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.