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proaching the south, the floras of these three great divisions of the globe differ more and more even in the same parallels of latitude, which shows that temperature alone is not the cause of the almost complete diversity of species that everywhere prevails. The floras of China, Siberia, Tartary, of the European district including central Europe and the coasts of the Mediterranean, and the oriental region, comprising the countries round the Black and Caspian Seas, all differ in specific character. Only twenty-four species were found by
. MM. Bonpland and Humboldt in equinoctial America, that are identical with those of the Old World; and Mr. Brown not only found that a peculiar vegetation exists in New Holland, between the thirty-third and thirty-fifth parallels of south latitude, but that, at the eastern and western extremities of these parallels, not one species is common to both, and that certain genera also are almost entirely confined to these spots. The number of species common to Australia and Europe are only 166 out of 4100, and probably some of these have been conveyed thither by the colonists. This proportion exceeds what is observed in southern Africa, and from what has been already stated, the proportion of European species in equinoctial America is still less.
Islands partake of the vegetation of the nearest
continents, but when very remote from land their floras are altogether peculiar. The Aleutian islands, extending between Asia and America, partake of the vegetation of the northern parts of both these continents, and may have served as a channel of communication. In Madeira and Teneriffe, the plants of Portugal, Spain, the Azores, and of the north coast of Africa are found, and the Canaries contain a great number of plants belonging to the African coast. But each of these islands possesses a flora that exists nowhere else, and St. Helena, standing alone in the midst of the Atlantic ocean, out of sixty-one indigenous species, produces only two or three recognised as belonging they scarcely form one-twentieth, and in Lapland one-thirtieth part. In approaching the equator, the ligneous exceed the number of herbaceous plants; in America, there are a hundred and twenty different species of forest-trees, whereas in the same latitude in Europe only thirty-four are to be found.
of the world. It appears from the investigations of Humboldt that between the tropics the monocotyledonous plants, such as grasses and palms, which have only one seed-lobe, are to the dicotyledonous tribe, which have two seed-lobes, like most of the European species, in the proportion of one to four; in the temperate zones they are as one to six; and in the arctic regions, where mosses and lichens, which form the lowest order of the vegetable creation, abound, the proportion is as one to two. nual monocotyledonous and dicotyledonous plants in the temperate zones amount to one-sixth of the whole, omitting the cryptogamia; in the torrid zone
Similar laws appear to regulate the distribution of marine plants. M. Lamouroux has discovered that the groups of algæ affect particular tempera- . tures or zones of latitude, though some few genera prevail throughout the ocean. The polar Atlantic basin, to the 40° of north latitude, presents a well-defined vegetation. The West Indian seas, including the gulf of Mexico, the eastern coast of South America, the Indian ocean and its gulfs, the shores of New Holland, and the neighbouring islands, have each their assemblage of distinct species. The Mediterranean possesses a vegetation peculiar to itself, extending to the Black Sea; and the species of marine plants on the coasts of Syria and in the port of Alexandria differ almost entirely from those of Suez and the Red Sea, notwithstanding the proximity of their geographical situation. It is observed that shallow seas have a different set of plants from such as are deeper and colder; and, like terrestrial vegetation, the algæ are most numerous towards the equator, where the quantity must be prodigious, if we may
judge from the gulf-weed, which certainly has its origin in the tropical seas, and is drifted, though not by the gulf-stream, to higher latitudes, where it accumulates in such quantities, that the early Portuguese navigators, Columbus and Lerius, compared the sea to extensively inundated meadows, in which it actually impeded their ships and alarmed their sailors. Humboldt, in his Personal Narrative, mentions, that the most extensive bank of sea-weed is in the northern Atlantic, a little west of the meridian of Fayal, one of the Azores, between the 25° and 36° of latitude. Vessels returning to Europe from Monte Video, or from the Cape of Good Hope, cross this bank nearly at an equal distance from the Antilles and Canary islands. The other occupies a smaller space, between the 22° and 26° of north latitude, about eighty leagues west of the meridian of the Bahama islands, and is generally traversed by vessels on their passage from the Caicos to the Bermuda islands. These masses consist chiefly of one or two species of Sargassum, the most extensive genus of the order Fucoideæ.
Some of the sea-weeds grow to the enormous length of several hundred feet, and all are highly coloured, though many of them must grow in the deep caverns of the ocean in total, or almost total darkness; light, however, may not be the only principle on which the colour of vegetables depends, since Humboldt met with green plants growing in complete darkness at the bottom of one of the mines at Freuberg.
It appears that in the dark and tranquil caves of the ocean, on the shores alternately covered and deserted by the restless waves, on the lofty mountain and extended plain, in the chilly regions of the north, and in the genial warmth of the south, specific diversity is a general law of the vegetable kingdom, which cannot be accounted for by diversity of climate; and yet the similarity though not identity of species is such, under the same isothermal lines, that if the number of species belonging to one of the great families of plants be known in any part of the globe, the whole number of the phanerogamous or more perfect plants, and also the number of species composing the other vegetable families, may be estimated with considerable accuracy.
Various opinions have been formed on the original or primitive distribution of plants over the surface of the globe, but since botanical geography became a regular science, the phenomena observed have led to the conclusion that vegetable creation must have taken place in a number of distinctly different centres, each of which was the original seat of a certain number of peculiar spe