54 H. A. GLEASON VEGETATIONAL HISTOEY OF MIDDLE WEST 



4. Location of maximum size of individuals. 



5. Continuity and convergence of lines of dispersal. 



6. Location of least dependence on a restricted habitat. 



7. Continuity and direction of individual variations or modifica- 

 tions radiating from the center of origin along the highways of dispersal. 



8. Direction indicated by biogeographical affinities. 



The Five Elements in the Floea of the Middle West. — Ap- 

 plying his principles to the vegetation of the Middle West, it is found 

 to be composed of five elements, not of equal importance, centering 

 respectively in the southern Appalachian Mountains, in the southern 

 Coastal Plain and the Mississippi Embayment, in the Ozark Moun- 

 tains, in the plains of Kansas and Nebraska, and in Canada east of the 

 Great Lakes. Four of these lie beyond the glaciated region far enough 

 to have suffered little or not at all from the advance of the ice, and 

 there is no reason to believe that they did not occupy these positions 

 continuously through the later glacial period. The last one is in the 

 glaciated region, and must have been developed since the retreat of the 

 ice from a temporary refuge farther south. For the first four, we are 

 probably safe in assuming that all subsequent migrations of their floral 

 elements have been on lines essentially radial, either from or toward 

 these centers, as ranges may have extended or retreated. For the fifth, 

 we are undoubtedly correct in assuming a general northward shift of 

 the whole flora following the ice retreat. 



Glacial vs. Modeen Climate. — The close similarity between the 

 interglacial and the modern floras, so far as evidence is at hand, indi- 

 cates that the glacial climate differed from the modern not so much 

 in kind as in degree. It may be expected accordingly that migrations 

 of the deciduous forests took place first in a northward direction from 

 their Appalachian center and later westward into the Middle West, 

 and that any eastward migration of a prairie flora took place toward 

 the north of our area, where there is still a marked distinction between 

 winter and summer rainfall and a generally smaller total amount, 

 rather than at the south where rainfall is more abundant and more 

 evenly distributed throughout the year. 



Further evidence may be drawn from the topography of the region. 

 Large and sharply marked moraines may indicate a rapid advance of 

 the glacial ice. Wide stream valleys leading from the moraines, with 

 great outwash deposits, may indicate rapid melting of the ice under 

 relatively high temperatures, and consequently in the adjacent region 

 a flora and vegetation correlated with such climate. Such evidence has 

 been used chiefly in connection with the Wisconsin glaciation, where 



