182 P. A. ØYEN [1914 



was able to relurn northward after the retreat ol' Ihe ice and 

 thus shows a considerable proportioii of woody species today. 

 In northern Europe, however, natural barriers have to a great 

 extent prevented this return, and the present flora of that region 

 seems to be descended from the remnant of the pre Glacial vege- 

 tation which survived on the unglaciated areas. The fact that 

 the proportion of herbs in the present north European flora is 

 hke that in northern or low alpine regions today provides us 

 with a clue as to climatic conditions during the height of the 

 glacial invasion« (The Journal of Geology, Chicago, Vol. 23, 1915, 

 pag. 305 — 306). De skranker, der sættes planters og dyrs utbre- 

 delse eller vandringer under saadanne forhold, kan være af for- 

 skjellig art. For planternes vedkommende har vistnok Davis 

 skildret det træffende paa følgende maate: »Oceans and moun 

 tain ranges are the chief visible barriers to the diffusion of 

 plants, but these are not so important as the invisible barriers 

 of climate« (W. M. Davis : Physical Geography, Boston a. London, 

 1900, pag. 54). Men naar vi i de svundne tiders efterlatte skiktserier 

 linder levninger efter en tidligere vegetation, maa vi ta tilbørligt 

 hensyn til muligens stedfundne transporter fra fremmede steder, 

 dels for værdsættelsen af den betydning, som disse plantelev- 

 ninger selv kan ha som ledende former, og dels for bedømmel- 

 sen af den betydning, som drivende plantedele kan ha hat som 

 transportmidler for andre, særlig mindre organismer. Thi de af 

 Ingvarson indgaaende utførte undersøkelser »0m drifveden i 

 Norra Ishafvet« (Kgl. svenska Vet.-Akad. Handl., B. 37, 1903, 

 Nr. 1) gir os et godt indblik i de ofte temmelig indviklede og 

 tilsjaielatende uregelmæssige transportretninger, som ofte strøm- 

 ningsforholdene i havet kan gi anledning til. 



Med hensyn til dyrenes vandringer turde kanske i store træk 

 følgende, som resultat af den blandt amerikanske forskere vundne 

 erfaring, ogsaa kunne finde en mere almindelig anvendelse: »It 

 appears that many, one might perhaps say most, invertebrates 

 are more readily transported across ocean barriers than verte- 

 brates, especially mammals, even making due allowance for 

 their greater antiquity« (Annals of the New York Academy of 

 Sciences, Vol. 24, 1915, pag. 317). Enkelte merkelige tilfælder 

 kan dog ogsaa her indtræff"e, saaledes som »Lyell gives an in- 

 teresting summary of the far-reaching effect of such an appa- 

 rently small and unimportant thing as the transportation of a 

 few polar bears by drift-ice to an island in northern seas before 

 the time of man, such as Iceland has seen since its colonisation 

 by Norwegians« (Logan Lobley: The Age of the World, 1914, 

 pag. 126). 



Men det er vistnok saa blandt dvrene, især inden den store 



