1st, Examining In Each Hemisphere, And At The Level Of The Coasts, The Vegetation On The Same Isothermal Line, Especially

Near convex or concave summits; 2nd, comparing, with respect to the form of plants, on the same isothermal line north

And south of the equator, the alpine branch with that traced in the plains; 3rd, comparing the vegetation on homonymous isothermal lines in the two hemispheres, either in the low regions, or in the alpine regions.) This phenomenon is one of the most curious in the history of organic forms. I say the history; for in vain would reason forbid man to form hypotheses on the origin of things; he still goes on puzzling himself with insoluble problems relating to the distribution of beings.

A gramen of Switzerland grows on the granitic rocks of the straits of Magellan.* (* Phleum alpinum, examined by Mr. Brown. The investigations of this great botanist prove that a certain number of plants are at once common to both hemispheres. Potentilla anserina, Prunella vulgaris, Scirpus mucronatus, and Panicum crus-galli, grow in Germany, in Australia, and in Pennsylvania.) New Holland contains above forty European phanerogamous plants: and the greater number of those plants, which are found equally in the temperate zones of both hemispheres, are entirely wanting in the intermediary or equinoctial region, as well in the plains as on the mountains. A downy-leaved violet, which terminates in some sort the zone of the phanerogamous plants at Teneriffe, and which was long thought peculiar to that island,* is seen three hundred leagues farther north, near the snowy summit of the Pyrenees. (* The Viola cheiranthifolia has been found by MM. Kunth and Von Buch among the alpine plants which Jussieu brought from the Pyrenees.) Gramina and cyperaceous plants of Germany, Arabia, and Senegal, have been recognized among those that were gathered by M. Bonpland and myself on the cold table-lands of Mexico, along the burning shores of the Orinoco, and in the southern hemisphere on the Andes and Quito.* (* Cyperus mucronatus, Poa eragrostis, Festuca myurus, Andropogos avenaceus, Lapago racemosa. (See the Nova Genera et Species Plantarum volume 1 page 25.)) How can we conceive the migration of plants through regions now covered by the ocean? How have the germs of organic life, which resemble each other in their appearance, and even in their internal structure, unfolded themselves at unequal distances from the poles and from the surface of the seas, wherever places so distant present any analogy of temperature? Notwithstanding the influence exercised on the vital functions of plants by the pressure of the air, and the greater or less extinction of light, heat, unequally distributed in different seasons of the year, must doubtless be considered as the most powerful stimulus of vegetation.

The number of identical species in the two continents and in the two hemispheres is far less than the statements of early travellers would lead us to believe. The lofty mountains of equinoctial America have certainly plantains, valerians, arenarias, ranunculuses, medlars, oaks, and pines, which from their physiognomy we might confound with those of Europe; but they are all specifically different.

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