The real character and importance of these movements have been appreciated by broad-minded historians. Thucydides elucidates the conditions leading up to the Peloponnesian War by a description of the semi-migratory population of Hellas, the exposure of the more fertile districts to incursions, and the influence of these movements in differentiating Dorian from Ionian Greece.[137] Johannes von Muller, in the introduction to his history of Switzerland, assigns to federations and migrations a conspicuous role in historical development. Edward A. Ross sees in such movements a thorough-going selective process which weeds out the unfit, or rather spares only the highly fit. He lays down the principle that repeated migrations tend to the creation of energetic races of men. He adds, "This principle may account for the fact that those branches of a race achieve the most brilliant success which have wandered the farthest from their ancestral home.... The Arabs and Moors that skirted Africa and won a home in far-away Spain, developed the most brilliant of the Saracen civilizations. Hebrews, Dorians, Quirites, Rajputs, Hovas were far invaders. No communities in classic times flourished like the cities of Asia created by the overflow from Greece. Nowhere under the Czar are there such vigorous, progressive communities as in Siberia."[138] Brinton distinguishes the associative and dispersive elements in ethnography. The latter is favored by the physical adaptability of the human race to all climates and external conditions; it is stimulated by the food-quest, the pressure of foes, and the resultant restlessness of an unstable primitive society.[139] The earth's surface is at once factor and basis in these movements. In an active way it directs them; but they in turn clothe the passive earth with a mantle of humanity. This mantle is of varied weave and thickness, showing here the simple pattern of a primitive society, there the intricate design of advanced civilization; here a closely woven or a gauzy texture, there disclosing a great rent where a rocky peak or the ice-wrapped poles protrude through the warm human covering. This is the magic web whereof man is at once woof and weaver, and the flying shuttle that never rests. Given a region, what is its living envelope, asks anthropo-geography. Whence and how did it get there? What is the material of warp and woof? Will new threads enter to vary the color and design? If so, from what source? Or will the local pattern repeat itself over and over with dull uniformity? [Sidenote: Geographical interpretation of historical movement.]