418 THE CHANGING GENERATIONS 



The effects of the glacial epoch. The cooling of climates that began 

 in the Miocene culminated in the Pleistocene glaciations, in which ice 

 sheets spread over about one-fourth of the land surface of the world. 

 Four glacial ages occurred, with warmer interglacial intervals during 

 which the ice melted away and climates became much as they are now. 

 With each advance of the ice sheets all vegetation was destroyed over 

 vast areas, which were later reexposed and repopulated. The ranges of 

 plants and animals shifted and changed shape, not only in the glaciated 

 territories but in the bordering regions as well. Many of the older and less 

 adaptable species became extinct or were reduced to mere isolated rem- 

 nants of what had been abundant and widespread populations. As yet 

 we do not know whether the ice ages are over or whether we are living 

 in one of the warm interglacial times. Parts of the Great Lakes region and 

 New England were still ice-covered as recently as 12,000 years ago; the 

 longest of the interglacial ages endured perhaps 100,000 years. 



