is strong.  Long residence in one territory is irksome, attachment is weak.  Therefore a small cause suffices to start the whole or part of the social body moving.  A temporary failure of the food supply, cruelty or excessive exaction of tribute on the part of the chief, occasions an exodus.  The history of every negro tribe in Africa gives instances of such secessions, which often leave whole districts empty and exposed to the next wandering occupant.  Methods of preventing such withdrawals, and therewith the diminution of his treasury receipts and his fighting force, belong to the policy of every negro chieftain.

[Sidenote:  Natural barriers to movement.]

The checks to this native mobility of primitive peoples are two:  physical and mental.  In addition to the usual barriers of mountains, deserts, and seas before the invention of boats, primeval forests have always offered serious obstacles to man armed only with stone or bronze axe, and they rebuffed even man of the iron age.  War and hunting parties had to move along the natural clearings of the rivers, the tracks of animals, or the few trails beaten out in time by the natives themselves.  Primitive agriculture has never battled successfully against the phalanx of the trees.  Forests balked the expansion of the Inca civilization on the rainy slope of the Andes, and in Central Africa the negro invaded only their edges for his yam fields and plantain groves.  The earliest settlements in ancient Britain were confined to the natural clearings of the chalk downs and oolitic uplands; and here population was chiefly concentrated even at the close of the Roman occupation.  Only gradually, as the valley woodlands were cleared, did the richer soil of the alluvial basins attract men from the high, poor ground where tillage required no preliminary work.  But after four centuries of Roman rule and Roman roads, the clearings along the river valleys were still mere strips of culture mid an encompassing wilderness of woods.  When the Germanic invaders came, they too appropriated the treeless downs and were blocked by the forests.[140] On the other hand, grasslands and savannahs have developed the most mobile people whom we know, steppe hunters like the Sioux Indians and Patagonians.  Thus while the forest dweller, confined to the highway of the stream, devised only canoe and dugout boat in various forms for purposes of transportation, steppe peoples of the Old World introduced the use of draft and pack animals, and invented the sledge and cart.

[Sidenote:  Effect of geographical horizon.]

Primitive peoples carry a drag upon their migrations in their restricted geographical outlook; ignorance robs them of definite goals.  The evolution of the historical movement is accelerated by every expansion of the geographical horizon.  It progresses most rapidly where the knowledge of outlying or remote lands travels fastest, as along rivers and thalassic coasts.  Rome’s location as toll-gate keeper