ses east and west against
the northern face of the Asiatic plateau, with low watersheds between,
the Russian explorer and sable hunter struck their eastward water trail
toward the Pacific. The advance, which under Yermak crossed the Ural
Mountains in 1579, reached the Yenisei River in 1610 and planted there
the town of Turuchansk as a sort of milestone, almost on the Arctic
Circle opposite the mouth of the Lower Tunguska, 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