530 GEOLOGY. 



merit, must have aided the shear of the crustal shell beneath it. This 

 may be among the reasons why fjords are so prevalent in these regions. 



THE LIFE OF THE HUMAN PERIOD. 



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

 cene appears to have passed by imperceptible gradations into that 

 of the present period. 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. We 

 say invasion of the human race advisedly, for whatever may be true 

 in the low latitudes, where the race perhaps came into its peculiar 

 function gradually, in the higher latitudes the apparition of man took 

 on the aspect of an invasion; indeed, from the point of view of other 

 living creatures, it came as an irresistible inundation. Thus far 

 man's dominance has been most pronouncedly a mid-latitude move- 

 ment, with less pronounced potency in the very high and the very 

 low latitudes, but even these latitudes are not likely long to escape 

 the overwhelming supremacy of the new dynasty. 



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 not only the greatest event of this class, 

 but may be studied to greater advantage than the similar event in 

 northwestern Europe, because of the uninterrupted thoroughfare between 

 low and high latitudes. Laporta has called attention to the barrier 

 interposed by the Mediterranean to the free re-peopling of Europe 

 after the ice-invasions. He notes that certain plants that abounded 

 in Europe before the ice-invasions, were forced across the Mediterra- 

 nean, or southeastward into Asia, and did not recross the barriers of 

 water and desert on the resumption of a congenial climate in Europe. 

 No such barrier intervened in North America. 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 abun- 

 dant evidence that open plains and arid climates had developed in 

 the western region in middle latitudes in the late Tertiary periods, 

 and that these were retained, with modifications and perhaps brief inter- 

 ruptions, throughout the glacial period and have become a present 

 inheritance. Among these evidences are the repeated drying-up of 



