162 DIRECTION OF DEVELOPMENT. 



"The five upland moor associations and their transitional forms described 

 in the preceding pages form a series, Sphagnetum, Eriophoretum, Sdrpetum, 

 Vacdnietum, and CaUunetum, showing a decreasing water content. . . . 

 The desiccation of the peat may be continued till the moor formation is com- 

 pletely destroyed. The first effect upon the vegetation is, as we have seen, 

 the disappearance of the cotton grass and the occupation of the peat surface 

 by the bilberry {Vacdnium myrtiUus), the crowberry (Empetrum nigrum), and 

 the cloudberry {Rvbus chamaemorus) . As the process of denudation continues, 

 this association gradually succumbs to changing conditions until the peat-hags 

 become almost or quite destitute of plants. The peat, being no longer held 

 together, is whirled about and washed away by every rainstorm or by the 

 waters of melting snow. 



"In the end, the retrogressive changes result in the complete disappearance 

 of the peat, and a new set of species begins to invade the now peatless surface." 



This is a convincing picture of the normal destructive action of erosion in 

 producing new areas for succession, and the apparent retrogression or degen- 

 eration of the moor thus resolves itself readily into the usual progressive move- 

 ment to the dwarf-shrub stage, and the more or less rapid destruction of the 

 latter, as well as the cotton-grass stage, by erosion. Destruction by erosion is 

 also the explanation of the "phase of retrogression" found in the dune succes- 

 sion when the "seaward face of the dunes is eaten away by the waves." 



Finally, Moss (1913) has extended the idea of retrogressive associations 

 to include, it would seem, the larger number of communities in the Peak dis- 

 trict of England. In discussing the degeneration of woodland (91), the author 

 himseK appears in doubt as to the natural occurrence of such a process. He 

 says: 



"There can be no doubt that a certain amoimt of the degeneration of the 

 woodland of this district has been brought about by the indiscriminate felling 

 of trees, the absence of any definite system of replanting, and the grazing of 

 quadrupeds. It is doubtful, however, if these causes are quite sufl&cient to 

 accoimt for so great a lowering of the upper limit of the forest as 250 feet 

 (76 in.) and for so general a phenomenon. ... It would appear to be true 

 that, in districts which are capable on climatic and edaphic grounds of sup- 

 porting woodland or true forest, the majority of examples of open scrub are to 

 be regarded as degenerate woods and retrogressive associations. (94) . . . 

 It would appear to be indubitable that woodland is frequently displaced by 

 associations of scrub, grassland, heath, and moor. In all parts of the British 

 Isles there has, within the historical period, been a pronoimced diminution of 

 the forest area, a ditninution which, in my judgment, is in addition to and 

 apart from any artificial deforestation or any change of climate. (96) . . . 

 The conversion of woodland into scrub and of scrub into grassland, heath, 

 or moor is seen not only on the Pennines, but in Wales, in the Lake District, 

 and in Scotland. . . . Such successions are not exceptional in this country, 

 but widespread and general; and whilst they are without doubt often due, 

 in part, to artificial causes, it is at least conceivable that this is not always and 

 wholly the case." (96) 



In an earlier paper (1907 : 44, 50), Moss states that ash-copse furnishes 

 "the preliminary stages of a naturally forming ash-wood, or sometimes a 

 vestige of a former extensive ash-wood," and apparently holds the opinion that 



