TAXONOMIC DIFFERENTIATION AS AN 



INDICATOR OF THE MIGRATORY HISTORY OF 



THE NORTH ATLANTIC FLORA WITH ESPECIAL 



REGARD TO THE SCANDES 



J. A. Nannfeldt 

 Institute of Systematic Botany, University of Uppsala, Uppsala, Sweden 



In this everchanging world the distribution of a taxon is determined not only 

 by the sum of all its ecological demands and reproductive properties but also 

 by historical factors. The environment is changing and so is also the genetical 

 constitution of every taxon, but it is only rarely that the changes in distribu- 

 tion can be proved by direct observations. 



To keep to the area and era closest to us, the North Atlantic area and the 

 Quaternary era, we know a good deal about, for example, our trees and their 

 immigration after the last glaciation from pollen-analysis and other fossil 

 remains. We know also a good deal about the first plants to invade south 

 Scandinavia after the retreat of the last ice-sheet. Already Nathorst and his 

 contemporaries found Dryas octopetala, Salix herbacea and other high- 

 mountain species, now growing in the Scandes. Iversen and Erdtman have 

 recently, by pollen-analytical methods, found a rich, very early steppe flora. 

 At least one of its species. Ephedra distachya, has long ago disappeared again 

 from Scandinavia. For at least one of the other steppe species, Centauiea 

 Cyaiius, it seems very improbable, to say the least, that the present-day 

 population of Scandinavia has any genealogical connection with that early 

 Post-glacial population. Also, as for Dryas and the other mountain plants now 

 growing in the Scandes, there are no proofs or even indications that there are 

 direct genealogical connections between those early populations of south 

 Scandinavia and the present-day populations of the Scandes. In the opinion of 

 numerous students, amongst them myself, there are none. 



About most of our species we know, in fact, nothing of that kind. 



One of the first distributional problems in the Scandes to attract the 

 interest of scientists was the presence of what has become known as a "West 

 Arctic Element" in the flora. These plants cannot reasonably have reached 

 their present areas from the south or the east. Neither can their occurrence in 

 the Scandes be explained by chance dispersal from a remote west. The 

 possibility and importance of such long-distance dispersal should of course 



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