602 PROCEEDINGS OF THE ACADEMY OF [Sept., 



h. Continuity and directness of individual variations, or modifica- 

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

 i. Direction indicated by biogeographic affinities.^ 



5. Old regions, botanically speaking, may be determined where the 

 number of specific forms of single genera is small, and new regions may 

 be determined where the number of species of single genera is ordinarily 

 very large. 



6. Drude's classification of endemic plants, as corresponding and 

 as relict, is of great assistance to the botanist in determining the age of 

 floristic elements. Plants are corresponding when the original con- 

 tinuous area of a variable species has been interrupted in such a way 

 as to form several smaller areas occupied by subspecies or new species, 

 while relicts are those species originally of extensive distribution able 

 to maintain themselves in a limited area only on account of changed 

 conditions of life. Having enunciated these general principles, I will 

 endeavor to apply them in the determination of the age of the different 

 floristic elements of eastern North America. 



All of eastern America north of the great terminal moraine which 

 marks the southern boundary of the great ice sheet, with the exception 

 of the nunataks, has been tenanted by plants which have migrated 

 into the territory abandoned by the great continental glacier. Geolo- 

 gists believe, from evidence afforded by the time that it has taken for 

 the river to cut the gorge at Niagara, that 10,000 or 15,000 years have 

 elapsed since the close of the glacial period. If their deductions are 

 sound, then the flora of the northern part of eastern America cannot 

 be older than 15,000 years at the outside. Some of its elements may 

 be much older, and we have reason to believe that many boreal plants 

 existed as such on the nunataks, which were unglaciated areas above 

 the great ice sheet. The first wave consisted of the distinctly glacial 

 flora, which skirted the border of the ice sheet. The second wave, 

 younger as a floristic element of the North, consisted of boreal plants, 

 many of which, as bog plants, tenanted the bogs and margins of the 

 glacial lakes that were formerly much more abundant in the North 

 than at present. These bog and tundra types pushed early into the 

 barren ground left by the retreating ice. The tundra was closely 

 followed by the coniferous forests on the western and eastern sides of 

 the glaciated areas and these trees constitute a third floristic element, 

 much younger in point of the time in which they have occupied the 

 North. These trees, and those forming a still younger element, sur- 

 rounded the bog plant societies which were trapped by the surround- 



' Adams, Chas. C, Biological Bulletin, III : 122. 



