EARLY MIGRATIONS OF THE DECIDUOUS FORESTS 



73 



The eastward advance of the prairies was either accompanied or 

 followed by some specific evolution among the species participating, 

 as indicated by such plants as Phymosia remota" Synthyris Bullii, and 

 Tetraneuris herbacea. These plants are now confined to the eastern 

 arm of the Prairie Province, but in each case have their nearest related 

 species much farther west. i 



There is no reason to suppose that the xerothermic period came to 

 a sudden close, which would require an equally sudden geological 

 event in explanation. In fact, it may have been most pronounced 

 during the Wisconsin glaciation and diminished in intensity ever since, 

 but not sufficiently to check the rapid advance of the prairies behind 

 the retreating coniferous forests. Neither can the time of maximum 

 advance of the prairies be correlated at present with the postglacial 



stages of the Great Lakes. 



i 



Early Migrations of the Deciduous Forests. — At some time an 

 amelioration of the climate began. This change was probably less 

 connected with temperature, which has doubtless changed but little in 

 our area since the Wisconsin glaciation, than with rainfall and atmos- 

 pheric humidity. The total rainfall probably increased considerably, 

 but the increase fell chiefly during the winter months and changed 

 the climate from one of summer rains, such as is now characteristic 

 of the prairies farther west, to one of fairly equable rainfall. This 

 change first made itself felt in the east and gradually progressed 

 toward the west. At k ,the present time, each hundred miles west of 

 the Wabash Kiver along the fortieth parallel shows a marked diminu- 

 tion in winter rainfall. 



The effect .of this climatic change was the retardation and eventual 

 cessation of the old successional relation between coniferous forest 

 and prairie, soon followed by a break in the equilibrium along the 

 southern and eastern margins of the prairies, leading to an advance 

 of the deciduous forests northward and westward. 



Two general routes were followed in this forest migration, a north- 

 ward and .westward route from the forests along the southern edge of 

 the prairies and a northward and then northwestward route from those 

 lying east of the easternmost extension of the prairies. Many species 

 were common to >oth regions and participated in both migrations ; 

 others were localized and show by their present distribution that they 

 entered the Middle West by only one of these routes. Both took ,place 

 simultaneously, and in some places both migrations eventually met, 



47 Clute, W. N. The rarest American Plant. Amer. Botanist 26: 127-129. 1920. 



