1915] 



WILLE — FLORA OF NORWAY 77 



In 1875, Axel Blytt (76) first brought forward his well- 

 known theory on the immigration of the flora of Norway 

 during alternate wet and dry periods. According to Blytt 's 

 theory, the wild plants of Norway should be arranged in the 

 following six groups: (1) the arctic (the mountain flora); 

 (2) the subarctic (the vegetation of mountain and wooded 

 slopes), which is more frequent in the arctic than in the 

 more southern, lower regions; (3) the boreal (the vegetation 

 of the rocky slopes covered with foliage trees), which has 

 its widest distribution in the low land, but not the coast 

 districts; (4) the Atlantic (Bergen coast vegetation), with 

 distribution in the coast district, especially between Stavanger 

 and Kristianssund ; (5) the sub-boreal, which occurs in the 

 southeast of the country, especially 'round the Kristiania 

 Fjord; and (6) the sub-Atlantic (Kristianssand coast vegeta- 

 tion), which has its widest distribution in the coast district 

 between Kragero and Stavanger. 



The arctic, boreal, and sub-boreal species of plants are 

 warmth-loving, continental plants, while the subarctic, At- 

 lantic, and sub- Atlantic keep chiefly to the coast districts and 

 are insular in character. The former have immigrated dur- 

 ing dry periods, the latter during damp periods, in the order 

 in which they have been placed. Blytt assumed that within 

 the period of history it is scarcely probable that any very 

 great changes have taken place in climate or vegetation, and 

 that the present is a dry period. 



Blytt ( '83) subsequently maintained that these changes of 

 climate were due to cosmic causes, namely alterations in 

 the eccentricity of the earth's orbit and alternate changes 

 in the earth's position with regard to the sun, occupying 

 periods of about 21,000 years. By the aid of this hypothesis 

 he calculated the period from the conclusion of the Glacial 

 Epoch down to the present time to be between 80,000 and 

 90,000 years. The damp and dry periods were thus of equal 

 duration, namely 10,500 years. 



As Blytt moreover started with the assumption that the 

 plants could advance only step by step in their migrations, 

 and could not be transferred direct from Denmark or England 



