MIGRATION AND EVOLUTION 75 



glacial period more or less mixed with, boreal elements, and this condi- 

 tion has been increased by their postglacial migration into a country 

 from which the coniferous forests were retreating and leaving numer- 

 ous relics behind, without an intervening prairie stage. Moving slowly 

 northward behind the glaciers, which occupied the Ontario basin long 

 after migration routes farther west were open, a portion of the species 

 concerned finally turned to the west and northwest and entered Michi- 

 gan, Wisconsin, and Minnesota, while others, such as Quercus Prinus, 

 moved scarcely west of the Appalachian region. 



It has not been possible so far to form an opinion as to the par- 

 ticular postglacial stage of the Great Lakes when this westward turn 

 took place. It may have been in a narrow strip between the prairies 

 and the coniferous forests, following the former to the north and 

 broadening out only after reaching relatively high latitudes, in which 

 case it could have occurred at an early postglacial stage, or it may be 

 a more recent movement, even as late as the Xipissing stage, in which 

 case it might have passed to the north of Lake Erie and entered 

 Michigan by crossing the Detroit and St. Mary's rivers. 



The Combined Effects. — The two migrations together may be com- 

 pared to a vast U, with its base in the southern Alleghenies, one side 

 extending as far west as the Ozarks, and the other passing northwest 

 to Minnesota. The northern migrants have shown little tendency to 

 move southward, especially toward the western part of their route, 

 while the southern ones have moved steadily northward until their 

 outposts have entered the area of the northern arm in Michigan and 

 Wisconsin, where oak, sassafras, and sycamore occur in the same area 

 with sugar maple, beech, and white pine. The base of the U has 

 gradually closed over Ohio and part of Indiana, chiefly through immi- 

 gration from the south. As a result there is little difference between 

 the forests of southern Michigan and those of any part of glaciated 

 Indiana. The valley of the Grand River in central Michigan is a 

 well marked division line between forests of the two types, according 

 to observations of B. E. Quick, and above it the deciduous forests are 

 derived mostly from the northern migration. 



Migration and Evolution. — The forest migration was accompanied 

 by specific evolution also, and various species have been described from 

 the forest region of the Middle West. In most cases their probable 

 evolution is in doubt, from lack of careful study, but Dr. F. W. Pennell 

 has kindly supplied certain cases among the Scrophulariaceae in which 

 the ancestry of the species or varieties seems reasonably certain. Thus 

 Agalinis paupercula of the glaciated region is considered as derived 



