52 H. A. GLEASON VEGETATIONAL HISTOKY OF MIDDLE WEST 



Historical Evidence of Migrations. — Historical evidence may 

 occasionally be used with considerable success, especially where civiliza- 

 tion has existed for a long period of time and written records preserve 

 trustworthy accounts of some of the original features of the vegeta- 

 tion. Thus Graebner 23 has been able to demonstrate that certain 

 heaths of northern Germany were originally beech forests. Tansley 21 

 has also made frequent use of historical records. For the Middle West, 

 such data cover little more than a single century and are correspond- 

 ingly restricted in value. There are, however, numerous early books 

 of travel which give fairly accurate accounts of the plant life. Their 

 descriptions of prairie fires, of the former extent of prairies, and of 

 the oak openings are in many cases excellent and may probably be 

 relied upon completely. They generally lack detail and seldom mention 

 many species by name, but are nevertheless better for phytogeographical 

 purposes than the floras compiled by early botanists. In almost every 

 county seat copies of the original land survey maps may be consulted. 

 On many of them various features of the vegetation are shown, such 

 as the occurrence of swamps, prairies, and barrens. In some cases the 

 nature of the forest may be approximated by reference to the accom- 

 panying field books, since the surveyors' monuments were usually 

 located with relation to certain described trees. 



Succession as Evidence of Migration. — For migrations still in 

 progress, the most valuable line of contemporary evidence is found in 

 the successional relations of the species toward the margin of their 

 ranges. Even there, succession can not generally be observed directly, 

 but numerous other considerations offer convincing evidence of the na- 

 ture of the process, such as the relation of the associations to physioV 

 graphic development, the reaction of the plant cover on the environment, 

 the presence and relative abundance of seedlings, or the relative age of 

 individuals. These have been used so freely by ecologists and such 

 a body of accepted data has been published that they need no dis- 

 cussion here. Nen iheless, a timely warning against the too general 

 application of the, Wiciples of succession has been sounded by Har- 

 vey 25 and deserves careful consideration. When, at or near the margin 

 of a range, a spec 1 '"' or tthe members of a flora of uniform geographical 

 distribution pari* * ates in succession as invading species, they are 



23 Graebner, P. Die Heide Norddeutschlands und die sich anschliessenden 

 Formationen in biogischer Betrachtung. Die Vegetation der Erde, vol. 5, Leipzig, 

 1901. 



24Tansley, A. G. Types of British vegetation. Cambridge, 1911. 



25 Harvey, Leroy H. Some phytogeographical observations in Lake County, 

 Michigan. Mich. Acad. Sci. Rep. 21: 213-217. 1920. 



