In the case of both primitive societies and those of a higher order the causes and the results of migration are often contradictory. These will depend on the state of civilization and the extremes of circumstance. When the desert blooms the farmer of the Piura Valley in northwestern Peru turns shepherd and drives his flocks of sheep and goats out into the short-lived pastures of the great pampa on the west. In dry years he sends them eastward into the mountains. The forest Indian of the lower Urubamba is a fisherman while the river is low and lives in a reed hut beside his cultivated patch of cane and yuca. When the floods come he is driven to the higher ground in the hills where he has another cultivated patch of land and a rude shelter. To be sure, these are seasonal migrations, yet through them the country becomes better known to each new generation of men. And each generation supplies its pioneers, who drift into the remoter places where population is scarce or altogether wanting.
Fig. 24—This stone hut, grass-thatched, is the highest permanent habitation in Peru, and is believed to be the highest in the world. Altitude of 17,100 feet (5,210 m.) determined by instrumental survey. The general geographic relationships of the region in which the hut is situated are shown in [Fig. 25]. For location see the topographic map, [204] .
Dry years and extremely dry years may have opposite effects. When moderate dryness prevails the results may be endurable. The oases become crowded with men and beasts just when they can ill afford to support them. The alfalfa meadows become overstocked and cattle become lean and almost worthless. But there is at least bare subsistence. By contrast, if extreme and prolonged drought prevails, some of the people are driven forth to more favored spots. At Vallenar in central Chile some of the workmen in extreme years go up to the nitrate pampa; in wet years they return. When the agents of the nitrate companies hear of hard times in a desert valley they offer employment to the stricken people. It not infrequently happens that when there are droughts in desert Chile there are abundant rains in Argentina on the other side of the Cordillera. There has therefore been for many generations an irregular and slight, though definite, shifting of population from one side of the mountains to the other as periods of drought and periods of rain alternated in the two regions. Some think there is satisfactory evidence to prove that a number of the great Mongolian emigrations took place in wet years when pasture was abundant and when the pastoral nomad found it easy to travel. On the other hand it has been urged that the cause of many emigrations was prolonged periods of drought when the choice lay between starvation and flight. It is evident from the foregoing that both views may be correct in spite of the fact that identical effects are attributed to opposite causes.