When the relief includes mountains, the character not only of the land but of the climate changes, and therewith the type of community.  Hence neighboring districts may produce strongly contrasted types of society.  Madison County of Kentucky, lying on the eastern margin of the Bluegrass region, contains the rich landed estates, negro laboring class and aristocratic society characteristic of the “planter” communities of the old South; and only twenty miles southeast of Richmond, the center of this wealth and refinement, it includes also the rough barren hill country of the Cumberland Plateau, where are found one-room cabins, moonshine stills, feuds, and a backward population sprung from the same pure English stock as the Bluegrass people.

[Sidenote:  Contrasted environments.]

Here is differentiation due to the immediate influences of environment.  The phenomenon reappears in every part of the world, in every race and every age.  The contrast between the ancient Greeks of the mountains, coasts and alluvial valleys shows the power of environment to direct economic activities and to modify culture and social organization.  So does the differences between the coast, steppe, and forest Indians of Guiana,[213] the Kirghis of the Pamir pastures and the Irtysh River valley, the agricultural Berbers of the Atlas Mountains and the Berber nomads of the Sahara, the Swiss of the high, lonely Engadine and those of the crowded Aar valley.

Contrasted environments effect a natural selection in another way and thereby greatly stimulate differentiation, whenever an intruding people contest the ownership of the territory with the inhabitants.  The struggle for land means a struggle also for the best land, which therefore falls to the share of the strongest peoples.  Weaklings must content themselves with poor soils, inaccessible regions of mountain, swamp or desert.  There they deteriorate, or at best strike a slower pace of increase or progress.  The difference between the people of the highlands and plains of Great Britain or of France is therefore in part a distinction of race due to this geographical selection,[214] in part a distinction of economic development and culture due to geographic influences.  Therefore the piedmont belts of the world, except in arid lands, are cultural, ethnic and often political lines of cleavage, showing marked differentiation on either side.  Isotherms are other such cleavage lines, marking the limits beyond which an aggressive people did not desire to expand because of an uncongenial climate.  The distinction between Anglo-Saxon and Latin America is one of zone as well as race.  Everywhere in North America the English stock has dominated or displaced French and Spanish competitors down to the Mexican frontier.