THE HUMAN OR PRESENT PERIOD 935 



shelves. It seems not improbable that valleys in the outer edge of 

 the continental shelf, and on the abysmal slope, are even deepened 

 and widened by tidal scour. 



THE LIFE OF THE HUMAN PERIOD 



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 

 laid waste by the ice-incursions, and the invasion of the human 

 race. Whatever may have been true in the low latitudes where 

 the human race perhaps came into ascendancy gradually, the 

 appearance of man in the higher latitudes was an invasion, and 

 from the point of view of other organisms, it was an irresistible 

 inundation. 



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

 northeastern half of North America by plants and animals after 

 the retreat of the last ice-sheet was a great event of its kind. Cer- 

 tain 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, so that the problem of re-peopling can be studied to 

 better advantage in our continent than in Europe. There was, 

 however, an ill-defined climatic barrier between 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 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 oncoming of the ice-sheets, the floras and faunas 

 were driven southward, as described in the history of that period. 



