226 BRITISH PLANTS 



and in the course of time, as the sand consolidates, one 

 association is replaced by another. The formation thus 

 has a past history, and, while it is still open, a future. 

 When once closed i.e., when the soil is entirely occupied 

 by close-growing vegetation it can only be further 

 altered by changing climatic or soil-conditions modifying 

 the biological nature of the habitat. In other words, a 

 formation can only comprise a number of associations whilst 

 it is open. In most cases we do not know the past history 

 of a formation, as in deciduous woods and some natural 

 pastures ; but on the moorland a study of the plant- 

 remains in the peat gives us a clue, not only to the past 

 vegetation, but the climate and other factors of the habitat 

 also (see p. 249), whilst in a sand-dune pasture the past 

 is being written for us in the vegetation of the younger 

 parts. 



The formations and associations of the ecologist are 

 closely analogous to the strata and zones of the geologist. 

 The chalk of England, for example, was formed under 

 definite conditions in a shallow sea ; but slight changes 

 must have taken place resulting in a corresponding 

 alteration in the fauna of the sea, but not sufficient to 

 affect the deposition of the chalk. The geologist sub- 

 divides this stratum into a series of zones, each charac- 

 terized by the dominance of one or more animals. The 

 zones merge into one another just as associations do, but 

 in both cases they represent relatively minor changes in 

 the surroundings. 



To sum up, then, the larger differences in habitat serve 

 to differentiate the formations ; the minor differences in each 

 habitat, the associations. No hard-and-fast rule can be 

 laid down as to what constitutes a larger or a minor 

 difference, any more than we can say what constitutes 

 a generic or a specific character in systematic botany. 



The Study of the Association. 



In England we rarely have to deal with natural asso- 

 ciations i.e., with associations undisturbed by man or 

 his domesticated animals. Traces only of the great 

 forests that once covered the country now remain, and 

 much of the woodland we see has been planted. Within 

 the cultivated area nearly all our grassland is artificail ; 



