Thus from absolute deserts, such as the floor of the deep sea or the regions surrounding the Poles, we pass to semi-deserts where plants are dotted thinly over the surface, and thence by degrees to closed vegetation of various types, where the plants elbow each other over the whole surface as they do in the grasslands spread around Farleton Fell, in the woods which adjoin them, and on the brown hillsides out to the north. But before we pass to the consideration of the conditions where favourable environment results in a closed vegetation, we may suggest for consideration the following point of view: that for any plant, or group of plants with similar requirements, much of the world is a desert--that is, a place where conditions are such that it cannot live. For each plant there exists, owing to long usage and slow adaptation to given surroundings, limiting conditions of life: where these conditions are exceeded, the desert supervenes. Thus, the salt-marsh is a desert to almost every plant of the mild open soil of hill or valley, just as the hills and valleys are deserts to most of the inhabitants of the salt-marsh. The alkaline soil of the rock crevices of Farleton Fell is fatal to some of the most abundant plants of the acid peaty soil of the hills, such as Ling (_Calluna vulgaris_) and Bilberry (_Vaccinium Myrtillus_). For another cause--the diminution of light--the deep woods are a desert for many plants of the sunny pastures, and _vice versa_. Plants vary very much as to their degree of adaptability to different soils and different climatic conditions. Some are highly specialized. Our salt-marsh flora, for instance, is, as regards most of its species, confined to its peculiar habitat. If on a map of Europe we coloured in its distribution we should find it formed a ribbon round the coast, except for a few dots where the plants have discovered inland salt springs or salt lakes, and have found their way to them. Most plants are more adaptable than these, and occupy a variety of habitats. The little Tormentil (_Potentilla silvestris_), for instance, flourishes equally on hot banks by the sea, in woods, and on mountain-tops. The more accommodating a plant is as regards habitat, the wider its distribution tends to be, both locally and in a broader sense. But wide range does not follow of necessity from adaptability to a variety of conditions: the problem of plant distribution is not so simple as that. One species may be spread right round the world, yet be always found in a special habitat; take the case, for instance, of the Yellow Bird’s-nest (_Monotropa Hypopitys_), a strange colourless, leafless plant, highly specialized, feeding, through the intermediary of a minute fungus which infests its roots (see p. 183), on the decaying leaves of deciduous woods in cold temperate regions, and yet found across Europe, Asia, and North America; while many other species, at home under very varied