be very imperfect, and would in fact scarcely lead to any definite results. A Flora, or descriptive catalogue of all the plants growing in one country, would prove a most unsafe guide to a person examining the botanical productions of another country situated at the same distance from the equator. There are other influences besides mere geographical position which have great weight in determining the diffusion of plants, none of which must be omitted from the account.

The great agents in promoting vegetation are heat, light, and moisture, to which must be added air and constitution of soil. Now, were the earth of uniform structure, level, and distributed into proportions of land and water everywhere the same, there would most likely ensue a regularity of climate governed by fixed and applicable laws; and, in that case, probably, vegetables would be arranged in zones, each having its defined limits, which it would rarely pass. But this is far from being the case: until we have examined all the features of any particular country, it is impossible to say what is its climate, and what are its vegetable productions. The existence of a mountain influences not only the vegetation of the mountain itself, but in a measure that of the surrounding country. Abundance of springs and rivers, not only irrigate the earth, but lower the temperature by evaporation. Extensive plains of sand occasion drought in the adjoining countries; for the parched winds which traverse them are, during their transit, exhausted of their moisture, and recruit themselves by absorbing a new supply from the moister countries at which they arrive. On the other hand, winds which have traversed the ocean, reach the shore overcharged with moisture, which they precipitate in rain, or deposit in dew. Again, as the water of the ocean varies much less in temperature than the atmosphere or the dry land, maritime countries are in general subject to cooler summers and warmer winters than inland countries; and since the density of water is diminished by heat as well as by excessive cold, a current of the ocean setting from a hot climate may occasion a high temperature in some land, which, from its geographical position, ought to be cold; or a current drifting along masses of ice may chill the air and retard the vegetation of a country, which we might expect to find clothed with perpetual verdure.

The general rule, then, that the abundance and luxuriance of botanical productions varies with the latitude, is modified by the causes I have just enumerated. Subject, however, to these limitations, it may be laid down as a principle, that the number

of species goes on progressively increasing, from the poles to the equator, and that in the torrid zone it reaches its maximum. The size of vegetables too, as well as the rate of growth, goes on increasing from the poles to the equator, and the number of woody species, as well as their proportion to herbaceous species, follows the same rule.

It is a law also of universal application, that temperature diminishes in a ratio, bearing a certain proportion to the elevation of any place above the earth's surface, that ratio being such as to give a difference of one degree, for from three to five hundred feet of elevation. If we suppose, for instance, a range of mountains to occupy the site of London: at an elevation of 1,800 feet, we should find the temperature of Edinburgh; at an elevation of 3,200 feet we should encounter that of the Orkneys; and at an elevation of 10,800 feet we should be assailed by the frosts of Spitzbergen. The plains of Sicily produce palms and the sugar-cane, but 100 feet above the level of the sea these plants cease; the belt above, extending for 1,000 feet, is occupied by oranges, olives, and rice; still higher up, rice, wheat, and maize cease, and are succeeded by oaks and chestnuts, which in their turn give place to fir, beech, and birch, and these, dwindling to shrubs, yield to leafless lichens, which last, at an elevation of 10,000 feet, finally disappear, and all vegetation is at an end.

The Tropics present yet greater contrasts: the forests of South America, to the elevation of 3,000 feet, are thick with leafy evergreen trees, palms, and arborescent ferns; and, as if the ground did not supply sufficient surface for the exuberant vegetation, the branches of the trees are laden with parasitic plants of exquisite structure, while the grasses, not as with us forming a verdant tapestry, grow in gigantic tangled masses. At an elevation of 3,250 feet, the palms disappear, but the tree-ferns continue for 2,000 feet more. Belt succeeds belt, each approaching more and more nearly the characters of the vegetation of colder regions, until at last even the Alpine plants disappear, and at an elevation of 15,600 feet, the limits of perpetual snow are attained.

Besides the variation which proceeds from decrease of temperature, there is little doubt that the degree of humidity in the air exercises an important influence; but so little is at present known of the laws which regulate the condition of the atmosphere in this respect, that we can only cursorily allude to this part of the subject. That temperature, however, is the main agent in promoting or retarding vegetation, is evident from the fact that the

own.

vegetable productions of hot climates can be successfully cultivated in cold ones by the aid of heat, and that the plants of cold climates may be cultivated in hotter climates by an artificial reduction of temperature. Still we may remark that the development of the vegetation of different families depends on some law distinct from that of temperature, the nature of which is wholly unknown, but which is probably connected with the original state of our planet. For instance, in tropical Africa palms are not very numerous, if compared with the much greater number in America. The Cape of Good Hope abounds in heaths, a tribe of which none have been discovered in the New World. On the other hand, there are 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 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 the West Indian Islands. Of those which have the power of adapting themselves to all climates, the instances are not so numerous, but they are very remarkable. In the woods of Georgia, in North America, there grows a rose, which, while all the other species of rose of that country are entirely different from those of other regions, is identical with a rose indigenous to China; though to the general Flora of that country, the Flora of North America has no resemblance. The brookweed (Samolus Valerandi) is found all over the world, from the frozen north to the burning south, associated here with daisies and buttercups, and there with Arctic saxifrages or waving palms. The yellow water crowfoot abounds on the banks of the Amazon, the Thames, and the Ganges. The cereal grasses, such as wheat, barley, &c., accompany man in all his wanderings, flourishing in every climate which is adapted to the requirements of the ox and horse. Above three hundred and fifty species are said to be common to Europe and North America; and even among the peculiar features of the Flora of New Holland, one hundred and sixty-six European species have been recognised.

We shall be able to give a better idea of the various forms

under which vegetation presents itself in the different parts of the world, by selecting a spot here and there, and sketching as it were the outline of a scene from nature, than by attempting a classification of plants according to the zones in which they predominate; for though perhaps it may not be perfectly faithful, it will at least be more suggestive than a tabular catalogue of

names.

ARCTIC REGIONS.

To begin, then, with those regions most scantily clothed with vegetation.

Two thousand feet above the line of never-melting snow, the surface of which is rarely moistened by the slanting rays of summer's mid-day sun, a perpendicular rock occasionally protrudes through the chilly mantle of the earth, and in the crevices of this, vegetable life first shows itself in the form of a few minute lichens, scarcely to be distinguished, and yet more difficult to be separated from the stone to which they cling. One thousand five hundred feet lower a few plants of Arctic Ranunculus (R. glacialis) and similar plants occur in like situations, where a few dark spots are to be seen; the spongy brown surface of the ground is thinly scattered with two or three saxifrages, a rush and a tufted Silene, plants identical with, or closely resembling, the products of our highest Scottish alps. Lower down, the number of these is sparingly increased, and with them are mixed a few dwarf berry-bearing shrubs, which, however, never ripen their fruit. As we descend these become productive; a few more flowering plants appear; the dwarf birch creeps along the ground, and willows attain the height of a foot or two. Then the cloudberry (Rubus Chamomorus) begins to ripen its fruit; a violet, an alpine arbutus, a stunted juniper bush, a speedwell, and even a dwarf fern is added to the Flora. The dry spots begin to be covered with reindeer lichen; the ling, the aspen, and the mountain ash appear. Forests stretch away in dreary succession, composed of gnarled, knotty, stiff beech trees, the loftiest of which may be looked down on by a man of ordinary stature. Scotch firs, with low stems and widely-extended branches, contrive to exist, and below them a coltsfoot, a sedge, and even a rose enjoy a brief summer. The spruce fir succeeds, not such as is "hewn on Norwegian hills to be the mast of some great admiral," but a mere slender pole covered from the ground with short drooping dark branches.

But this is a highly-coloured picture of Arctic scenery in its predominant character, especially as it exists near the coasts.

There, for thousands of dreary acres, no tree interrupts the uniform line of the horizon; no shrub dares to show itself above the level of the turfy vegetation; all woody remains are closely prostrated to the ground, and only maintain life by seeking shelter among the mosses and lichens. One of the most recently-discovered islands, about twelve miles in circumference, afforded but eight species of plants to the botanists who explored it; a hepatica, two mosses, a grass, a wormwood, a scurvy grass, a saxifrage, and a lichen. In more-favoured districts, the prevailing vegetation is composed of low tufted herbaceous plants, with fleshy leaves and large flowers; a prodigious number of small berry-bearing shrubs, which afford food to the natives and refreshment to the mariner; mosses, of which one affords wicks for lamps, and another, a bed and coverlet for children; a species of garlic and a polygonum (P. viviparum), well-known to Scotch botanists, which produces roots, bearing, when roasted, some resemblance to a potato.

The sub-Arctic region is distinguished from the Arctic, by producing vegetable forms which grace the plains of the temperate zone, such as the currant, rose, lupin, aconite, larkspur, violet, and others. The gaiety of the vegetation is in some localities very striking; many flowers are large, their colours bright; and though white and yellow are predominant, other tints are not uncommon. Among one hundred and eighty-nine species found on the N.W. coast of Africa, fifty-three were greenish-yellow, fifty white, thirty-six yellow, twenty-five purple, fourteen blue, ten rose-colour, one red, scarlet being entirely wanting.

ANTARCTIC REGIONS.

We have already seen that the proximity of any place to a great body of water tends greatly to equalize the temperature, of that place, diminishing the intensity of its heat in summer, and the rigour of its cold in winter. Now, a simple glance at a map of the world will suffice to show that the mass of all the great continents lies in the northern hemisphere, the arctic circle traversing more than four times as much land as it does water. The consequence is that in the Arctic Regions the winter is intensely cold, while the summer is bright and hot. In the southern hemisphere, on the other hand, America is the only continent which stretches as far as the fifty-sixth degree of latitude, and the Antarctic circle traverses water only, or, at least, what scattered islands there are in its vicinity are removed from the influence of any tracts which enjoy a continental climate, and are so inconsiderable in them