THE GENERAL NATURE OF PLANT MIGRATIONS 41 



of these are intimately concerned with migration, the latter factor por- 

 traying its progress and the former its limitation. Neither factor alone 

 can account completely and satisfactorily for the present range of any 

 species. This is the resultant of its range at any period in the past 

 and its opportunities for subsequent migration; one exposure in the 

 long film of an activity which has been continued since the origin of 

 the species. If logs in the glacial drift show that Pinus Strobus 

 formerly existed in central Illinois and experiment proves that it can 

 thrive there now, historical evidence must be used to show why it left : 

 modern conditions may show whether and why it is not returning. 



Vegetational History and Human History. — The history of the 

 vegetation of the Middle West, as of every other portion of our con- 

 tinent, is a history of repeated migrations of diverse floristic elements, 

 arriving in the region from various directions, persisting there for 

 various lengths of time, and finally retreating under the pressure of 

 environmental changes which made their position no longer tenable. 

 This history has been complicated by the extinction of old and the 

 differentiation of new species, by the mingling of species of originally 

 different history, and by the persistence of isolated relics of early 

 migrations. In all these respects, vegetational history exhibits a strik- 

 ing parallelism to human history. Thus Europe has witnessed during 

 the last two millenniums the extinction or absorption of the early 

 Iberians, the persistence of the Basques, the arrival and retreat of the 

 Moors and Turks, and an enormous evolution in culture. Furthermore, 

 the underlying causes of vegetational and human history are similar, 

 even though the human species exhibits an exceedingly complex rela- 

 tion to its environment. The geography of man has long been recog- 

 nized as merely the present condition of his history, and the relations 

 of both to natural conditions and processes have been well summarized. 7 

 Unfortunately, no such summary exists for plants. 



Outline. — It has therefore been necessary to preface this history of 

 plant life in the Middle West, presented in Part IV., by a discussion 

 of the conditions and nature of plant migrations in Part II., and by a 

 resume of the evidence used in determining past migrations in Part III. 



Part II. The General Nature of Plant Migrations 

 A great body of literature dealing with the migration of plants has 

 appeared in the last century and a half. This has been so well sum- 

 marized bv Clements 8 that it needs no further discussion here. Most 



i Semple, Ellen C. Influences of geographic environment. New York, 1911. 

 s Clements, Frederic E. The development and structure of vegetation. Studies 

 in the vegetation of the state III. Botanical Survey of Nebraska VII. Lincoln, 1904. 



