THE DESCENT OF MAN. 373 



mote regions of the earth before the prehistoric epoch, the dispersion 

 of the human race must have been coincident with the ice age. It is 

 with the climatic and topographic condition of the glacial period, 

 therefore, that we have to do in determining the original routes of 

 migration. According to the astronomical explanation of glacial 

 phenomena, which best accords with the geological facts as far as they 

 are known, the Northern and Southern Hemispheres were alternately 

 subjected to frigid conditions. Owing, however, to the disposition 

 of the land-masses of the globe, glacial influences were more wide- 

 spread in the north than in the south. When the glaciers proceeded 

 from the arctic regions, the climate of the northern continents grew 

 cold, the thermal equator moved somewhat south of the geographical 

 equator and the southern peninsulas became predominately tropical. 

 When, on the other hand, the ice advanced from the antarctic regions, 

 the highlands of the southern peninsulas were glaciated, and, as the 

 thermal equator moved north of the geographical equator, the northern 

 continents enjoyed an equable climate, ranging from tropical to tem- 

 perate conditions and devoid of great seasonal variations. Several 

 such glacial cycles appear to have elapsed during quaternary times. 

 After the third advance of the ice from the north, however, the glacia- 

 tion of the hemispheres became less severe and the genial conditions 

 more permanent, until, towards the end of the great ice-age, the glaciers 

 were confined to the arctic and antarctic regions, and the globe became 

 divided as at present into temperature zones. 



As the ice advanced for the first time from the arctic regions, the 

 temperature of the Northern Hemisphere became gradually colder, until 

 during the early part of the pleistocene period, continental glaciers 

 spread down over central Europe and North America. The increasing 

 frigidity of the Eurasian continent at this time was doubtless sufficient 

 to deter dispersion from the Indo-Malaysian cradle-land towards the 

 north. Mountain ranges also hindered progress in this direction, for 

 the Himalayas must have acted as a barrier towards Asia and tended 

 to deflect the lines of migration east and west along the central lati- 

 tudes. Immediately preceding and during the first glacial epoch, there- 

 fore, climatic and topographic influences combined to confine the original 

 course of dispersion within the Indo-Mediterranean-Malaysian belt and 

 the lower peninsulas of the Old World. This southern portion of the 

 Eastern Hemisphere, it should be noticed, is separated from the north- 

 ern continental area by a broken mountain range, running from the 

 Himalayas to the Pyrenees. These mural masses protected the low- 

 lying lands along the southerly slopes of the mountains from the in- 

 creasing cold of the glaciated north, so that in spite of the fact that 

 the thermal equator ran somewhat south of the geographical equator 

 at this time, tolerable climatic conditions prevailed everywhere below 



