DETERMINING MIGRATION CENTERS 53 



extending their range. Similar behavior toward the center of the range 

 may indicate only a general closing-up process, representing the later 

 slower stages of a migration and not implying any general extension 

 of range in the ordinary sense of the word. 



The known successional tendencies of the three major types of vegeta- 

 tion of the Middle West have frequently been described, especially 

 the tendency of the deciduous forests to succeed the prairies toward 

 the west and the coniferous forests toward the north. Project this 

 tendency into the future and it points to a still wider extension of this 

 type of forest, with a corresponding restriction of prairie and coni- 

 ferous forest. Project it into the past and it reveals a former condi- 

 tion of restricted deciduous forest and of a larger extent of prairie 

 and coniferous forest. Obviously, such reasoning must not be carried 

 too far, and its results must always be checked by the existence of relic 

 species or relic colonies, as discussed in a following paragraph. 



The stratigraphic sequence of fossils has been used with great suc- 

 cess as one line of evidence in Europe. 26 While similar evidence has 

 already yielded important results in America, the necessary data for 

 extensive generalizations are usually lacking or deficient. The careful 

 examination of the structure of peat has scarcely more than begun in 

 this country, and the pioneer work of Dachnowski 27 may lead to impor- 

 tant results. 



Geological events must have necessitated certain migrations and by 

 their very nature give a general idea of them. Thus it is incredible 

 that beech-maple forests could have occupied northern Michigan during 

 the maximum advance of the Wisconsin glaciers, which reached some 

 400 miles farther south. A northward movement of these trees and 

 their associated species to their present location must have taken place 

 after the retreat of the ice, but the details of this migration must be 

 discovered by other lines of evidence. 



Determining Migration Centers. — Adams 5 has advanced certain 

 criteria for determining the migration-center of a ' >als, which are in 

 many cases equally applicable to plants. They ar^ th two omissions, 

 as follows : • t 



1. Location of greatest differentiation of a type.*? , 



2. Location of dominance or great abundance or Individuals. 



3. Location of synthetic or closely related forms. 



2« Lewis, F. J. The plant remains in the Scottish peat mosses. Part I. The 

 Scottish southern uplands. Trans. Roy. Soc. Edinburgh 41: part 3, 699-723. 1905. 



27 Dachnowski, Alfred P. Peat deposits and their evidence of climatic changes. 

 Bot. Gaz. 72: 57-89. 1921. 



