[footnote] *Humboldt, 'Aphorismi ex Physiologia Chemica Plantarum', in the 'Flora Fribergensis Subterranea', 1793, p. 178.
Among these we may reckon many of the marine Algae — Cladoniae and mosses, which extend over the desert steppes of Northern Asia — grasses, and cacti growing p 347 together like the pipes of an organ — Avicennim and mangroves in the tropics — and forests of Coniferae and of birches in the plains of the Baltic and in Siberia. This mode of geographical distribution determines, together with the individual form of the vegetable world, the size and type of leaves and flowers, in fact, the principal physiognomy of the district,* its characteracter being but little, if at all, influenced by the ever-moving forms of animal life, which, by their beauty and diversity, so powerfully affect the feelings of man, whether by exciting the sensations of admiration or horror.
[footnote] *On the physiognomy of plants, see Humboldt, 'Anischten der Natur', bd. ii., s. 1-125.
Agricultural nations increase artificially the predominance of social plants, and thus augment, in many parts of the temperate and northern zones, the natural aspect of uniformity; and while their labors tend to the extirpation of some wild plants, they likewise lead to the cultivation of others, which follow the colonist in his most distant migration. The luxuriant zone of the tropics offers the strongest resistance to these changes in the natural distribution of vegetable forms.
Observers who in short periods of time have passed over vast tracts of land, and ascended lofty mountains, in which climates were ranged, as it were in strata one above another, must have been early impressed by the regularity with which vegetable forms are distributed. The results yielded by their observations furnished the rough materials for a science, to which no name had as yet been given. The same zones of regions of vegetation which, in the sixteenth century, Cardinal Bembo, when a youth,*described on the declivity of Aetna, were observed on Mount Ararat by Tournefort.
[footnote] *Aetna Dialogus.' 'Opuscula', Basil., 1556, p. 53, 54. A very beautiful geography of the plants of Mount AEtna has recently been published by Philippi. See 'Linnaea', 1832, s. 733.
He ingeniously compared the Alpine flora with the flora of plains situated in different latitudes, and was the first to observe the influence exercised in mountainous regions, on the distribution of plants by the elevation of the ground above the level of the sea, and by the distance from the poles in flat countries. Menzel, in an inedited work on the flora of Japan, accidentally made use of the term 'geography of plants'; and the same expression occurs in the fanciful but graceful work of Bernardin de St. Pierre, 'Etudes de la Nature'. A scientific treatment of the subject began, however, only when the geography of plants was intimately associated with the study of the distribution p 348 of heat over the surface of the earth, and when the arrangement of vegetable forms in natural families admitted of a numerical estimate being made of the different forms which increase of decrease as we recede from the equator toward the poles, and of the relations in which, in diffrent parts of the earth, each family stood with reference to the whole mass of phanerogamic indigenous plants of the same region. I consider it a happy circumstance that, at the time during which I devoted my attention almost exclusively to botanical pursuits, I was led by the aspect of the grand and strongly characterized features of tropical scenery to direct my investigations toward these subjects.
The study of the geographical distribution of animals, regarding which Buffon first advanced general, and, in most instances, very correct views, has been considerably aided in its advance by the progress made in modern times in the geography of plants. The curves of the isothermal lines, and more especially those of the isochimenal lines, correspond with the limits which are seldom passed by certain species of plants, and of animals which do not wander far from their fixed habitation either with respect to elevation or latitude.*
[footnote] *[The following valuable remarks by Professor Forbes, on the correspondence existing between the distribution of existing faunas and floras of the British Islands, and the geological changes that have affected their area, will be read with much interest; they have been copied, by the author's permission, from the 'Survey Report', p. 16: "If the view I have put forward respecting the origin of the flora of the British mountains be true — and every geological and botanical probability, so far as the are is concerned, favors it — then must we endeavour to find some more plausible cause than any yet shown for the presence of numerous species of plants, and of some animals, on the higher parts of Alpine ranges in Europe and Asia, specifically identical with animals and plants indigenous in the regions very far north, and not found in the intermediate lowlands. Tournefort first remarked and Humboldt, the great organizer of the science of natural history geography, demonstrated, that zones of elevation on mountains correspond to parallels of latitude, the higher with the more northern or southern, as the case might be. It is well known that this correspondence is recognized in the general 'facies' of the flora and fauna, dependent on generic identities. But when announcing and illustrating the law that climatal zones of animal and vegetable life are mutually repeated or represented by elevation and latitude, naturalists have not hitherto sufficiently (if at all) distinguished between the evidence of that law, as exhibited by 'representative species' and by 'identical'. In reality, the former essentially depend on the law, the latter being an 'accident' not necessarily dependent upon it, and which has hitherto not been accounted for. In the case of the Alpine flora of Britain, the evidence of the activity of the law, and the influence of the accident, are inseparable, the law being maintained by a transported flora, for the transmission of which I have shown we can not account by an appeal to unquestionable geological events. In the case of the Alps and Carpathians, and some other mountain ranges, we find the law maintained partly by a representative flora, special in its region, i.e., by specific centers of their own, and partly by an assemblage more or less limited in the several ranges of identical species, these latter in several cases so numerous that ordinary modes of transportation now in action can no more account for their presence than they can for the presence of a Norwegian flora on the British mountains. Now I am prepared to maintain that the same means which introduced a sub-Arctic (now mmountain) flora into Britain, acting at the same epoch, originated the identity, as far as it goes, of the Alpine floras of middle Europe and Central Asia; for, now that we know the vast area swept by the glacial sea, including almost the whole of Central and Northern Europe, and belted by land, since greatly uplifted, which then presented to the water's edge those climatal lconditions for which a sub-Arctic flora — destined to become Alpine — was specially organized, the difficulty of deriving such a flora from its paarent north, and of diffusing it over the snowy hills bounding this glacial ocean, vanishes, and the presence of identical species at such distant pooints remain no longer a mystery. Moreover, when we consider that conditions during the epoch referred to, the undoubted evidences of Continental observers, on the boounds of Asia by Sir Roderick Murchison, in America by Mr. Lyell, Mr. Logan, Captain Bayfield, and others, and that the botanical (and zoological as well) region, essentially northern and Alpine, designated by Professor Schouw that 'of saxifrages and mosses,' and first in his classification, exists now only on the flanks of the great area which suffered such conditions; and that, though similar conditions reappear, the relationship of Alpine and Arctic vegetation in the southern hemisphere, with that in the northern, is entirely maintained by 'representative', and not by identical species (the general truth of my explanation of Alpine floras, including identical species, becomes so strong, that the view proposed acquires fair claims to be ranked as a theory, and not considered merely a convenient or bold hypothesis.">[ — Tr.
The p 349 elk, for instance, lives in the Scandinavian peninsula, almost ten degrees further north than in the interior of Siberia, where the line of equal winter temperature is so remarkably concave. Plants migrate in the germ; and, in the case of many species, the seeds are furnished with organs adapting them to be conveyed to a distace through the air. When once they have taken root, they become dependent on the soil and on the strata of air surrounding them. Animals, on the contrary, can at pleasure migrate from the equator toward the poles; and this they can more especially doo where the isothermal lines are much inflected, and where hot summers succeed a great degree of winter cold. The royal tiger, which in no respect differs from the Bengal species, penetrates every summer into p 350 the north of Asia as far as the latitudes of Berlin and Hamburg, a fact of which Ehrenberg and myself have spoken in other works.*