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Show 84 .AGENCY OF MAN IN TilE [Ch. V. in four years as to become one of the commonest weeds in the island*· The most remarkable proof, says Decandolle, of the extent to which man is unconsciously the instrument of dispersing and naturalizing species, is found in the fact, that in New Holland, America, and the Cape of Good Hope, the a~original European species exceed in number all t~e ot~er~ whtch have come from any distant regions, so that, m thts mstance, the influence of man has surpassed that of all the other causes which tend to disseminate plants to remote districts. Although we are but slightly acquainted, as yet, with the extent of our instrumentality in naturalizing species, yet the facts ascertained afford no small reason to suspect that the number which we introduce unintentionally, exceeds all those transported by design. Nor is it unnatural to suppose that the functions, which the inferior beings extirpated by man once discharged in the economy of nature, should devolve upon the human race. If we drive many birds of passage from different countries, we are probably required to fulfil their office of carrying seeds, eggs of fish, insects, molluscs, and other creatures, to distant regions; if we destroy quadrupeds, we must r~place them, not merely as consumers of the animal and vegetable substances which they devoured, but as disseminators of plants, and of the inferior classes of the animal kingdom. We do not mean to insinuate that the same changes which man brings about, would have taken place by means of the agency of other species, but merely that he supersedes a certain number of ~gents, and so far as he disperses plants unintentionally, or against his will, his intervention is strictly analogous to that of the species so extirpated. We may observe, moreover, that if, at former periods, the animals inhabiting any given district have been partially altered by the extinction of some species, and the introduction of others, whether by new creations or by immigration, a change must have taken place in regard to the particular plants con- • Principles of Botany, 11. 389. Ch. V.] DISPERSION OF PLANTS • 85 veyed about with them to foreign coun t ri. es. A s 1~0 r exampI e when o. ne set of migratorY bI' r d s I·S subst·i tuted for another, the' co. untries from and to which seeds are t ransporte d are x. mme-dta. t ely changed. Vi. cissitudes' therefore ' a na 1o gous t o t h ose whiC. h . man has occasiOned, may have previous! y a tte n d e d t h e sprmgi~g up of new relations between species in the vegetable and ammal worlds. . It rna~ also be rem~rked~ tha~ if man is the most active agent m enlar~mg, so als~ Is he m circumscribing the geographical boundaries of partiCular plants. He promotes the migration of some, he :etards that of other species, so that while in many respects he appears to be exerting his power to blend and ~onfound the ~arious provinces of indigenous species, he is, In other ways, mstrumental in obstructing the fusion into on group of the inhabitants of contiguous provinces. e ThusJ for example, when two botanical regions exist in the same. great continent, such as the European region, comprehendmg ~he central parts of Europe and those surrounding the Mediterranean, and the Oriental region, as it has been termed, ~mbracing the countries adjoining the Black Sea and the Caspian, the interposition between these of thousands of squ~res mi~es of cultivated lands, opposes a new and powerful barne: agamst the mutual interchange of indigenous plants. Botamsts are wel~ aware that garden plants naturalize and diffuse th.emselves with great facility in comparatively unreclaimed countnes, but spread themselves slowly and with difficult · d' . . ym Istricts highly cultivated. There are many obvious cau £o r th '~ ~ d'ffi ses 1 erence; by drainage and culture the natural variety of stations is diminished, and those stray individuals by which the passage of a species from one £t station to another is effetted, are no sooner detected by the agriculturist than the a~e uprooted as weeds. The larger shrubs and tr:es, in pa:. tiCu~ar, can scar~ely. ever escape observation, when they have attamed a certam size, and will rarely fail to be cut down if unprofitable. The same observations are applicable to the interchanO'e of 0 |