68o THE HUMAN PERIOD 



extreme desert conditions and excessive heights are prohibitive, that 

 semi-arid conditions of varying and precarious intensities lead to 

 nomadic habits, sparse distribution, and limited social and civic 

 evolution, while well- watered plains and fertile valleys, under 

 congenial skies, invite fixed habitation and the development of 

 stable civil and social institutions. Excessive humidity, dense 

 forests, and extreme ruggedness of surface tend to limitation and 

 repression among primitive peoples. Early in the history of the 

 race, it is presumed that a warm climate was more favorable than 

 a severe one. From these considerations and from historical 

 evidence arises the presumption that the primitive centers of evolu- 

 tion of the race were somewhere in the open or diversified parts of 

 the warm tract of the largest of the continents. From this, or from 

 some analogous tract in that quarter of the globe, there seem to 

 have been divergent movements to all habitable lands. 



A basal factor in the early evolution of civilization was the 

 productiveness of the soil. The advance from hunting and fishing 

 and herding was dependent essentially on agriculture, and was 

 therefore influenced largely by the fertility of the soil and suitable 

 climatic conditions. Loss of soil-fertility has been one cause which 

 has forced the migration of centers of civilization. In lower lati- 

 tudes the upland soils are mostly the residue produced by the 

 decomposition of the underlying rocks and not removed by erosion. 

 With cultivation, wash and wind-drift are accelerated, and unless 

 protective measures are employed, as has not been the case usually, 

 the soils are carried away, and barrenness succeeds fertility. There 

 are areas in the Orient, once well settled, where nothing grows ex- 

 cept such plants as find a foothold in the crevices of the rock. In 

 some places, soils underlain by sandy subsoils have been washed 

 away, leaving barren wastes. Sands from the exposed subsoil have 

 then been driven by the wind over adjacent fertile tracts, making 

 them barren. The explanation of much of the former richness and 

 present poverty of Oriental peoples no doubt lies in this simple proc- 

 ess. Impoverishment of soil threatens many peoples to-day, and 

 is in process of actual realization. This is one of the fields in which 

 conservation is most important. 



In glaciated lands the soil-factor has a character quite its own. 

 i. Near the centers of ice radiation the old soils were worn away, 

 and new soils have not developed in equal amount in their stead. 

 Reduced fertility is the result. These areas lie chiefly in high 



