From what has now been said, it would seem that the forms assumed by vegetation in different latitudes are dependent upon particular conditions of climate and soil, and that it is to variations of these conditions that we are to ascribe the difference between the Flora of the equator and of the polar regions. And this is no doubt true: but there are, nevertheless, some plants which have a remarkable power of adapting themselves to all climates and circumstances; and there are others which readily naturalize themselves in climates similar to their own. Of the latter, examples present themselves at every step; all the hardy plants of our gardens may in some sort be considered of this nature; for although they do not grow spontaneously in the fields, they flourish almost without care in our gardens. The pine apple has gradually extended itself, eastward from America, through Africa, into the Indian Archipelago; where it is now as common as if it were a plant indigenous to the soil; and in like manner the spices of the Indies have become naturalized on the coast of Africa and in the West Indian Islands. Of the former description the instances are not numerous, but they are very remarkable. In the woods of Georgia, in North America, grows the Rosa lævigata, which, while all the other species of rose of that country are entirely different from those of other regions, is identical with the R. sinica of China; to the Flora of which country, that of North America has no resemblance. Samolus Valerandi is found all over the world, from the frozen north to the burning south; associated here with Amentacea and similar northern forms, and there mixed with palms and the genuine denizens of the tropics. Above 350 species are said to be common to Europe and North America, and even among the peculiar features of the Flora of New Holland, Mr. Brown recognised 166 European species. The presence of many of such strangers may undoubtedly be referred to the agency of man, by whom they have been transported from climate to climate, along with corn and by other means; as, for example, at Pont Juvenal, near Montpellier, the vicinity of which abounds with Barbary plants; the seeds of which are known to have been brought across the Mediterranean along with the Barbary wool, which is disembarked at that station. In like

manner the various kinds of corn have been carried about from country to country for the service of mankind, until their real home has become doubtful. Medicago sativa abounds in Chili, whither it has been transported by the Spaniards; and instances in abundance of similar cases could be produced. But it must not thence be inferred, that all cases of species growing in places far away from their kindred forms, are to be referred to migration: for this, the agency of man, of animals, of seas, of wind, and of torrents, will doubtless have done a great deal; but none of the causes, nor any other with which I am acquainted, will explain the identity of the Calypso borealis, Orchis viridis, and Betula nana of North America and of Europe; of the Potamogetons common to Europe and New Holland; of the Rosa, already adverted to, as common to North America and China; of the wide diffusion of Samolus valerandi, and, most especially, of the identity of the cryptogamic plants of various countries, plants incapable of cultivation, unconnected with the purposes of man, and of all others, the most difficult of transport under any form. To us it appears that such plants must have been originally created in the places where they now exist; the contingent circumstances under which they are found having been favourable to the particular mode of vegetable developement which was necessary for their formation. And this may, I think, be admitted, as a circumstance connected with the original creation of the world, without having recourse to the theory of some philosophers, that Nature exercises at this time the power of producing plants without parents; a subject upon which Professor Link remarks, that "we find buried in the earth the remains of plants which formerly existed, but which are now unknown. New forms have, therefore, been produced by nature different from the first. Wherever a salt spring breaks out at a distance from the sea, its vicinity immediately abounds with salt plants, although none grew there before. When lakes are drained a new kind of vegetation springs up: thus, when the Danish island of Zeland was drained, Vilny observed Carex cyperoides springing up, although that species is naturally not a native of Denmark, but native of the North of Germany. Hence it is easy to infer, that some plants have been produced at one time and others at another, some earlier

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and some later and to this cause may be attributed the smaller number of species found upon islands than upon continents, the former having been produced the latest. Perhaps plants change from one to the other, as certain organic bodies when young belong to a less perfect class than when they are older. On the naked rocks we find Lichens, on the mud Confervæ, in ancient strata the remains of Monocotyledonous plants, in more recent strata those of Dicotyledons."

In concluding this important and very interesting subject, I must refer the reader who is desirous of further information to the writings of Mr. Brown in the appendix to Captain Flinders's Voyagers, and Captain Tuckey's Expedition to the Congo; to M. De Candolle's Essay upon the Geography of Plants, published in the 18th volume of the Dictionnaire des Sciences Naturelles; to the numerous writings of Baron Humboldt; and to the observations upon the subject by Professor Schouw, as translated in Brewster's Edinburgh Journal.

BOOK VII.

MORPHOLOGY; OR, OF THE METAMORPHOSIS of ORGANS.

THAT part of botany which treats of the gradual transmutation of leaves into the various organs of a plant, which shows that bracteæ are leaves affected by the vicinity of the fructification, that the calyx and corolla are formed by the adhesion and verticillation of leaves, that the filament is a form of petiole, and the anther of lamina; and, finally, that the ovarium itself is a convolute leaf, with its costa elongated into a style, and the extremity of its vascular system denuded under the form of stigma, is called morphology.

This doctrine has already been treated of in this work, in connection with the different organs of which mention has been made; but it is so curious and important as to deserve especial mention. It seems to have originated with Linnæus, was deeply entered upon by the celebrated poet Göthe, has been universally adopted in Germany, and been partially received in France and England; in the former country by Du Petit Thouars, De Candolle, and others; and in this kingdom by most of the botanists of the present day.

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The first idea of the subject appeared in the second volume of the tenth edition of the Systema Naturæ, published in 1759, in which Linnæus thus expresses himself: "Leaves are the creation of the present year, bractea of the second, calyx of the third, petals of the fourth, stamens of the fifth, and the stamens are succeeded by the pistillum. This is apparent from ornithogalums, luxuriant plants, proliferous plants, double flowers, and cardui."