328 THE CANADIAN NATURALIST. [June 



cross to Greenland and Europe ; — or that Greenland should con- 

 tain actually much fewer species of European plants than have 

 found their way eastwards from Lapland by Asia into Western 

 and Eastern Arctic America ; — or that the Scandinavian vege- 

 tation should in every longitude have migrated southward across 

 the tropics of Asia and America, whilst the typical genera of Asia 

 and America which have found their way into the arctic regions 

 have remained restricted to these continents. 



It appears to me difficult to account for these facts, unless we 

 admit Mr. Darwin's* hypothesis, first, that the existing Scan- 

 dinavian flora is of great antiquity, and that previous 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 longi- 

 tude, and even across the tropics into the south temperate zone ; 

 and that on the succeeding warmth of the present epoch, those 

 species that survived 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 American 

 and Asiatic arctic types to their own peculiar longitudinal 

 zones; and that far greater difficulty, the representation of the 

 same arctic genera by most closely allied species in different 

 longitudes. To this representation, and the complexity of its 

 character, I shall have to allude when indicating the sources of 

 difficulties I have encountered, whether in limiting the polar 

 species, or in determining to what southern forms many are most 

 directly referable. Mr. Darwin's hypothesis accounts for many 

 varieties of one plant being found in various alpine and arctic 

 regions of the globe, by the competition into which their common 

 ancestor was brought with the aborigines of the countries it 

 invaded : different races survived 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 



* This theory of a southern migration of northern types heing due to 

 the cold epochs preceding and during the glacial, originated, I believe, 

 with the late Edward Forbes ; the extended one, of their transtropical 

 migration, is Mr. Darwin's, and is discussed by him in his ' Origin of 

 Species,' chap. xi. 



