THE PRAIRIE-GRASS FORMATION 77 



forced by the invasion of the deciduous forests. Such conditions are 

 visible at the Dells of the Wisconsin, the dunes at the head of Lake 

 Michigan, and in various places in southern and southwestern Michi- 

 gan. The conclusion from both conditions is that in the western and 

 central part of our area successions between the coniferous forests and 

 the prairie were mostly completed before the arrival of deciduous trees, 

 while toward the east the deciduous forests arrived before the prairies 

 or during their advance upon the boreal associations. 



Development of the Prairie-grass Formation. The prairie flora 

 also changed considerably during this advance of the deciduous forests. 

 Scores of species, which had been inhabitants of sand dunes, rocky soil 

 and barrens, or the more xerophytic upland woods, migrated faster than 

 the trees and became firmly established in the prairie associations. The 

 only restrictions on this movement were that the plants should be herbs, 

 dying to the ground every year and thereby escaping the effect of the 

 arid xerothermic winter, and adapted to compete for space with grasses, 

 which were the dominant plants of the prairies. Among these emigrants 

 were numerous southeastern grasses which, having the same vegetative 

 form, mingled easily with the western species and became dominant 

 with them. 



As the xerothermic period finally drew to a close, the western species 

 were at a disadvantage. They were unable to withstand a large increase 

 in moisture, while the species of eastern derivation could exist with a 

 much gTeater rainfall. The latter accordingly became more and more 

 abundant, until they practically dominated the prairies. In fact, the 

 four most important grasses of the Illinois prairies, Andropogon furca- 

 tus, A. scoparius, Sorghastrum nutans, and Spartina Michauxiana, are 

 all of eastern origin. The western species became reduced in number of 

 individuals or restricted to extreme habitats where edaphic conditions 

 compensated for the increased rainfall. 



Here we have the origin of the flora of the eastern arm of the Prairie 

 Province, early recognized by Pound and Clements as distinct from that 

 of the western plains, and designated by them the prairie-grass forma- 

 tion. Just as the original western prairies were segregated from the 

 arctotertiary forests by climatic differentiation, so the xerothermic 

 period produced an analogous vegetation thousands of years later from 

 the same stock. In the later case, however, the process was one of select- 

 ive migration, rather than of extermination and evolution, and most of 

 the southeastern species now characteristic of the prairie-grass formation 

 still live also in the forested region of the southeastern states. Never- 

 theless, the movement was doubtless accompanied by evolution in many 

 instances: Asclepias Meadii, a typically prairie species, seems to be 



