294 A NATURALIST IN THE GREAT LAKES REGION 



Without going back unduly into the past, it is very evident 

 that when the last glacial advance slowly came on and buried 

 the Great Lakes region almost in its entirety under thousands of 

 feet of ice, the plant and animal life existent here before the 

 advance began must have been obliterated, except in so far as 

 it could retreat southward into congenial territory. Moreover, 

 there must have been a complete change in the fauna and flora 

 even before the glacier arrived. For as it progressed there 

 preceded it a change in climate. The rigors of the winters 

 increased, the summers became brief and cool. Many of the 

 plants and animals flourishing here before the ice sheet began its 

 southward movement must have been driven out and replaced 

 with forms more characteristic of the north. Evergreens took 

 the place of the deciduous forests, and the animals changed 

 accordingly. As the glacier approached still nearer, the conifer- 

 ous forests extended farther south, while here trees were becoming 

 dwarfed. Then this region became treeless, and tundras covered 

 with hardy grasses, mosses, and lichens occupied it, similar to those 

 existent in the far north on the border of the regions of perpetual 

 snow. Finally this tundra region moved south, and the glacier 

 itself came on. Probably neither the tundra zone bordering the 

 glacier nor the coniferous forest zone beyond it were very wide 

 when the glacier reached its maximum extension, for it was melt- 

 ing along its front, so the climate there could not have been very 

 rigorous. The deciduous forest probably covered much of the 

 southeastern portion of the continent and the arid plains and 

 short grass regions much of the southwestern portions as now. 



When the last glacial period was passing off and the great 

 ice sheet was beating a slow retreat, the land, with topography 

 much changed, was open to plant and animal occupation. The 

 great tundras, the areas of stnnted trees, and the coniferous 

 forests largely followed the glacier northward, leaving a few 

 scattered remnants here and there. The climate of our region 

 again became congenial to the deciduous forests, the prairie 

 plants, and their accompanying animals. 



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