200 HOOKER ON ARCTIC PLANTS. 



American types should terminate so abruptly on the west coast 

 of Baffin's Bay, and not cross to Greenland and Europe ; or that 

 Greenland should contain 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 Scandina- 

 vian vegetation should in every longitude have migrated across the 

 tropics [of Asia and America, whilst those typical plants of these 

 continents which have found their way into the Arctic regions 

 have there remained restricted to their own meridians. 



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

 admit Mr. Darwin's hypotheses,* first, that the existing Scandi- 

 navian 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 tliis 

 Scandinavian vegetation was driven southward in every longitude, 

 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 northwards, 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 for 

 what is a far greater difficulty, the representation of the same 

 Arctic genera by most closely allied species in different longitude!;-. 



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 longi- 

 tudes ; 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, Avhose 

 geographical limits overlap, and whose members very probably 

 occasionally breed together. 



Nor is the application of this hypothesis limited to this inquiry ; 

 for it offers a possible explanation of a general conclusion at which I 

 had previously arrived,"]* and which I shall have again to discuss here, 

 viz., that the Scandinavian flora is present is every latitude of the 

 globe, and is the only one that is so ; and it also helps to explain 

 another class of most interesting and anomalous fticts in Arctic dis- 



* This theory of a southern migration of northern types being due to tlio 

 cold epochs preceding and during the Glacial oiigiuated, I belicA'e, >vitli the 

 late Edward Forbes ; the extended one of their transtropical migratioa is 

 Mr. Darwin's, and is discussed by him in his " Origin of Species," c'lnp. xi. 



t Introd. essay to the "Flora of Tasmania," p. ciii. 



