Delaware Rivers, or planted them on islands either furnishing or commanding extensive trade, such as Ceylon, Mauritius, the East Indies, or the Dutch holdings in the Antilles. Much farther down in the cultural scale we find the fisher tribes of Central Africa extending their villages from point to point along the equatorial streams, and the river Indians of South America gradually spreading from headwaters to estuary, and thence to the related environment of the coast. The Tupis, essentially a water race, have left traces of their occupation only where river or coast enabled them to live by their inherited aptitudes.[205] The distribution of the ancient mounds in North America shows their builders to have sought with few exceptions protected sites near alluvial lowlands, commanding rich soil for cultivation and the fish supply from the nearby river. Mountaineer folk often move from one upland district to another, as did the Lombards of Alpine Pannonia in their conquest of Lombardy and Apennine Italy, where all their four duchies were restricted to the highlands of the peninsula.[206] The conquests of the ancient Incas and the spread of their race covered one Andean valley after another for a stretch of one thousand five hundred miles, wherever climatic and physical conditions were favorable to their irrigated tillage and highland herds of llamas. They found it easier to climb pass after pass and mount to ever higher altitudes, rather than descend to the suffocating coasts where neither man nor beast could long survive, though they pushed the political boundary finally to the seaboard. [Map page 101.] [Sidenote: Movement to better geographic conditions.] The search for better land, milder climate, and easier conditions of living starts many a movement of peoples which, in view of their purpose, necessarily leads them into an environment sharply contrasted to their original habitat. Such has been the radial outflow of the Mongoloid tribes down from the rugged highlands of central Asia to the fertile river lowlands of the peripheral lands; the descent of the Iran pastors upon the agricultural folk of the Indus, Ganges and Mesopotamian valleys, and the swoop of desert-born conquerors upon the unresisting tillers of well-watered fields in all times, from the ancient Hyksos of the Nile to the modern Fulbe of the Niger Valley. [Sidenote: Southward and westward drifts in the northern hemisphere.]