90 



The Irish Naturalist. 



August, 



longer stationary period a retreat commenced and the 

 plants and animals began to assume their present distri- 

 bution. 



Some of the more important evidence regarding the 

 immigration of southern animals and plants into Scan- 

 dinavia during the Climatic Optimum may be now referred 

 to. The peat-bogs and mosses of those lands, with their 

 abundant fossil remains of trees, herbs, and animals have 

 long been known as important recent geological deposits 

 and investigations into their fossil contents show definite 

 changes of type as we leave the arctic conditions of the Ice 

 Age and approach "-he present day. 



The high-arctir ^getation of the Yoldia Clays, charac- 

 terized hy Dryas octopetala and Salix polaris, gradually 

 disappeared and was replaced in certain localities by a 

 sub-arctic flora with Betula odorata and Juniperus com- 

 munis. After the ice-sheet had melted, the temperature 

 is considered to have risen rapidly and the plant life having 

 responded to the rise, the flora of the wood-meadows soon 

 replaced the arctic and sub-arctic vegetation. Then came 

 the Birch, Poplar and Fir, the latter becoming the dominant 

 forest tree until the Littorina period, when it began to be 

 replaced by the Oak. This Oak period is considered to 

 coincide in general with the Littorina period and the fact 

 that most of the charcoal found in the kitchen-middens was 

 derived from the Oak enables us to date its maximum 

 distribution at about the time of the Neolithic period. It 

 should be understood that this succession of forest types. 

 Birch, Fir, Oak and Beech is not now considered as important 

 and accurate as a time index as in the earlier studies of the 

 peat-bogs and mosses of Scandinavia, and that while the 

 general succession of Birch, Pine, Oak and Beech in post- 

 Glacial time is a well proved fact, their use as definite 

 evidence of a precise point in post-Glacial time is now 

 regarded as somewhat obsolete. Among the trees and 

 shrubs certain species have been selected for detailed 

 investigation, two of which may be referred to. 



The most striking case is that of the Hazel (Corylus 

 Avellana) concerning the post-Glacial distribution of which 



