Effects of tbe Glacial period 235 



The glacial period is not only closely re- 

 lated to the topography of North America and 

 parts of Europe in the changing of river beds, 

 the formation of lakes, the transportation of 

 rock, the grinding down of mountains and 

 spreading the debris over thousands of miles 

 in extent, but it is related in an intimate way 

 to many of the sciences, such as botany and 

 zoology. A study of the flight of animals and 

 plants in front of the great advancing ice 

 sheet is a subject of intense interest. The 

 migration of great forests would seem to be an 

 impossible thing when viewed from the stand- 

 point of a casual observer. It is true that in- 

 dividual trees could not take themselves up 

 and move forward in advance of the oncoming 

 ice, but they could and did send their children 

 on ahead, and when the ice had overtaken the 

 children there were still the children's children 

 ad infinitum. 



By an examination of the map it will be 

 seen that the land gathers about the north 

 pole, while the south pole is surrounded chiefly 

 by great oceans. As we have hinted before, in 

 preglacial times the temperate zone extended 

 much farther north than it does to-day, and 

 north of that there was an arctic zone (which 

 to-day is largely covered with ice sheets), 

 where forests, plants, and animals flourished 

 that were fitted for an arctic climate. When 

 the glacial period set in and the ice sheet be- 



