DR. HOOKER ON THE DISTRIBUTION OF ARCTIC PLANTS. 253 



dinavian plants should, under existing conditions of sea, land, and temperature, have 

 not only found their way westward to Greenland, by migration across the Atlantic, but 

 should have stopped short on its west coast, and not crossed to America ; or that so many 

 American types should terminate as abruptly on the west coast of BaflS.n'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 Scandinavian 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 Scandinavian 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 dming the advent of the glacial period this Scandina- 

 vian 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 

 retm-ned northward, accompanied by aborigines of the countries they had invaded during 

 thek 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 lon- 

 gitudes. 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 dkectly re- 

 ferable. 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, after- 

 wards 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 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 t and shall have 

 again to discuss here— viz. that the Scandinavian flora is present in 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 facts in arctic distribution, at which I have now arrived 

 from an examination of the vegetation of the several polar districts, and especially of that 

 of Greenland. 



* This theory of a southern migration of northern types being 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. 



t Introd. Essay to the ' Flora of Tasmania,' p. ciii. 



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