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.