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Show 70 VEGETATION OF ISLANDS. [Ch. V. · t f an uninterrupted land communication, the diver-exts ence o sity in the specific character of t!1e respective vegetations is almost as striking. Thus there lS found one assemblage of species in China, another in the countries border~ng the Dlac~ Sea and the Caspian, a third in those surroundmg the Medt. terranean, a fourth in the great platforms of Siberia and Tartary, and so forth. The distinctness of the groups of indigenous plants, in the same parallel of latitude, is greatest where continents are disjoined by a wide expanse of ocean. In the northern hemisphere, near the Pole, where the extremities of Europe, Asia and America unite or approach near to one another, a con· siderable number of the same species of plants are found, common to the three continents. But it has been remarked, that these plants, which are thus so widely diffused in the Arctic regions, are also found in the chain of the Aleutian islands, which stretch almost across from America to Asia, and which may probably have served as the channel of communication for the partial blending of the Floras of the adjoining regions. It has, indeed, been found to be a general rule, that plants found at two points very remote from each other, occur also in places intermediate. In islands very distant from continents, the total number of plants is comparatively small; but a large proportion of the species are such as occur nowhere else. In so far as the Flora of such islands is not peculiar to them, it contains, in general, species common to the nearest main lands *'. The islands of the great southern ocean exemplify these rules; the easternmost containing more American, and the western more Indian plants t. Madeira and Teneriffe contain many species, and even entire genera, peculiar to them; but they have also plants in common with Portugal, Spain, the Azores, and the north-west coast of Africa :t:. * Prichard, vol. i. p. 36. Brown, Appendix to Flinders. t Forster, Observations, &c. l Humboldt, Pers. N arr., vol. i. p. 270 of the translation, Prichard, Pbyl• Hist, of Mankind, vol. i.~p. 37. Ch. V.] DISTINCT BOTANICAL REGIONS. 71 In the Canaries, out of five hundred and thirty-three species of phanerogamous plants, it is said that three hundred and ten are peculiar to these isles, and the rest identical with those of the African continent; but in the Flora of St. Helena which is so far distant, even from the western shores of Afric~, there have been found, out of sixty-one native species, only two or three which are to be found in any other part of the globe. Decandolle has enumerated twenty great botanical provinces inhabited by indigenous or aboriginal plants ; and although many of these contain a variety of species which are common t~ several others, and sometimes to places very remote, yet the hnes of demarcation are, upon the whole, astonishingly well defined*· Nor is it likely that the bearing of the evidence on which these general views are founded will ever be materially affected, since they are already confirmed by the examination of seventy or eighty thousand species of plants. The entire change of opinion which the contemplation of these phenomena has brought about is worthy of remark. The first travellers were persuaded that they should :find, in distant regio~s, ~h~ plants of their own country, and they took a pleasure m g1vmg them the same names. It was some time before this illusion was dissipated; but so fully sensible did botanists at last become of the extreme smallness of the number of phrenogamous plants common to different continents, that the ancient Floras fell into disrepute. All grew diffident of the pretended identifications, and we now find that every naturalist is inc.lined to examine e~ch supposed exception with scrupulous seventyt. If they adm1t the fact, they begin to speculate on the mode whereby the seeds may have been transported from one country into the other, or inquire on which of two continents ~he. ~lant was indigenous, assuming that a species, like an md1vtdual, cannot have two birth-places. The marine vegetation is less known, but we learn from * See a farther subdivision 'by which twenty-seven provinces are made b M Alph. Decandolle, son of Decandolle. Monogr. des Campanul6es. Paris; 1:30.' t Deca.ndolle, Essai EMmen. de G6og. Botan, p. 45. |