DISTRIBUTION OF ARCTIC PLANTS. 123 



south European and African Distribution ; central and south 

 Asiatic Distribution. The theory upon which the facts are 

 collocated and discussed, and which they are thought strongly 

 to confirm, is that of Edward Forbes, which was completed, 

 if not indeed originated, by Darwin : — " first, that the exist- 

 ing Scandinavian flora is of great antiquity, and that pre- 

 vious to the glacial epoch it was more uniformly distributed 

 over the Polar Zone than it is now ; secondly, that during 

 the advent of the glacial period this Scandinavian vegetation 

 was driven southward in every longitude, and even across the 

 tropics into the south temperate zone ; and that, on the suc- 

 ceeding warmth of the present epoch, those species that sur- 

 vived both ascended the mountains of the warmer zones, and 

 also returned northward, accompanied by aborigines of the 

 countries they had invaded during their southern migration. 

 Mr. Darwin shows how aptly such an explanation meets the 

 difficulty of accounting for the restriction of so many Ameri- 

 can and Asiatic arctic types to their own peculiar longi- 

 tudinal zones, and for what is a far greater difficulty, the 

 representation of the same arctic genera by closely allied 

 species in different longitudes. . . . Mr. Darwin's hypothesis 

 accounts for many varieties of one plant being found in va- 

 rious alpine and arctic regions of the globe, by the competi- 

 tion into which their common ancestor was brought with the 

 aborigines of the countries it invaded. Different races sur- 

 vived the struggle for life in different longitudes ; and these 

 races again, afterwards converging on the zone from which 

 their ancestor started, present there a plexus of closely allied 

 but more or less distinct varieties, or even species, whose 

 geographical limits overlap, and whose members, very prob- 

 ably, occasionally breed together." A further advantage 

 claimed for this hypothesis is, that it explains a fact brought 

 out by Dr. Hooker in a former publication, namely : " that 

 the Scandinavian flora is present in every latitude of the 

 globe, and is the only one that is so." 



Moreover, Dr. Hooker discovers in the flora of Greenland a 

 state of things explicable upon this hypothesis, but hardly by 

 any other, namely : its almost complete identity with that of 



