ZONES OF VEGETATION , ETC . 
179 
vegetation , i.e. zones occupied by similar types of vegetation, 
with the same periods of growth and with similar general 
adaptation to their environment. These divisions are purely 
climatic-ecological, and in dealing with them the systematic 
relationships of the plants are neglected. 
In a similar way the zones themselves may be divided 
upon an ecological basis into smaller areas, possessing similar 
soil, climate, and other conditions, and occupied by the 
same kind of plants, grouped in the same general way. 
Such grouping of plants as make up or occupy an alpine 
meadow, a beechwood, a salt marsh, a cultivated meadow, 
a heath, a peatmoor, forms the unit. It is sometimes called 
a formation , but this word has been used in so many senses 
that it is better avoided, and the term plant-association or 
plant-society used. Given a certain combination of climatic 
and other ecological factors, soil, &c. a given area will be 
occupied by a definite association of certain plants, which 
will bear a fairly definite numerical relationship to one 
another. Some species will usually be very abundant, or 
dominant , as for instance the heather, Calluna vulgaris , on 
most dry moors or heaths in Scotland ; others will be sub- 
dominant^ as the Ericas in the same association, while others 
again will be comparatively rare, though usually occurring in 
similar proportions in similar areas. If the conditions of 
life change, as in passing from a level to a slope, from a 
slope to a rocky precipice, from well drained to wet land, 
or from one soil to another, the grouping of the plants 
changes also, and the association is modified, or passes 
over into another association more or less rapidly according 
to the rate of change of the conditions. Or, without change 
of locality, the conditions may be changed by the introduc- 
tion of a new species of plant from abroad, which may prove 
able to establish itself in the association and thus modify 
the proportions and interrelationships of the other species. 
Or again, the invasion of grazing animals may keep down 
some species, and encourage others. 
Many of the species in an association will compete 
strenuously with one another, as they will be ecologically 
similar, using the same food-stuffs in similar proportions, or 
relying on the same insects for pollination. Others will be 
dependent species, e.g. parasites upon their hosts, sapio- 
12 — 2 
