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. As the great process of European colonization has permeated the earth and multiplied its population, not only the best land but the amount of this has commenced to differentiate the history of various European nations, and that in a way whose end cannot yet be definitely predicted. The best lands have fallen to the first-comers strong enough to hold them. People who early develop powers of expansion, like the English, or who, like the French and Russians, formulate and execute vast territorial policies, secure for their future growth a wide base which will for all time distinguish them from late-comers into the colonial field, like Germany and Italy. These countries see the fecundity of their people redounding to the benefit of alien colonial lands, which have been acquired by enterprising rivals in the choice sections of the temperate zone. German and Italian colonies in torrid, unhealthy, or barren tropical lands, fail to attract emigrants from the mother country, and therefore to enhance national growth. [Sidenote: Two-type populations.]