8 Introduction 



not pastured the shrubs and trees of the formation re- 

 colonise the open land^ and woodland is regenerated \ 



Besides these degenerative processes, due to human 

 interference, there are others due to "natural" causes, 

 which are for the most part little understood. 



The natural process of the development of formations 

 on new soils has also to be taken into con- 

 development sideration. The formation which is to occupy 

 or primary a new soil rarely springs into being fully 

 succession in constituted. It normally passes through a 

 orma i . g^^^Q^ of phases of development (primary 

 succession), each phase exhibiting a definite 'plant-associa- 

 tion (see below). Clements has given a very interesting 

 account of this primary succession of associations (called 

 by him "formations") in the case of several American 

 formations; while the primary successions met with on 

 sand-dunes, on salt-marshes, and on the borders of inland 

 lakes where peat accumulates, have long been familiar 

 to European plant-geographers. Clements^ has shown 

 that primary succession follows definite laws, and has 

 pointed out that the pioneer species generally form an asso- 

 ciation in which but few species occur and which does not 

 completely cover the soil (open association), while the 

 succeeding associations are mixed in character and the 

 final association is closed and often dominated by a single 

 species, or by a few similar species. 



Primary succession must take place universally where 

 new land habitable by plants is formed, but in a country 

 like our own, where geological changes are slow and 

 insignificant, there is little opportunity for observing it 



^ This very brief summary of the relation of woodland, scrub and 

 grassland or heathland omits all reference to complicating factors. Some 

 of these are discussed under the different formations in Part n. 



^ The Development and Structure of Vegetation, Lincoln, Neb., U.S.A., 

 1904; Research Methods in Ecology, Lincoln, Neb., 1905. See also Moss, 

 op. cit. 



