ON THE PLANT REMAINS IN THE SCOTTISH PEAT MOSSES. 349 



conditions. In connection with the spread of birch over this area, it might be pointed 

 out that the Arctic plant bed at the base of this peat represents a very late phase of 

 the glacial epoch. 



When these Arctic plants flourished the period of maximum glaciation had long 

 passed away, and a mild, interglacial epoch had been followed by a partial return to 

 o-lacial conditions in the Southern Uplands and to severer glaciation in the Highlands. 

 Thereafter mild interglacial conditions had again supervened, only to be succeeded by a 

 relapse to cold conditions, with, tundras in the South of Scotland and local glaciation in 

 the Highlands. It was during the passing away of the latter period that the Arctic 

 plants now under consideration spread over the district. When the last ice-sheet had 

 disappeared and was succeeded by a mild interglacial climate, Britain became joined to 

 the Continent and the immigration of our present flora took place. The later return to 

 cold conditions marked by the Mecklenburgian or Fourth glacial stage did not result in 

 the appearance of a general ice-sheet, but produced only glaciation in the north of 

 Britain and over hilly or mountainous ground. These later stages then probably did 

 not destroy any species of the British flora, but only modified its distribution. The 

 Mecklenburgian stage would result in a driving of temperate plants to the southern 

 parts of Britain, with a corresponding southward extension of Arctic plants. The Lower 

 Turbarian would likewise permit of a southward extension of the present Arctic 

 members of the flora, only less marked than the last ; whilst the Upper Turbarian or 

 Sixth glacial stage was so slight that it would scarcely cause any change in the dis- 

 tribution of the flora except over ground lying at considerable elevation in the north. 

 The greater part of the British flora, then, immigrated during the interglacial period 

 following on the disappearance of the last ice-sheet, and its subsequent history has been 

 one long series of changes as regards distribution. The explanation of the present 

 distribution of the British Flora must be sought for not only in respect to the present 

 climatic conditions and soil characters, but also in regard to the constant "shuffling" 

 which has been caused by climatic changes during the later phases of the glacial epoch. 

 Tn other words, the present broad distribution of the flora is the resultant of both 

 historical and ecological factors ; and in considering the broad distribution of the flora 

 as a whole over Britain, the historical factor may be the more important of the two. A 

 simple instance of this is the absence of some members of the British flora from Ireland. 

 These plants are not excluded from Ireland through any inability to thrive there, but 

 from their failure to immigrate during the last land connection with Britain. In the 

 same way, the present grouping of certain plant associations in Britain may have been 

 influenced by the great changes in distribution which took place during the Fifth 

 glacial stage, when Arctic plants formed a close covering over low ground in the 

 Southern Uplands of Scotland ; and again during the succeeding forest period (upper 

 forest zone), when some of the wettest peat mosses were covered with pine forest and 

 Calluna, and plant associations characteristic of bog or swamp conditions were driven to 

 the far West. 



