The larger and more uniform a climatic district, the more conspicuously do even slight elevations form climatic islands, like the Harz Mountains in the North German lowlands.  A land of monotonous relief has a uniform climate, while a region rich in vertical articulations is rich also in local varieties of climate.[1254] A highland of considerable elevation forms a cold district in the Temperate Zone, a temperate one in the Tropics, and a moist one in a desert or steppe.  Especially in arid and torrid belts does the value of elevation for human life increase.

[Sidenote:  Altitude zones of economic and cultural development.]

The highlands of Mexico, South America and the Himalayan rim of India show stratified zones of tropical, temperate, and arctic climate, to which plant, animal and human life conform.  The response is conspicuous in the varying density of population in the successive altitude zones.  Central Asia shows a threefold cultural stratification of its population, each attended by the appropriate density, according to location in steppe, piedmont and mountain.  The steppes have their scattered pastoral nomads; the piedmonts, with their irrigation streams, support sedentary agricultural peoples, concentrated at focal points in commercial and industrial towns; the higher reaches of the mountains are occupied by sparse groups of peasants and shepherds, wringing from upland pasture and scant field a miserable subsistence.  The same stratification appears in the Atlas Mountains, intensified on the southern slope by the contrast between the closely populated belt of the piedmont and the wandering Tuareg tribes of the Sahara on the one hand, and the sparse Berber settlements of the Atlas highlands on the other.  The long slope of Mount Kilimanjaro in German East Africa descends to a coastal belt of steppe and desert, inhabited by Swahili cattle-breeders.  Its piedmont, from 1000 feet above the plain up to 2400 feet, constitutes a zone of rich irrigated plantations and gardens, densely populated by peaceful folk of mingled Bantu and Hamitic blood.  At 6000 feet, where forests cease, are found the kraals, cattle, sheep and goats of the semi-nomadic Masai of doubtful Hamitic stock, who raid the coastal lowlands for cattle, and purchase all their vegetable food from the tillage belt.[1255] [See maps page 105 and 487.]

[Illustration:  DENSITY OF POPULATION IN ITALY.]

This stratification assumes marked variations in the different geographical zones.  In Greenland life is restricted to the piedmont coastal belt; above this rises the desert waste of the ice fields.  Norway shows a tide-washed piedmont, containing a large majority of the population; above this, a steep slope sparsely inhabited; and higher still, a wild plateau summit occupied in summer only by grazing herds or migrant reindeer Lapps.  Farther south the Alps show successive tiers of rural economy, again with their appropriate density of settlement.  On their