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SOME COMPARISONS OF THE LICHEN FLORAS OF 

 EURASIA AND NORTH AMERICA 



By R. Heber Howe, Jr. 



In studying many of the filamentous and foliose lichen-species 

 of North America I have been struck with the interesting corre- 

 lation of species-distribution found on the two continents of 

 North America and Eurasia. This correlation is so often over- 

 looked, that new varieties and even species are being described 

 without a sufficient study of the distributional problems that 

 present themselves, though as Dr. Darbishire has pointed out, 

 these species may be "alike only in their external morphology," 

 and "may have been separately derived from some common 

 ancestor." It, nevertheless, should make one wary of describing 

 new species even if excused by any such theoretical probability. 



That the lichen flora of western Europe and the western 

 coast of North America is closely allied, — as that of eastern 

 Asia and eastern portions of the United States is, in some striking 

 instances, at least undeniable. I am told that the same analogy 

 is apparent in other groups, i. e., the mosses.* 



Climate, Elevation, Etc. 



There are evidently several underlying causes that develop 

 lichen species; just what these are, and their relative importance, 

 is still to be explained. We have climatic conditions, the three 

 most important factors of which for lichens, as for all plants, 

 are moisture and sunlight, and the variability of temperature 

 due to elevation or latitude. The character of the soil (of no 

 concern in the species here discussed) plays, no doubt, an im- 

 portant role. The proximity of the sea also, it would seem, 

 has a definite influence. All of these factors, however, fail, it 

 appears, to explain entirely the curious occurrence of a given 

 species on both continents. A combination of them all is more 

 likely the answer. 



[* Gray, Hooker and many recent writers have discussed the well-known rela- 

 tionship of the flora of eastern Asia and eastern North America. See Torreya 

 for January 1914, p. 8. — Ed.] 



