Page:Popular Science Monthly Volume 60.djvu/381

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373
THE DESCENT OF MAN.

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 widespread 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 temperate 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 glaciation 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 latitudes. Immediately preceding and during the first glacial epoch, therefore, 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 northern 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 increasing 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