396 PART IV. VEGETABLE TAXONOMY. 



which elsewhere has so greatly modified manv species and caused 

 the extinction of others. 



During the later Cretaceous and early Tertiary, a climate 

 considerably warmer than the present must have prevailed over 

 the northern hemisphere, for a flora similar to that now found in 

 the latitude of the great lakes prevailed as far north as the Arctic 

 Circle. Moreover, there appears to have been a greater uni- 

 formity of distribution over the northern hemisphere than at 

 present, which implies not only a more equable climate but a 

 common origin for the floras. Furthermore, the fossil remains 

 of plants afford evidence that there was a gradual lowering of 

 the temperature toward the close of the Tertiary. 



Plants of the Quarternary. The changes which took 

 place in the flora of this age, are largely such as may be attrib- 

 uted to the great refrigeration of the glacial epoch. As the 

 glaciers slowly plowed their way southward, the plants as gradu- 

 ally retreated before them until, in the northern portion of the 

 United States, as far south as the Ohio River, the temperate 

 flora had been supplanted by an arctic or sub-arctic one. 



Later, when the invading ice and its accompanying cold had 

 withdrawn to the north and a milder temperature had returned, 

 the arctic flora also mostly withdrew to its northern home, and 

 the temperate plants returned to their former habitat. 



This, however, did not occur without considerable loss. 

 Although the advance and retreat of the ice must have occupied 

 some thousands of years, or at least were so slow as not to inter- 

 fere with the migration of species by the ordinary natural means, 

 yet, in some cases, when the ice advanced, lakes, inland seas or 

 other formidable obstacles blocked the path of retreat for the 

 plants, and some species were thus hemmed in and destroyed. 



This was especially true in Europe, for here the impassable 

 Mediterranean lay in the path of retreat, and many species of 

 plants were there cut off and exterminated. This is doubtless the 

 reason of the poverty of the European flora as compared with 

 that of eastern North America, and it also accounts for the fact 

 that some types which in Tertiary times were abundant in both 

 Europe and America, now survive only in America. This is the 

 case with the Sequoias, the Tulip-tree and the Sassafras, already 

 mentioned. 



