The proximity of a large town, especially if it is a centre of manufacture, is a notorious factor in the reduction of the native flora: not only by the thoughtless and wanton destruction carried out by its inhabitants, but more subtly by the deposition of soot, and by the poisoning of the air by sulphurous and acid fumes. The higher Cryptogams, such as Mosses and Hepatics, are particularly susceptible in this respect, and vanish along with the more delicate Seed Plants. Mining centres are specially destructive of plant life, since, in addition to other drawbacks, the soil is often buried under masses of excavated material containing poisonous substances. If there is a purgatory for plants, it is surely found in such areas. Other examples of the multitudinous ways in which human activities disturb and destroy native plant life will occur to the reader--the burning of moors in order to improve them as pasturage; in recent years the tarring of roads, which kills the pleasant wayside herbage and poisons the streams into which the road drainage is carried; and so on. The indictment is an overwhelming one, and, as said in the first chapter, the flora is now everywhere so altered that we can gain some idea of its original aspect only by a study of isolated fragments and much-adulterated samples. But if the debit side of the account, as presented by the lover of nature, is heavy, it must not be forgotten that there are many items to man’s credit. Though our country’s vegetation has lost in scientific interest, it has gained vastly in both economic and æsthetic value by the introduction of useful and ornamental plants from all the Temperate regions of the world; and besides, a large number of species have followed in man’s footsteps, and, taking advantage of the disturbance of the native flora caused by his operations, endeavour with more or less success to establish a footing in the country. Before we trespass on the domains of arboriculture, horticulture, or agriculture, under which heads the cultivation of useful or ornamental plants divides itself, some consideration is required of those plants which, quasi-wild, are usually included in accounts of the vegetation under the head of aliens, denizens, colonists, and so forth. These constitute a quite considerable proportion of the total number of species found in any area which has felt the influence of man. For instance, in the county of Dublin, which, owing to its diversified surface--sea-cliff, sands, moorland, woodland, and cultivation--and its favourable climate--the warmest and driest in the country--possesses the largest flora of any similar area (354 square miles) in Ireland, the list of about 760 “wild” plants includes some 170, or over one-fifth of the whole, whose presence is attributable, directly or indirectly, to human activities. We may compare these figures with those drawn from a study of the flora of Kent, which faces