THE EVIDENCE OF PLANT MIGEATIONS 51 



tion, such as the area here discussed, fall in this group, and in them 

 floras of different origin are still relatively distinct and adjustments 

 of range are actively in progress. On the other hand, in regions which 

 have enjoyed relatively uniform environmental conditions for a long 

 period of time, migrations are more generally completed and vegeta- 

 tion is in more nearly a static condition, except in those successional 

 series which are related to' continuous physiographic processes such as 

 base-leveling. This is now the case in parts of the tropics, and was 

 formerly the case over wide areas of the North Temperate zone in the 

 later Cretaceous and earlier Tertiary periods, when the flora and vegeta- 

 tion of North America and Eurasia were remarkably uniform. 



Summary of Paet II. — 1. Migration of a species depends on an 

 environmental change within or beyond its range, 



2. The rate of migration depends on the rate of environmental 

 change or the migration-capacity of the plant. 



3. The present status of migration, as shown by the range of the 

 species, depends on its rate and on the time available for it. 



4. The present range of a species is not necessarily final, even with- 

 out further environmental change. 



5. Migration proceeds most rapidly along routes with nearly con- 

 tinuous habitats. 



6. Species of similar environmental demands migrate together. 



7. Isolated areas of a species or of vegetation are to be interpreted 

 as results of retreating migration. 



8. Long continued uniformity of environment leads to floristic uni- 

 formity; recent environmental changes to floristic segregation and 

 to new migrations. 



Part III. The Evidence of Plant Migrations 



The entire distance which may have been travelled by the native 

 plants of the Middle West in their migrations since the close of the 

 last glaciation seldom exceeds 500 miles. Since the time available 

 for this movement may be 10,000 to 40,0G$:*years, this requires an 

 average annual migration of only 66 to 264 feet. In the case of trees, 

 a juvenile period of 20 years would still necessitate a movement during 

 one generation of not more than a mile. The discovery of migrations 

 still in progress is accordingly removed from direct observation, except 

 in the case of a few species, and any conclusions must depend entirely 

 upon historical or indirect evidence. 



