STABLE AND MIGRATORY PLANT FORMATIONS 69 



eminently fulfilled in the past, peat has taken possession of 

 the ground almost irrespective of the nature of the under- 

 lying rocks, and, in extreme cases, even of the physiography. 

 Moorland peat overlies clays and sands, sandstones and 

 limestones, as well as all kinds of metamorphic and igneous 

 rocks. To the south and east, where the past climate has 

 been less frequently favourable for peat growth, the moor- 

 land is more and more restricted by physiographic and 

 edaphic factors. The greater part of the moorland is at 

 present in a retrogressive phase owing to a late change in 

 climate (31) (26). This retrogression has been accelerated 

 locally by physiographic factors, and the peat has been 

 entirely or partially removed from considerable tracts, includ- 

 ing the better-drained slopes and wind-swept cols and summits, 

 especially in areas which have developed a system of under- 

 ground drainage, as porous debris and limestone pavement. 



The moorland associations, over large areas, owe their 

 present persistence to an edaphic inheritance of thick peat, 

 formed under more favourable conditions for peat accumula- 

 tion in the past. This cloak of peat, and a highly acid 

 drainage arising from its degradation, act as a barrier to the 

 invasion of the moorland region by other plant-associations 

 to which the present climatic conditions are not entirely 

 unfavourable. Pine-forest is capable of colonising the 

 retrogressive bogland, but the spread of the pine during the 

 late retrogressive phase was perhaps prevented by early 

 human interference. 



A large part of the rest of the country was once covered 

 by woodland. It possessed the stations best protected from 

 the wind, and where the summer temperatures were highest, 

 i.e. the southern and eastern lowlands, and the northern and 

 western valleys. In the north and west woodland becomes 

 more and more restricted to the warm, sheltered valleys and 

 steep slopes where peat has found no lodgment. 



The dominance of tree-species in the woodlands has 

 depended on their specific capabilities of resisting unfavour- 

 able climatic and edaphic conditions on the one hand, and 

 competition on the other. The past climate and the method 

 of dissemination of the seed have probably been the chief 

 regulators of their periods of immigration, while competition, 



