Within any one climatic region--say within the British Islands--the _soil_ in which the plants grow is the controlling factor in determining the character of the plant population. And while a classification by _plant form_--such as woodland, grassland--is often convenient, when we come to analyze the various plant associations which colonize the ground, it will be found that similarity of form-type does not necessarily imply affinity as regards either physiological conditions or floristic constituents. Thus, a Beech wood on the Chalk has really no affinity with an Oak wood on the Coal-measures, save that they are both woods: they shelter plant groups of quite different composition, one a constituent association of the Limestone Formation, and the other of the Formation of Clays and Loams, according to modern English classification. Similarly, the Hazel copse which covers the screes of Farleton Fell has no close relation to the Hazel copses along the Westmorland becks, although the dominant plant--the Hazel--is the same in both cases: soil is the controlling factor, and the one is related to the limestone vegetation of the hill above, the other to the vegetation of the loams and peaty soils of the adjoining mountain-side. In the British Isles the leading plant formations are those of clays and loams, of sands and sandstones, of siliceous soils, of calcareous soils, of peat, of marsh, of lakes and rivers, of salt-marsh, sand dune, and shingle beach; also, governed by the climatic factor, alpine vegetation stands somewhat apart. While the vegetation of some of these, such as salt-marsh or peat, usually presents a uniform aspect, others, such as the clays, sands, and limy soils, display each a characteristic type of woodland and of grassland, as well as other variants, dependent on the composition, depth, and wetness of the soil, the degree of exposure, and so on: these form the _associations_ which together constitute the formation. Each association, if the plants composing it be examined, will be found to consist of an assemblage of species, large and small, brought together by their superior fitness for the particular conditions which prevail. There are mostly in each association one or more dominant species--such as the trees of an Oak wood, or the Heather of a moor--which by their abundance or vigorous growth control the association. The shelter which they give may protect some of the members of the community: the shade which they cast may keep out other plants which otherwise would invade the ground. The association will include some species specially adapted to the particular conditions which prevail, and perhaps not found elsewhere in the area; these are the indicator plants of the association, which give it its special character, and which will help us to identify the association should we encounter it again; there will be others--dependent species--which are attracted by the shade, or shelter, or other advantages which the growth