EARLY GLACIAL STAGES 59 



reason for believing that such genera could not have produced species 

 adapted to a temperate climate then, just as Phoradendron, Diospyros, 

 and Tripsacum have at present, so that the actual shift of climate north- 

 ward may have been only a few hundred miles. 



These three floristic and vegetational types have since maintained 

 their identity, and the history of the vegetation of the Middle West is 

 concerned almost exclusively with them. A fourth floristic element, 

 the Coastal Plain flora of the southeastern states, was likewise segre- 

 gated from the arctotertiary forests, probably during the late Tertiary 

 or Pleistocene, and enters the southern portion of our region, but has 

 probably never occupied a more extensive territory than at present. 

 The Coastal Plain flora has been considerably modified by the immi- 

 gration, accompanied by evolution, of numerous tropical forms, enter- 

 ing the region via Texas or the Florida peninsula. Some of these have 

 extended still farther north and have invaded other floras as well. 



Early Glacial Stages. — There is no present evidence concerning 

 the migrations of vegetation during the jSTebraskan and Kansan glacial 

 advances, at least one of which entered our area, or during the corre- 

 sponding Aftonian and Yarmouth interglaeial stages, and the earliest 

 evidence at hand deals with its location during the Illinoian period at 

 the time of maximum advance of the ice. 



The southern boundary of Illinoian glaciation extends in a generally 

 southwesterly direction across Ohio and Indiana and lies on the north- 

 ern slope of the Ozark uplift in southern Illinois. It then turns north- 

 ward, following in a general way the present course of the Mississippi 

 river to the Wisconsin border, displacing the river westward in eastern 

 Iowa, but passing to the east of the driftless area in northwestern 

 Illinois. Its deposits in Wisconsin are almost wholly covered by 

 those of a later period and its further extent in the Middle West is 

 unknown. 



The pre-Illinoian flora of the territory covered by this ice sheet must 

 have been destroyed or have migrated south, west, or in both directions. 

 The immediate problems are to ascertain the distance beyond the glacial 

 boundary to which it was forced and the location of the extraglacial 

 floras during the time of maximum ice advance. Several features of 

 present distribution seem to cast some light on the questions. 



The Mississippi Embayment. — The Mississippi embayment of south- 

 ern Indiana, Illinois, and Missouri is occupied by a large number of 

 species which reach here their northern limits in the extensive flood- 

 plains of the Ohio, Wabash, and Mississippi rivers, without migrating 

 farther to the north along these floodplains into glaciated territory. 



