a long eastern tributary.  Up this they passed to the Lena in 1627, thence to Bering Sea by the Kolima and Anadyr rivers, because these arctic fields yielded sable, beaver and fox skins in greatest quantity.[643] The Lena especially, from its source down to its eastern elbow at Yakutsk, that great rendezvous of Siberian fur traders, was a highway for trapper and Cossack tribute-gatherer.[644] From the sources of the Yenisei in Lake Baikal to the navigable course of the Amur was a short step, taken in 1658, though the control of the river, which was claimed by China, was not secured till two hundred years later.[645] [See map page 103.]

As the only highways in new countries, rivers constitute lines of least resistance for colonial peoples encroaching upon the territory of inferior races.  They are therefore the geographic basis of those streamers of settlement which we found making a fringe of civilization across the boundary zone of savagery or barbarism on the typical colonial frontier.  Ethnic islands of the expanding people cluster along them like iron filings on a magnetized wire.  Therefore in all countries where navigable rivers have fixed the lines of expansion, as in the United States, the northern part of the Russian Empire, and the eastern or colonial border of Germany and Austria, there is a strong anthropo-geographic resemblance in the frontiers of successive decades or centuries.  But in arid or semi-arid regions like South Africa, the western plains of North America, the steppes of Russian and Chinese Turkestan, the river highway motif in expansion is lost in a variety of other geographic and geologic factors, though the water of the streams still attracts trail and settlement.

[Sidenote:  Determinants of routes in arid or semi-arid lands.]

A river like the Nile, lower Volga, Irtysh or Indus, rising in highlands of abundant rainfall but traversing an arid or desert land, acquires added importance because it furnishes the sole means of water travel and of irrigation.  The Nile has for ages constituted the main line of intercourse between the Mediterranean and Equatorial Africa.  The Tigris, Euphrates, Indus, and the Niger where it makes its great northern bend into the Sahara near Timbuctoo,[646] attest the value to local fertility and commerce inherent in these rivers of the deserts and steppes.  Such rivers are always oasis-makers, whether on their way to the sea they periodically cover a narrow flood-plain like that of the Nile, or one ninety miles wide, like that of the Niger’s inland delta above Timbuctoo;[647] or whether they emerge into a silent sea of sand, like the Murghab of Russian Turkestan, which spreads itself out to water the gardens of Merv.