earth.]

The important characteristic of plains is their power to facilitate every
phase of historical movement; that of mountains is their power to
retard, arrest, or deflect it. Man, as part of the mobile envelope of
the earth, like air and water feels always the pull of gravity. From
this he can never fully emancipate himself. By an output of energy he
may climb the steepest slope, but with every upward step the ascent
becomes more difficult, owing to the diminution of warmth and air and
the increasing tax upon the heart.[1186] Maintenance of life in high
altitudes is always a struggle. The decrease of food resources from
lower to higher levels makes the passage of a mountain system an ordeal
for every migrating people or marching army that has to live off the
country which it traverses. Mountains therefore repel population by
their inaccessibility and also by their harsh conditions of life, while
the lowlands attract it, both in migration and settlement. Historical
movement, when forced into the upheaved areas of the earth, avoids the
ridges and peaks, seeks the valleys and passes, where communication with
the lowlands is easiest.

[Sidenote: Inaccessibility of mountains.]

High massive mountain systems present the most effective barriers which
man meets on the land surface of the earth. To the spread of population
they offer a resistance which long serves to exclude settlers. The
difficulty of making roads up steep, rocky slopes and through the
forests usually covering their rain-drenched sides, is deterrent enough;
but in addition to this, general infertility, paucity of arable land,
harsh climatic conditions, and the practical lack of communication with
the outside world offer scant basis for subsistence. Hence, as a rule,
only when pressure of population in the lowlands becomes too great under
prevailing economic methods, do clearings and cabins begin to creep up
the slopes. Mountains are always regions of late occupation. Even in the
Stone Age, we find the long-headed race of Mediterranean stock, who
originally populated Europe, distributed over the continent close up to
the foot of the high Alps, but not in the mountains themselves, and only
scantily represented in the Auvergne Plateau of France. The inhospitable
highlands of Switzerland, the German Alps, and the Auvergne received
their first population later when the Alpine race began to occupy
western Europe.[1187] The _Mittelgebirge_ of Germany were not s