life of the sea, the fisheries of the continental shelf which
are factors in the food quest and the distribution of settlements.
Moreover, the ocean floor enters into the problem of laying telegraph
cables, and thereby assumes a certain commercial and political
importance. The name of the Telegraph Plateau of the North Atlantic,
crossed by three cables, points to the relation between these and
submarine relief. So also does the erratic path of the cable from
southwestern Australia to South Africa via Keeling Island and Mauritius.

Submarine reliefs have yet greater significance in their relation to the
distribution of the human race over the whole earth; for what is now a
shallow sea may in geologically recent times have been dry land, on
which primitive man crossed from continent to continent. It is vital to
the theory of the Asiatic origin of the American Indian that in Miocene
times a land bridge spanned the present shallows of Bering Sea. Hence
the slight depth of this basin has the same bio-geographical
significance as that of the British seas, the waters of the Malay
Archipelago, and the Melanesian submarine platform. The impressive fact
about "Wallace's Line" is the depth of the narrow channel which it
follows through Lombok and Macassar Straits and which, in recent
geological times, defined the southeastern shore of Asia. In all these
questions of former land connection, anthropo-geography follows the lead
of bio-geography, whose deductions, based upon the dispersal of
countless plant and animal forms, point to the paths of human
distribution.

[Sidenote: Mean elevations of the continents.]

The mean elevation of the continents above sea level indicates the
average life conditions of their populations as dependent upon relief.
The 1010 meters (3313 feet) of Asia indicate its predominant highland
character. The 330 meters (1080 feet) representing the average height of
Europe, and the 310 meters (1016 feet) of Australia indicate the
preponderance of lowlands. Nevertheless, anthropo-geography rarely lends
itself to a mathematical statement of physical conditions. Such a
statement only obscures the facts. The 660 meters (2164 feet) mean
elevation of Africa indicates a relief higher than Europe, but gives no
hint of the plateau character of the Dark Continent, in which lowlands
and mountains are practically negligible features; while the almost
identical figure (650 meters or 2133 feet) for both North and South
Am