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 26 Empire until the ever-mounting crest of the barbarian torrent burst through defences, which there were no longer the men or the military spirit to defend. Walls and ramparts have now passed away as Frontiers of dominions, just as they are becoming obsolete as defences of cities. Occasionally in some remote corner to which the tides of human movement have not penetrated, their survivals are found. The Tibetans thought that they could bar their mountain plateaux to the Indian army by a stone wall built across a valley. A more practical and modern analogy has been traced in the Customs Hedge or Frontier made of thorny bushes and trees, which until a few years ago was stretched for 2,500 miles round the territories of British India to keep out contraband salt from the Native States.

But a commoner and more widely diffused type of ancient Frontier was that of the intermediary or Neutral Zone. This may be described as a Frontier of separation in place of contact, a line whose distinguishing feature is that it possesses breadth as well as length. Sometimes it was a razed or depopulated or devastated tract of country; at others a debatable strip between the territories of rival powers: or, again, a border territory subject to and defended by one party, though exposed to the ravages of the other. Between Korea and China there existed, till beyond the middle of the last century, a broad uncultivated and uninhabited tract over 5,000 square