historical movements,--intrusions, whether peaceful or hostile, into its own land, and encroachments upon neighboring territory necessitated by growth. Back of all such movements is natural increase of population beyond local means of subsistence, and the development of the war spirit in the effort to secure more abundant subsistence either by raid or conquest of territory. [Sidenote: Evolution of the Historical Movement.]