Mosses are sometimes found in an isolated state as single individuals, but they are far oftener found in a social condition. It is a peculiarity of the family to grow in tufts or clusters, the appearance of which is always distinct and well-marked in different species, and often affords a specific character. This disposition to grow together, which is exhibited in no other plants so strongly, redeems them from the insignificance of their individual state, and enables them to modify in many places the appearance of the general landscape. As social plants they often cover vast districts of land. Along with the lichens they give a verdant appearance to the desert steppes of Northern Europe, Asia, and America. Mixed with grass they luxuriate in parks, lawns, and meadows, particularly in moist, low-lying situations. They spread in large patches over the ground in woods and forests; and at a certain elevation on mountain ranges they take exclusive possession of the soil, forming immense beds into which the foot sinks up to the ankles at every step, bleached on the surface by the sunshine and rain, blackened here and there by dissolving wreaths of snow which lie upon them through all the summer months, and gradually decomposing underneath into black vegetable mould. The plants whose peculiarities have been described in the preceding pages are called Urn Mosses, their fructification being urn-shaped, furnished with teeth and closed with a lid. There is another large class called Scale-Mosses, so closely allied to the true mosses that they are frequently confounded even by an educated eye. There are upward of a hundred species of scale mosses indigenous to Great Britain and Ireland, some of which are so small as to be scarcely visible and others much larger than any of the true mosses. With the exception of a few prominent species, which are found in every moist wood and on every shady rock, they are somewhat local and limited in their distribution, many of them being remarkably rare and confined to remote and isolated localities. The greatest number of species occurs in the tropics; and nowhere do they luxuriate so much as in the dark woods and mountain ravines of New Zealand. Some of them grow in the bleakest spots in the world, and are to be found even at a higher altitude than the urn-mosses on the great mountain ranges of the globe. They form the faintest tint of green on the edges of glaciers and on the bare, storm-seamed ridges of the Alps and Andes, where not a tuft of moss or a trace of other vegetation can be seen; and this almost imperceptible film of verdure, when cleansed from the earth and moistened with water, presents under the microscope the most beautiful appearance.