226 BRITISH PLANTS 
and in the course of time, as the sand consolidates, one 
association is replaced by another. The formation thus 
has @ 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 
arts. 
4 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—7.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 ; 
