First of all is to be noted an evolution in the mobility of peoples. In the lower stages of culture mobility is great. It is favored by the persistent food-quest over wide areas incident to retarded economic methods, and by the loose attachment of society to the soil. The small social groups peculiar to these stages and their innate tendency to fission help the movements to ramify. The consequent scattered distribution of the population offers wide interstices between encampments or villages, and into these vacant spaces other wandering tribes easily penetrate. The rapid decline of the Indian race in America before the advancing whites was due chiefly to the division of the savages into small groups, scattered sparsely over a wide territory. Hunter and pastoral peoples need far more land than they can occupy at any one time. Hence the temporarily vacant spots invite incursion. Moreover, the slight impedimenta carried by primitive folk minimize the natural physical obstacles which they meet when on the march. The lightly equipped war parties of the Shawnee Indians used gorges and gaps for the passage of the Allegheny Mountains which were prohibitive to all white pioneers except the lonely trapper. Finally, this mobility gets into the primitive mind. The _Wanderlust_ 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.]