NEW Y 



BO! AN1< 

 40 H. A. GLEASON VEGETATIONAL HISTORY OF MIDDLE WEST 



Part I 



The Dynamic Viewpoint. — Many years have elapsed since Engler 

 published his famous and fundamental "Leitende Ideen," which he 

 had adopted as a basis for his discussion 1 [of the Tertiary and tPledsr 

 tocene development of the vegetation of the earth. Here, in thirty-six 

 concise paragraphs, he summarized the general principles controlling 

 the distribution of species and of species-groups. The first two of them 

 serve as a summary for all : 



"1. Die gegenwartige Verbreitung der Pflanzen ist nicht bios bedingt 

 durch die jetzt auf der Erde herrschenden klimatischen Bedingungen 

 und die Bodenverhaltnisse. 



" 2. Ein wahres Verstandniss der Verbreitung der Pflanzen ist nur 

 dann moglich, wenn man die allmalige Entwicklung derselben zu 

 ermitteln sucht." 



Engler was apparently inspired to his studies of vegetational history 

 and development, at least in part, by the classic essays of Asa Gray. 2 ' 3 '* 

 His lead was followed by numerous European botanists, who, assisted 

 by the co-operation of palaeontologists and glacial geologists, have in 

 the past two decades contributed greatly to the knowledge of the phyto- 

 geographical history of central and western Europe. Later American 

 botanists, on the other hand, neglected such lines of investigation almost 

 completely and their phytogeographical activities were confined, as is 

 natural in relatively new regions, to descriptions of floras and the de- 

 termination of modern distribution. It remained for Adams 5 ' 6 to 

 revive developmental biogeography in America and to show that the 

 distribution of a species can be satisfactorily explained only through 

 the location of its origin and the nature of its subsequent migrations. 



General Control of Distribution. — The distribution of plants, 

 as shown by Engler's principles quoted above, depends, in general 

 terms, on modern environment and earlier developmental history. Both 



i Engler, Adolph. Versuch einer Entwieklungsgeschichte der Pflanzemvelt, insbe- 

 sondere der Florengebiete, seit der Tertiarperiode. Leipzig, 1879. 



2 Gray, Asa. The flora of Japan. Scientific Papers 2: 125-141. (1859) 1889. 



3 Gray, Asa. Sequoia and its history. Scientific Papers 2: 142-173. (1872) 

 1889. 



4 Gray, Asa. Forest geography and archaeology. Scientific Papers 2: 204-233. 

 (1878) 1889. 



e Adams, C. C. Southeastern United States as a center of geographical distribu- 

 tion of flora and fauna. Biol. Bull. 3: 115-131. 1902. 



e Adams, C. C. Postglacial origin and migrations of the life of the northeastern 

 United States. Journ, of Geog. 1: 303-310, 352-357. 1902. 



