THE HUMAN OR PRESENT PERIOD. 531 



Lakes Bonneville and Lahontan, the distinctively arid topographies 

 of the west — the mesas, buttes, and canyons that only an arid environ- 

 ment can develop — the evolution of the xerophytic floras that have 

 been transmitted to the present stage, and the special faunas adapted 

 to and dependent on these xerophytic floras. The aridity that gave 

 rise to these physiographic and biologic evolutions probably had its 

 center in the zone of descending atmospheric currents which should 

 normally have lain near the thirtieth degree of latitude, but which, 

 in this hemisphere, is now, and probably was then, shifted to the north- 

 ward by the configuration of the great bodies of land and water. The 

 pre-glacial arid tracts seem to have had a distribution in the western part 

 of our continent not unlike that of to-day, while the eastern half of 

 the continent was then, as now, more moist, and covered with forests 

 rather than herbaceous vegetation. With the invasion of the ice of the 

 glacial period, the floras and faunas were forced southward, as described 

 in the story of that period, but differentially in the two sections. In the 

 west, the northern life was driven by ice behind, hemmed in by mountain 

 and other barriers at the sides, and resisted by arid tracts in front. 

 The arid tracts were themselves forced to retire in some measure, but 

 the lateral restraint of biotic migration became increasingly formidable 

 as glaciers gathered on the mountain heights and occupied the passes. 

 As the trends of the mountains' were mainly north and south, they 

 demarked a series of meridional tracts which directed the life migra- 

 tions. There was therefore but little of the east-and-west intermigra- 

 tion that might otherwise have prevailed. Even on the plains east 

 of the mountains, the climatic differences seem to have appreciably 

 restrained east and west migration. 



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

 driven southward in the more unrestrained way already described, 

 but for the greater part they kept within the eastern humid tract. 



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

 moved northward, each biotic zone, arctic, subarctic, cold-temperate, 

 and temperate, expanding as it went. It was as though the life-zones were 

 elastic bodies which had been compressed to narrow limits about the 

 edge of the advancing ice, and then recovered their normal breadth 

 as the ice-pressure was withdrawn. The arctic or tundra flora and 

 fauna that had probably been crowded into an almost vanishing zone 

 fringing the ice-sheet, moved northward through about 20° of latitude, 



