84 H. A. GLEASON VEGETATIONAL HISTORY OF MIDDLE WEST 



sonia tomentosa,, and Tanacetum liuronense. Two other species, Salix 

 syrticola and Solidago Gillmani, are endemic to the Great Lakes, but 

 the distribution of their nearest relatives indicates that they are of the 

 same ultimate geographical origin. 



The most plausible explanation for the occurrence of these species so 

 far inland from their maritime range is that they migrated along the 

 shores of the Champlain Sea during the Algonkian or Nipissing stages 

 of the Great Lakes, and entered the lake region along the Kirkfield or 

 North Bay outlets. 33 This explanation agrees with the present center 

 of abundance of the lake shore species near the north end of Lake Huron 

 and, since the ice fields were not far away during this period, with their 

 prevailingly northern maritime distribution.* 



Summary of Part IV and Conclusion. — 1. The principal vegeta- 

 tional and floristic elements of the Middle West were differentiated 

 during the Tertiary Period and have continuously maintained their 

 present relative position. 



2. Each glacial advance has induced extensive migrations toward the 

 south and west, but in them the coniferous and arctic floras have been 

 greatly narrowed and the deciduous forests have lived uninterruptedly 

 in the Ohio valley. 



3. Extensive readjustments of distribution have taken place in each 

 interglacial stage. During the Sangamon, most or all of the Middle 

 West was probably occupied by deciduous forests. 



4. Climate in the Middle West beyond the Wisconsin glaciation was 

 semi-arid during the time of maximum ice advance, thereby restricting 

 the boreal floras greatly toward the west. 



5. A xerothermic period followed the retreat of the Wisconsin ice, 

 and caused the eastward migration of prairies as far as Ohio, succeed- 

 ing the coniferous forests as the latter migrated north. 



6. A gradual increase in rainfall was accompanied by a westward 

 migration of deciduous forests. This was composed of two elements, 

 one moving from the Ohio valley and one from the Allegheny Moun- 

 tains. At the close of this period, forests occupied a much greater 

 proportion of the Middle West than at present. 



7. With the arrival of the Indian and prairie fires, prairies again 

 began to encroach on the forests and reduced the forested area to its 

 condition at the beginning of the last century. 



8. During the nineteenth century extensive afforestation took place, 

 continuing until the land was placed under cultivation. 



* Since this paper went to press, D. C. Peattie has discussed this subject in 

 more detail in Rhodora 24: 57-70, 80-88. 1922. 



