3 i8 SCIENCE PROGRESS 



trated to these outlying islands. After the close of the Lower 

 Forestian the general type of vegetation over these peat mosses 

 seems to have remained very uniform through the lower peat 

 bog, second arctic stage, and the Upper Forestian to the present 

 day. During those periods the mosses in the Hebrides must 

 have resembled the tundra of Northern Siberia described by 

 Sewell (5). The observations from the Shetlands illustrate the 

 same feature, for although the second arctic bed is present, yet the 

 Upper Forest does not seem to have spread over the peat areas. 

 Shortly stated, the farther one passes to the edge of the con- 

 tinental plateau the more uniform does the flora appear to have 

 remained, since the later stages of the glacial period. This may 

 imply either that the climatic conditions have been more uniform 

 in those regions, or that the successive waves of vegetation 

 passing over the mainland did not penetrate there on account 

 of natural barriers to immigration. The peat does not afford 

 any direct evidence upon these points, for whilst the second 

 arctic bed was being formed on the mainland, the peat mosses 

 of the Hebrides were covered with marsh consisting of 

 Phragmites communis and Equisetnm, which might well occur 

 under sub-arctic or under temperate conditions. 



The Upper Peat Bog is developed in all districts, and the flora 

 is generally very similar to that of the lower peat bog. In some 

 of the Highland areas, such as the Spey-Findhorn watershed, 

 Findhorn-Nairn watershed, Coire Bog, Cape Wrath district, 

 Rannoch Muir, remains of small Betida alba shrubs occur in 

 the lower parts of this zone. The wood is very local in 

 occurrence, whilst the main mass of the peat is formed of 

 Scirpus ccespitosus, Eriophorum angustifolium, E. vaginatum, and 

 Sphagnum. Evidently these small clumps of shrubby birch 

 were soon displaced, as no wood is met with above the first 

 plot or so of the upper peat bog. In the Hebrides and the 

 lowland mosses of Wigtownshire the upper peat bog is simply 

 a continuation of the lower peat bog, there being no layer of 

 arctic plants between. 



The Upper Forest Bed. — This zone is confined to the mainland 

 of Scotland, from the lowland mosses of Wigtownshire to the 

 valley of the Dionard south of the Kyle of Durness, near Cape 

 Wrath. Whilst the flora exhibits remarkable uniformity, some 

 interesting and important modifications appear as the bed is 

 traced northwards through Scotland. In the areas of the south 



