forest are modified considerably. The larger mammals and a number 

 of the birds can no longer be found, and their places are filled by other 

 species which have become more numerous. 



GI^NERAL HISTORY 01^ THE SAND PRAIRIE VEGETATION 



Certain plant ecologists believe that after the close of the Illinois 

 glaciation, and again after the retreat of the Wisconsin ice-sheet, the 

 climate of the interior was extremely arid; that the conifers of the 

 ice margin were succeeded directly by prairie plants of intensely xero- 

 phytic nature ; and that there was consequently a great eastern exten- 

 sion of the prairie. The mesophytic prairie-grass of the eastern prairie 

 region had not then reached its present development. There was sub- 

 sequently an increase of moisture in the eastern states and in the Mis- 

 sissippi valley. This resulted in the east, in an advance of the decidu- 

 ous forest from the southeast, and in the Mississippi valley, in the 

 development of the mesophytic prairie formation. West of this region 

 little or no change from the former arid conditions took place ; the 

 xerophytic prairie formation of the sand-hills therefore was changed 

 little if at all. This partial histoi"y of the eastern prairies is supported 

 by a mass of evidence which does not properly belong in this account. 

 The evidence is taken from the distribution of relic colonies of west- 

 em and northern plants, from the present successional tendencies of 

 our vegetation, and from our knowledge of glacial and climatic history. 



If we accept this explanation of vegetational changes in the past, 

 we see at once that after and during the Wisconsin glaciation, the 

 xerophytic grassland, which was then at its maximum development, 

 must have covered most of Illinois south and west of the conifer mar- 

 gin, which never extended far beyond the terminal moraine. But it 

 was at about the same time that the sand deposits of the Illinois River, 

 and similar deposits of the Rock River and the Mississippi, were left 

 uncovered by the diminishing streams. The xerophytic prairie was 

 therefore the original vegetation of the sand deposits, for it was the 

 only vegetation near enough at hand for invasion. This prairie was 

 continuous in distribution from its present location in the sand-hills 

 of the West to central Illinois and much farther east. It has persisted 

 in the West because the arid climate has remained unchanged. It has 

 persisted in the sand deposits of Illinois because physical conditions of 

 the soil, evaporation, and exposure have been such as to keep out in- 

 vaders not adapted to these conditions. The kinship between the sand 

 prairie of Illinois and the sand prairie of the West is shown in the 

 plants and in the animals. Many animals reach the eastern limit of 



