78 H. A. GKLEASON VEGETATIONAL HISTORY OF MIDDLE WEST* 



a derivative of Asclepias amplexicaulis of a more eastern range; a vari- 

 etal evolution from Agalinis termifolia, illustrates the same geograph- 

 ical relation, according to Dr. F. W. Pennell, and other similar cases 

 might be cited. 



The numerous ponds of the eastern arm of the Prairie Province were 

 also formed at this time, in response to increasing rainfall in a region 

 physiographically immature. They were of necessity colonized almost 

 completely by species of southeastern origin, since true hydrophytes 

 were not ^ound in the western vegetation. 



Extent of Foeest Migration in the Prehistoric Period. The 

 northern arm of the forest advance lay chiefly or wholly in a region not 

 reached by .the prairies. There is no reason to believe that it has 

 suffered any retardation to the present time, except as caused within 

 the past century by commercial developments in lumbering and agri- 

 culture. 



The southern migration, on the other hand, was quite different in 

 nature, due to a less favorable climate and to the character of the vegeta- 

 tion encountered. It has already been noted that it followed the water 

 courses, most of which, as tributaries of the Mississippi, offer convenient 

 highways leading from the center of forest preservation into the area 

 of xerothermic prairies. It has also been noted that the longitudinal 

 advance following the streams was more rapid than the lateral advance 

 at right angles to them. The forests therefore soon assumed the form 

 of long branching strips, following the rivers from their mouths toward 

 their sources. Through the slow lateral advance these strips were 

 widened and eventually became confluent across the interfluvial prairies, 

 frequently isolating portions of the prairie in the process. 



The nature of the forest advance was determined by the location of 

 sufficient environmental change to permit the migration. Since the 

 unfavorable environment was chiefly due to conditions of rainfall and 

 atmospheric humidity, the advance followed the lines of topographic 

 relief, where some shelter was offered from the drying winds and the 

 subsurface water stood at a greater height, and the water courses where 

 edaphic moisture compensated for low rainfall. The rolling surface of 

 the moraines offered favorable conditions for trees of the xerarch series, 

 and they were soon colonized by forests of oak and hickory, reaching the 

 moraines by the intersecting streams and spreading across from one 

 stream valley to another. On the other hand, the wide alluvial bottom 

 lands of the larger rivers, notably the Missouri, Mississippi, and Illinois, 

 seem to have resisted forest invasion, and on them the forests were 

 limited to relatively narrow strips along the channel and the abandoned 

 oxbows, alternating with strips of prairie. This condition, recorded 



