678 THE HUMAN PERIOD 



post-glacial formations remain beneath the sea, and are not avail- 

 able for study. The general character of the formations being made 

 will be readily inferred from what has been said in earlier chapters. 



LIFE 



In the seas, and on the land in the tropics, the life of the Pleis- 

 tocene period appears to have passed by imperceptible gradations 

 into that of the present. In the higher latitudes the transition 

 was marked by two exceptional features, the re-peopling of the 

 lands depopulated by the ice, and the invasion of the human race. 



Re-peopling the glaciated areas. The re-peopling of the north- 

 western half of North America by plants and animals after the re- 

 treat of the last ice-sheet was a great event of its kind. Certain 

 plants that abounded in Europe before the glacial period were 

 forced across the Mediterranean, or southeastward into Asia, and 

 did not recross the barriers of water and desert when the climate 

 of Europe became mild again. No such barrier intervened in North 

 America. There was, however, an ill-defined climatic barrier be- 

 tween the arid plain region of the southwest and the humid forest 

 region of the southeast. There is abundant evidence that open 

 plains and arid climates had developed in the middle latitudes of 

 the west by the later part of the Tertiary, and that these have 

 persisted, perhaps with brief interruptions, till now. The pre- 

 glacial arid tracts of the west seem to have been distributed much as 

 now, while the eastern half of the continent was more moist, and 

 covered with forests. 



As the floras -and faunas of the western mountain region were 

 driven south by the ice, they were hemmed in by mountain barriers 

 at the sides, and resisted by arid lands in front. As the trend of the 

 mountains was mainly north and south, they defined a series of 

 meridional tracts which directed the life migrations. 



In the eastern half of the continent, forests and forest-life were 

 driven southward in a more unrestrained way, but for the most part 

 they kept within the eastern humid tract. 



Following the last ice-retreat, the life of each of these sections 

 moved northward, expanding as it went. The arctic or tundra flora 

 and fauna that had. probably been crowded into a narrow zone 

 fringing the ice-sheet, moved northward through about 20, and 

 expanded to a breadth of 600 or 700 miles in the northern part of 

 the continent, and occupied the arctic islands not covered by 



