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migration: e.g. the physiological qualities of the plants themselves. There are 

 probably several stages in the development of every plant species. Young 

 ones are plastic ecologically as well as morphologically. At this stage the 

 species are able to migrate long distances if climatic and geographic factors 

 allow it, and they easily form new taxa under the influence of different 

 conditions. 



(a) (b) 



Fig. 2 (a). The extent of ice-shields during the Great glaciation. (Compiled after 



several authors.) 



(b). The extent of ice-shields during the Last glaciation. (Compiled after 

 several authors.) 



"Mature species" have a stabilized morphology, but they are still plastic 

 enough ecologically. Thus, if they get the opportunity to migrate, they may 

 reach distant regions, but do not change in subsequent periods even if they are 

 isolated for a long time. These are the species with disjunctive areas, which 

 mostly are relics from former periods. "Old species" lose even their ecological 

 plasticity, they cannot spread even over short distances, their energy seems to 

 suffice only for holding their former area. If their life conditions change, they 

 die out. 



The question of the survival of the Arctic flora in situ is, 1 think, positively 

 established by biological as well as by geological arguments. An example is 

 Bear Island, which, though small and situated far to the north, was only 



