132 P. A. Ø YEN [1914 



was able to return northward after the retreat of Ihe ice and 

 thus shows a considerable proportion of woody sj)ecies loday. 

 In norlhern Europe, however, natural harriers have lo a greal 

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

 seems to be descended from the remnant of the i)re Ghicial vege- 

 tation which survived on the unglaciated areas. The faet that 

 the proportion of herbs in the present north European flora is 

 hke that in northern or low alpine regions loday provides us 

 with a chie as to climatic conditions during Ihe 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 

 skildrel del træffende paa følgende maate: »Oceans and moun 

 tain ranges are the chief visible harriers to the diffusion of 

 piants, but these are not so imporlant as the invisible harriers 

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

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

 finder levninger efter en tidligere vegelalion, maa vi la tilbørligt 

 hensyn til muligens stedfundne transporler 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 utførle undersøkeiser »Om drifveden i 

 Norra Ishafvet (Kgl. svenska Vel. Akad. Handl., B. 37, 1903, 

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

 tilsynelatende uregelmæssige transi)orlretninger, soiu 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, inverlebrates 

 are more readily transported across ocean harriers 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 indtræffe, saaledes som »Lyell gives an in- 

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

 rently small and unimportanl Ihing as the transporlation of a 

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

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

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

 pag. 126). 



Men det er vistnok saa blandt dvrene, isa^r inden den store 



