56 II. A. GLEASON VEGETATIONAL HISTORY OF MIDDLE WEST 



intervals within, the normal migration-capacity of the species, or were 

 entirely continuous over the whole area. Stated in more general terms, 

 the present occurrence of relic colonies indicates the past extent of 

 the vegetation of which they are a type. For the same reason, isolated 

 stations of a single relatively immobile species indicate a former more 

 nearly continuous distribution, and evidence from this source is of 

 greater importance when the species in question is regularly associated 

 with some peculiar or unusual habitat. 



A combination of evidence from successional tendencies and relic 

 colonies indicates both the direction and the extent of prehistoric migra- 

 tions. The full extent may not be indicated, because the isolated 

 colonies may have disappeared completely from the more remote parts 

 of their original range. In such cases isolated stations of single species 

 may give some not entirely untrustworthy idea of it. The farther 

 back in time plant migrations are traced, the fewer relic colonies may 

 be expected and the more dependence must be placed on individual 

 species. 



No one of these various lines of evidence is sufficient in itself to 

 build up a history of vegetational movements in the Middle West. 

 All of them must be used together and the deductions from all woven 

 into a history which accounts for present conditions satisfactorily and 

 is plausible in itself. The development of this history must be com- 

 menced with the present, where the latest migrations are shown by 

 successional relations. The extent of these recent events may be esti- 

 mated and checked by relic colonies, which in turn point to earlier 

 migrations. These must be checked by glacial topography, by infer- 

 ences concerning glacial climates, and by the distribution of plants 

 along the moraines. Still farther back, plant distribution and migra- 

 tions must be deduced from fossils, from the location of the moraines, 

 from the known physiological requirements of the plants, and through 

 the use of Adams' criteria. 



The vegetational history which follows in Part IV has been con- 

 structed in this way. Observed conditions and recorded vegetational 

 changes during the historical period have given evidence on migrations 

 then in progress and permitted deductions as to those of the later 

 prehistoric period. Conclusions as to the preceding stages have been 

 reached through use of the various lines of evidence outlined above 

 and carried back with decreasing detail and assurance to the advance 

 of the Illinoian glaciers. 



Summary of Part III. — 1. Migrations in progress now or in the 

 recent past are indicated by historical evidence or by observation of 

 successions. 



