258 SUCCESSION IN EUKASIA. 



sand-dune formation (339) consists of 5 associations: (1) strand plants, (2) 

 Agropyretum juncei, (3) Ammophiletum arenariae, (4) fixed dune, (5) dune 

 marshes. Oliver (360) has recognized three groups of communities at 

 Blakeney Harbour, viz, (1) shingle-beach, (2) sand-dune, (3) salt-marsh 

 (c/. pp. 260-261). 



Adamson (1912 : 339) has made a thorough study of a woodland in Cam- 

 bridgeshire, in the course of which he has determined the successional relations 

 of the characteristic societies: 



"The ground flora of the ash-oak association, on the calcareous clay, is 

 divided into four societies. (I) Spiraea ulmaria society: range of light-values 

 0.005-0.01, water-content high (summer average 36 to 43 per cent.) ; in spring 

 Spiraea is not very important, the dominating plants being Primula elatior, 

 Anemone, Viola, etc., but later the Spiraea forms an almost pure ground vege- 

 tation; in winter, mosses form the most conspicuous portion of the plant- 

 covering. This society is divided into a shade form and an open type, showing 

 interesting contrasts in the ground vegetation. (II) Spiraea ulmaria and 

 Deschampsia caespitosa society; light, 0.03 to 0.6, water-content as in I. Here 

 Spiraea is still the dominant plant, but becomes associated with numerous 

 others intolerant of deep shade; with light not more than 0.05 Spiraea is 

 solely dominant, and all stages are found between this and the co-dominance 

 of the two species which occurs at about 0.2 light-intensity; mosses are less 

 abundant than in I. Where the light is stronger Deschampsia becomes more 

 prominent locally to the exclusion of the Spiraea, forming a Deschampsia dry 

 sub-society — other sub-societies are distinguished. (Ill) Mercurialis perennis 

 society: sharply separated from I and II, especially by the strikingly different 

 water-content (22-26 per cent.), light varying; in the drier parts Sdlla non- 

 scripta is abundant or even locally dominant. (IV) Fragaria vesca society: 

 water-content about 30 per cent., light conditions much as in II; this essen- 

 tially mixed society is intermediate between II and III and passes imperceptibly 

 into the other societies by increase or decrease of moisture; in dense shade 

 Circaea lutetiana becomes dominant." 



Elgee (1912) has discussed the various types of moors in northeastern 

 Yorkshire, and has given incidentally some indications of development. As 

 to the origin of the moorland, the conclusion is reached that the geological 

 history and geographical distribution of the chief moorland plants prove that 

 the moors were formed in preglacial times, probably towards the close of the 

 Pliocene period. The history of the moorland flora is summed up in the 

 following stages : 



(1) Evolution of Vaccinium spp., Eriophorum, Empetrum, etc., in a northern 

 land in Pliocene times, and a gradual dispersal of these species southwards 

 with the approach of the Ice Age; (2) origin of Calluna and Erica spp. in 

 southwestern Europe and their dispersal north and east during the Pliocene 

 period; (3) advent of the Ice Age with survival of most of the northern species 

 on the driftless area — Erica cinerea, E. tetralix, Myrica gale, and Pteris aquilina, 

 however, probably driven from the district; (4) post-glacial re-entrance of 

 these four plants, and development of moors from the Arctic plant communities 

 of the uplands and upon the bare ground ; (5) a warmer and drier climate with 

 a decline of wet moors and the growth of trees in the slacks, gills, and dales 

 and on slopes and parts of the higher moors; (6) an increased rainfall with an 

 acceleration of moor formation, and a destruction of the birch and oak woods 

 in the slacks and gills by the development of peat bogs; (7) the present moors, 

 where peat formation and destruction counterbalance one another. 



