Such mountain regions attain independent historical importance when their population increases enough to form the nucleus of a state, and to acquire additional territory about the highland base either by conquest or voluntary union, while they utilize their naturally protected location and their power to grant safe transit to their allies, as means to secure their political autonomy.  Therefore to mountain regions so often falls the role of buffer states.  Such were medieval Burgundy and modern Savoy, which occupied part of the same territory, Navarre which in the late Middle Ages controlled the important passway around the western end of the Pyrenees, and Switzerland which commands the passes of the central Alps.  The position of such mountain states is, however, always fraught with danger, owing to the weakness inherent in their small area and yet smaller allowance of productive soil, to their diverse ethnic elements, and the forces working against political consolidation in their deeply dissected surface.  Political solidarity has a hard, slow birth in the mountains.

[Sidenote:  Transition forms of relief between highlands and lowlands.]

In view of the barrier character of mountains, a fact of immense importance to the distribution of man and his activities is the rarity of abrupt, ungraded forms of relief on the earth’s surface.  The physiographic cause lies in the elasticity of the earth’s crust and the leveling effect of weathering and denudation.  Everywhere mountains are worn down and rounded off, while valleys broaden and fill up to shallow trough outlines.  Transition forms of relief abound.  Human intercourse meets therefore few absolute barriers on the land; but these few reveal the obstacles to historical movement in perpendicular reliefs.  The mile-high walls of the Grand Canon of the Colorado are an insuperable obstacle to intercourse for a stretch of three hundred miles.  The glacier-crowned ridge of the Bernese Alps is crossed by no wagon road between the Grimsel Pass and the upper Rhone highway around their western end, a distance of 100 kilometers (62 miles).  The Pennine Alps have no pass between the Great St. Bernard and the Simplon, a distance of 90 kilometers (54 miles).

[Sidenote:  Importance of transition slopes.]

Gentle transition slopes or terrace lands facilitate almost everywhere access to the lowest, most habitable and therefore, from the human standpoint, most important section of mountains.  They combine the ease of intercourse characteristic of plains with many advantages of the mountains, and especially in warm climates they unite in a narrow zone both tropical and temperate vegetation.  The human value of these transition slopes holds equally of single hills, massive mountain systems, and continental reliefs.  The earth as a whole owes much of its habitability to these gently graded slopes.  Continents and countries in which they are meagerly developed suffer from difficulty