938 GEOLOGY 



eastern hemisphere, perhaps southern Asia or northern Africa, as 

 the place of his appearance. There are some grounds for the in- 

 ference that the earliest developments of those qualities that gave 

 him dominance were associated with the open tracts of the .sub- 

 tropical zone, rather than with the forests of the equatorial belt. 

 Subsequent history, as well as the nature of the case, teach us that 

 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, 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 virile 

 evolution and radiation of the race lay 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 glol >o, 

 there seem to have been divergent movements to all habitable 

 lands of the earth. 



A basal factor in the early evolution of civilization was the 

 productiveness and availability of the soil. The passage from the 

 condition of hunters and fishers, scattered in adjustment to the 

 distribution of game and shifting with its changes, or from that of 

 simple herders in sterile tracts roaming with the changes of pasture, 

 was dependent essentially on agriculture, and was therefore in- 

 fluenced largely by the fertility of the soil and suitable climatic 

 conditions. And so, conversely, among the agencies that have 

 forced the migration of centers of civilization, loss of soil-fertility 

 is one of the more important. In the lower latitudes, the upland 

 soils are usually the residue left by the decomposition of the under- 

 lying rocks which has not been removed by surface-wash. With 

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

 tective measures are employed, as has not been the case usually. 

 the soils are swept away, and barrenness succeeds productive!!. 



