For the same reason they leave their boundaries undefined; a mile nearer or farther, what does it matter?  Moreover, their fitful or nomadic occupation of the land leads to oscillations of the frontiers with every attack from without and every variation of the tribal strength within.  Their unstable states rarely last long enough in a given form or size to develop fixed boundaries; hence, the vagueness as to the extent of tribal domains among all savage peoples, and the conflicting land claims which are the abiding source of war.  Owing to these overlapping boundaries—­border districts claimed but not occupied—­the American colonists met with difficulties in their purchase of land from the Indians, often paying twice for the same strip.

[Sidenote:  Common boundary districts.]

Even civilized peoples may adopt a waste boundary where the motive for protection is peculiarly strong, as in the half-mile neutral zone of lowland which ties the rock of Gibraltar to Spain.  On a sparsely populated frontier, where the abundance of land reduces its value, they may throw the boundary into the form of a common district, as in the vast, disputed Oregon country, accepted provisionally as a district of joint occupancy between the United States and Canada from 1818 to 1846, or that wide highland border which Norway so long shared with Russia and Sweden.  In South America, where land is abundant and population sparse, this common boundary belt is not rare.  It suggests a device giving that leeway for expansion desired by all growing states.  By the treaty of 1866, the frontier between Chile and Bolivia crossed the Atacama desert at 24 deg.  South Latitude; but the zone between 23 deg. and 25 deg. was left under the common jurisdiction of the two states, for exploitation of the guano deposits and mineral wealth.[362] A common border district on a much larger scale is found between Brazil and the eastern frontier of French Guiana.  It includes a belt 185 miles (300 kilometers) wide between the Oyapok and Arawary rivers, and is left as a neutral district till its fate is decided by arbitration.[363] All these instances are only temporary phases in the evolution of a political frontier from wide, neutral border to the mathematically determined boundary line required by modern civilized states.

[Sidenote:  Tariff free zones.]

Even when the boundary line has been surveyed and the boundary pillars set up, the frontier is prone to assert its old zonal nature, simply because it marks the limits of human movements.  Rarely, for instance, does a customs boundary coincide with a political frontier, even in the most advanced states of Europe, except on the coasts.  The student of Baedecker finds a gap of several miles on the same railroad between the customs frontier of Germany and France, or France and Italy.  Where the border district is formed by a high and rugged mountain range, the custom houses recede farther and farther from the common political line upon the ridge, and drop down the slope to convenient points, leaving between them a wide neutral tariff zone, like that in Haute Savoie along the massive Mont Blanc Range between France and Italy.