"The lonely island of St. Helena, for example, at the time of its discovery in 1501, produced about sixty vegetable species. Its flora now comprises seven hundred and fifty species. The faculty of spontaneous reproduction supposes a greater power of accommodation than we find in most domesticated plants. Although every wild species affects a habitat of a particular character, it will grow under conditions extremely unlike those of its birthplace. The seven hundred new species which have found their way to St. Helena within three centuries and a half were probably not in very large proportion designedly introduced there by human art. As a general rule, it may be assumed that man has intentionally transferred fewer plants than he has accidentally into countries foreign to them. Tares follow the wheat. The weeds that grow among the cereal grains, and form the pest of the kitchen-garden, are the same in America as in Europe. Some years ago, the author made a collection of weeds in the wheat-fields of Upper Egypt, and another in the gardens on the Bosphorus. Nearly all the plants were identical with those that grow under the same conditions in New England. The change from one locality to another is effected by a thousand casual circumstances. The upsetting of the wagon of an emigrant in his journey across the Western plains may scatter upon the ground the seeds he designed for his garden. The herbs which fill so important a place in the rustic _materia medica_ of the Eastern States spring up along the prairie-paths just opened by the caravan of the settler. The _hortus siccus_ of a botanist may accidentally sow seeds from the foot of the Himalayas on the plains that skirt the Alps. It is a fact frequently observed, that exotics transplanted to foreign climates suited to their growth escape from the flower-garden, and naturalize themselves among the spontaneous vegetation of the pastures. The straw and grass employed in packing the sculptures of Thorwaldsen were scattered in the court-yard of the museum in Copenhagen, where they are deposited, and the next season there sprang from the seeds no less than twenty-five species of plants belonging to the Roman Campagna. In the campaign of 1814, the Russian troops brought in the stuffing of their saddles seeds from the banks of the Dnieper to the valley of the Rhine, and even introduced the plants of the Steppes into the environs of Paris. The Turkish armies in their incursions into Europe brought Eastern vegetables in their train, and left the seeds of Oriental wall-plants to grow upon the ramparts of Buda and Vienna. The Canada thistle is said to have sprung up in Europe two hundred years ago from a seed which dropped out of the stuffed skin of a bird."