[1175] James Bryce, Impressions of South Africa, p. 422.  New York, 1897.

[1176] Deuteronomy, VII, 1-3.

[1177] Ratzel, History of Mankind, Vol.  III, p. 184.  London, 1896-1898.

[1178] Boyd Alexander, From the Niger to the Nile, Vol.  I, pp. 190-197.  London, 1907.

[1179] H. Barth, Travels in North and Central Africa, Vol.  I, pp. 202, 277-281.  New York, 1857.

[1180] Ratzel, History of Mankind, Vol.  III, p. 173.  London, 1896-1898.

[1181] Ibid., Vol.  III, Chapter on Islam, pp. 195-204.

[1182] George Adam Smith, Historical Geography of the Holy Land, pp. 28-30.  New York, 1897.  L. March Phillipps, In the Desert, pp. 101-105.  London, 1905.

[1183] E.A.  Freeman, Historical Geography of Europe, pp. 114-116.  London, 1882.

[1184] George Adam Smith, Historical Geography of the Holy Land, pp. 88-90.  New York, 1897.

[1185] Jeremiah, Chap.  XXXV, 6-14.

CHAPTER XV

MOUNTAIN BARRIERS AND THEIR PASSES

[Sidenote:  Man as part of the mobile envelope of the 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