1915] 



WILLE — FLORA OP NORWAY 79 



Corylus Avellana, Tilia europaea, Ulmus montana, etc. Here 

 we come to the transition to the next flora. 



(4) Quercus Flora, which immigrated during the warmest 

 period after the Glacial Epoch, when the mean summer 

 temperature was about 2.5° C. higher than at the present 

 day. In addition to Quercus pedunculata and Q. sessiliflora, 

 there immigrated during this period Acer platanoides, Frax- 

 inus excelsior, Hedera Helix, Viscum album, and a great num- 

 ber of warmth-loving plants, which have since kept to the 

 warm slates and limestones. 



As the last immigrants during the steady decrease of the 

 summer temperature, Gunnar Andersson gives 



(5) The Fagus Flora and (6) the Picea excelsa Flora. 

 What is new in this theory is that there is assumed to have 



been only one period with higher temperature since the 

 Glacial Epoch. This, too, is supported by the results at which 

 W. C. Brogger ('00) has arrived in his investigations of the 

 Quaternary fossil mollusc fauna in the south of Norway. 



Since then, the question of the immigration of the flora into 

 Sweden has been treated in a series of papers by R. Ser- 

 nander ('10), who rather inclines to A. Blytt's theory, and 

 in Norway by J. Holmboe ('03), who subscribes to that of 

 Gunnar Andersson. 



The geological basis, however, upon which all investigations 

 of the immigration of the flora into the Scandinavian penin- 

 sula must be built, has of late years undergone considerable 

 alteration. A number of recent discoveries of fossil plants 

 also give new points of support. There is still, however, 

 uncertainty concerning many points, so that the opinions of 

 geologists and phytogeographers by no means coincide. 



In the first place, by counting the layers in stratified clay 

 deposits in Sweden, Gerhard de Geer ('08) has succeeded in 

 proving that not more than about 12,000 years have elapsed 

 since the ice-cap of the last Glacial Period extended as far as 

 Skaane in the south of Sweden. The ice had taken about 

 4,000 years to withdraw thus far from its southernmost 

 boundary in Germany, and it afterwards took as much as 

 about 3,000 vears to withdraw to a range of terminal moraines 



