Where nature presents barriers to far-ranging food-quests, man is forced to advance from the natural to the artificial basis of subsistence; he leaves the chase for the sedentary life of agriculture.  Extensive activities are replaced by intensive ones, wide dispersal of tribal energies by concentration.  The extensive forests and grassy plains of the Americas supported abundant animal life and therefore afforded conditions for the long survival of the hunting tribes; nature put no pressure upon man to coerce him to progress, except in the small mountain-walled valleys of Peru and Mexico, and in the restricted districts of isthmian Central America.  Here game was soon exhausted.  Agriculture became an increasing source of subsistence and was forced by limited area out of its migratory or essartage stage of development into the sedentary.  As fields become fixed in such enclosed areas, so do the cultivators.  Here first population becomes relatively dense, and thereby necessitates more elaborate social and political organization in order to prevent inner friction.

The geographically enclosed district has the further advantage that its inhabitants soon come to know it out to its boundaries, understand its possibilities, exploit to the utmost its resources, and because of the closeness of their relationship to it and to each other come to develop a conscious national spirit.  The population, since it cannot easily spread beyond the nature-set limits, increases in density.  The members of the compact society react constantly upon one another and exchange the elements of civilization.  Thus the small territory is characterized by the early maturity of a highly individualized civilization, which then, with inherent power of expansion, proceeds to overleap its narrow borders and conquer for itself a wide sphere of influence.  Hand in hand with this process goes political concentration, which aids the subsequent expansion.  Therefore islands, oases, slender coastal strips and mountain valleys repeatedly show us small peoples who, in their seclusion, have developed a tribal or national consciousness akin in its intensity to clan feeling.  This national feeling is conspicuous in the English, Japanese, Swiss and Dutch, as it was in the ancient city-states of Greece.  The accompanying civilization, once brought to maturity in its narrow breeding place, spreads under favorable geographic conditions over a much larger space, which the accumulated race energy takes for its field of activity.  The flower which thus early blooms may soon fade and decay; nevertheless the geographically evolved national consciousness persists and retains a certain power of renewal.  This has been demonstrated in the Italians and modern Greeks, in the Danes and the Icelanders.  In the Jews it has resisted exile from their native land, complete political dissolution, and dispersal over the habitable world.  Long and often as Italy had to submit to foreign dominion, the idea of the national unity of the peninsula was never lost.