The chief features of mountain relief reappear on a large scale in the continents, which are simply big areas of upheaval lifted above sea level.  The continents show therefore homologous regions of lowlands, uplands, plateaus and mountains, each district sustaining definite relations to the natural terrace above or below it, and displaying a history corresponding to that of its counterpart in some distant part of the world, due to a similarity of relations.  This appears first in a specialization of products in each tier and hence in more or less economic interdependence, especially where civilization is advanced.  The tendency of conquest to unite such obviously complementary districts is persistent.  Hence the Central Highland of Asia is fringed with low peripheral lands like Manchuria, China, India and Mesopotamia, into whose history it has repeatedly entered as a disturbing force.  All the narrow Pacific districts of the Americas from Alaska to Patagonia are separated by the Cordilleras from the lowlands on the Atlantic face of the continents; all reveal in their history the common handicap arising from an overwhelming preponderance of plateau and mountain and a paucity of lowlands.  Colombia, Ecuador and Peru have in the past century been stretching out their hands eastward to grasp sections of the bordering Amazon lowlands, where to-day is the world’s great field of conflicting boundary claims.  Chile would follow its geographical destiny if it should supplement its high, serrated surface by the plateaus and lowlands of Bolivia, as Cyrus the Persian married the Plateau of Iran to the plains of the Tigris and Euphrates, and Romulus joined the Alban hills to the alluvial fields of the Tiber.

[Sidenote:  Anthropo-geography of lowlands.]

Well-watered lowlands invite expansion, ethnic, commercial and political.  In them the whole range of historical movements meet few obstacles beyond the waters gathering in their runnels and the forests nourished in their rich soils.  Limited to 200 meters (660 feet) elevation, lowlands develop no surface features beyond low hills and undulating swells of land.  Uniformity of life conditions, monotony of climate as of relief, except where grades of latitude intervene to chill or heat, an absence of natural boundaries, and constant encouragement to intercourse, are the anthropo-geographic traits of lowlands, as opposed to the arresting, detaining grasp of mountains and highland valleys.  Small, isolated lowlands, like the mountain-rimmed plains of Greece and the Aegean coast of Asia Minor, the Nile flood-plain, Portugal, and Andalusia in Spain, may achieve precocious and short-lived historical importance, owing to the fertility of their alluvial soils, their character as naturally defined districts, and their advantageous maritime location; but while in these restricted lowlands the telling feature has been their barrier boundaries of desert, mountains and sea, the vast level plains of the earth have found their distinctive and lasting historical importance in the fact of their large and unbounded surface.