E. V. Wulff ^158 — Historical Plant Geography 



Still richer in species is the third group, also connected with arid, 

 continental climatic conditions. In it we find a new sub-boreal ele- 

 ment, viz.: Spiraea filipendula, Geranium sanguineum, Thymus chamae- 

 drys, Rhamnus cathartica, Fragaria collina, etc. 



In addition to these three groups of species, there are two others 

 which, in contrast to the preceding, prefer humid habitats near the sea- 

 shore. The first of these Blytt calls an Atlantic flora, inhabiting the 

 most humid places along the shore. The number of species belonging 

 to this flora become less and less, the farther from the shore and the 

 more continental the habitat conditions. To this group belong: 

 Ilex, Digitalis purpurea, Erica Tetralix, etc. The second is a sub-Atlantic 

 flora, concentrated in the southernmost parts of the shore. Here we 

 find, for instance, Gentiana pneumonanthe, Cladium mariscus, and Teu- 

 crium scorodonia. 



Lastly, the sixth group comprises the subarctic flora distributed 

 throughout the entire country. It also is linked with humid climatic 

 conditions, for its species do not avoid the coast regions. 



The species composing these groups are frequently found in mixed 

 communities, but in some places the species of one and in others those 

 of another group predominate to such an extent that there can be no 

 doubt that each group possesses a specific history of origin. 



In view of the fact that the Scandinavian peninsula during the Ice 

 Age could have retained only insignificant remnants of its pre-glacial 

 vegetation, since it was almost entirely covered by the great ice-sheet, 

 it is clear that its present vegetation could have penetrated its terri- 

 tory only from places farther south and only after the ice-sheet had 

 begun its retreat. Moreover, each of the above-mentioned floras con- 

 stitutes a group of species that simultaneously invaded those parts of 

 Scandinavia from which the ice had receded. 



If we now keep in mind that these floristic groups constitute com- 

 binations of xerophytic or hydrophytic elements, the conclusion natu- 

 rally follows that the climate of the post-glacial period underwent great 

 changes, periods of cold, dry climate alternating with periods of warm, 

 humid climate. Such alternations of cHmate occurred repeatedly, and 

 during each respective climatic period there took place an invasion into 

 Norway of a corresponding group of species. Each group of new- 

 comers, adapted to the climatic conditions prevaiHng at the time, 

 crowded out their predecessors that had invaded the peninsula during 

 the preceding climatic period. The latter, however, did not entirely 

 disappear; some were preserved in sheltered places, where the climatic 

 conditions were such as to secure them from competition with the new 

 invaders. 



There remains now only to establish in what sequence Norway was 

 restocked with vegetation, and, consequently, in what sequence there 

 took place the alternate periods of cold, dry and warm, humid climate. 

 The answer to this question we find in the peat-beds of Scandinavia, 

 the plant remains in the successive layers of which testify very posi- 

 tively to a definite sequence in the succession of floras. These peat- 

 beds, according to Blytt, show that three times during the course of 

 their formation there were dry periods, lasting presumably for thou- 

 sands of years, when the formation of peat ceased and the peat-bogs 



