Note 1186 — Chapter XV · —Mountain Barriers And Their Passes

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.