isotherms.  Excessive heat lays its retarding touch upon everything, while a prevailing aridity (rainfall less than 10 inches or 25 centimeters), except on the narrow windward slope of the eastern mountains, gives the last touch of climatic monotony.  The coastal belt of Cape Colony and Natal raise tropical and sub-tropical products[1431] like all the rest of the continent, while the semi-arid interior is committed with little variations to pastoral life. [See maps pages 484 and 487.] Climatic monotony, operating alone, would have condemned South Africa to poverty of development, and will unquestionably always avail to impoverish its national life.  South African history has been made by its mines and by its location on the original water route to India; the first have dominated its economic development, and the latter has largely determined its ethnic elements—­English, Dutch, and French Huguenots, while the magnet of the mines has drawn other nationalities and especially a large Jewish contingent into the urban centers of the Rand.[1432] In the background is the native Kaffir and Hottentot stocks, whose blood filters into the lower classes of the white population.  The diversity of these ethnic elements may compensate in part for the monotony of climatic conditions, which promise to check differentiation.  However, climatic control is here peculiarly despotic.  We see how it has converted the urban merchants of Holland and the skillful Huguenot artisan of France into the crude pastoral Boer of the Transvaal.

In contrast to South Africa, temperate South America has an immense advantage in its large area lying outside the 20 deg.C. isotherm, and in the wide range of mean temperatures (from 20 deg.C. to 5 deg.C.) found between the Tropic of Capricorn and Tierra del Fuego.  Climate and relief have combined to make the mouth of the La Plata River the site of the largest city of the southern hemisphere.  Buenos Ayres, with a population of over a million, reflects its large temperate hinterland.

[Sidenote:  The effects of Arctic cold.]

Frigid zones and the Tropics alike suffer from monotony, of Arctic the one of cold and the other of heat.  The Arctic climatic belt, extending from the isotherm of 0 deg.C. (32 deg.F.) to the pole, includes inhabited districts where the mean annual temperature is less than -15 deg.C. (or 5 deg.F.), as at the Greenland village of Etah on Smith’s Sound and the Siberian town of Verkhoyansk.  Here the ground is covered with, ice or snow most of the year, and permanently frozen below the surface.  Animal and plant life are reduced to a minimum on the land, so that man, with every poleward advance of his thin-strung settlements, is forced more and more to rely on the sea for his food.  Hence he places his villages on narrow strips of coast, as do the Norse of Finmarken, the Eskimo and the Tunguse inhabiting the Arctic rim of Asia.  Products of marine animals make the basis of his domestic economy.  Farther inland, which means farther south, all tribes live by hunting and fishing.  The Eurasian Hyperboreans find additional subsistence in their reindeer herds, which they pasture on the starchy lichen (Cladonia rangiferina) of the tundra. [See maps pages 103, 153.]