INTRODUCTION 75 



This is coincident, as a rule, with the two other 

 factors. It has the effect of entirely altering the 

 water content, and aquatic plants or hygrophiles 

 thus give place to types intermediate between them 

 and terrestrial plants or mesophytes, in their demand 

 for moist conditions. As in the last two cases 

 drainage has mainly disturbed areas that have been 

 most affected. The Fenland is the most extensive 

 area which has been altered by drainage. Tracts 

 originally covered by aquatic or fen formations have 

 become converted into grassland and arable. But 

 by purely natural causes, either in the past or in 

 comparatively recent times, marsh and aquatic 

 vegetation and fenland have been constantly chang- 

 ing places, and this is due largely to natural silting 

 up by the natural formation of shingle beaches on 

 the coast and changes of level. 



In other lowland areas aquatic vegetation has 

 become neutral grassland or rush associations, 

 marshes have succeeded aquatic formations, and 

 bog or fen has succeeded marshland, as a greater 

 thickness of peat has accumulated. 



Other factors of human origin are the preservation 

 of game and the formation of artificial plantations, 

 especially of conifers, which, except in the case of 

 the Pine,"^ are not native. A similar artificial effect 

 has been caused by peat-cutting and by furze or 

 heather-burning in upland areas for fuel or for game 

 preservation or allied purposes. 



* The Larch found in ancient plant-beds has been re-introduced. 



