590 CHAPTER XXXVIII. LAWS WHICH REGULATE THE GEOGRAPHICAL DISTRIBUTION OF SPECIES. Analogy of climate not attended with identity of species-Botanical geography – Stations Habitations - Distinct provinces of indigenous plants Vegetation of islands - Marine vegetation - In what manner plants become diffused -Effects of wind, rivers, marine currents - Agency of animals- Many seeds pass through the stomachs of animals and birds undigested - Agency of man in the dispersion of plants, both voluntary and involuntary - Its analogy to that of the inferior animals. NEXT to determining the question whether species have a real existence, the consideration of the laws which regulate their geographical distribution is a subject of primary importance to the geologist. It is only by studying these laws with attention, by observing the positions which groups of species occupy at present, and inquiring how these may be varied in the course of time by migrations, by changes in physical geography, and other causes, that we can hope to learn whether the duration of species be limited, or in what manner the state of the animate world is affected by the endless vicissitudes of the inanimate. Different regions inhabited by distinct species.-That different regions of the globe are inhabited by entirely distinct animals and plants, is a fact which has been familiar to all naturalists since Buffon first pointed out the want of specific identity between the land quadrupeds of America and those of the Old World. The same phenomenon has, in later times, been forced in a striking manner upon our attention, by the examination of New Holland, where the indigenous species of animals and plants were found to be, almost without exception, distinct from those known in other parts of the world. But the extent of this parcelling out of the globe amongst different nations, as they have been termed, of plants and animals-the universality of a phenomenon so extraordinary and unexpected, may be considered as one of the most interesting facts clearly established by the advance of modern science. Scarcely fourteen hundred species of plants appear to have been known and described by the Greeks, Romans, and Arabians. At present, more than three thousand species are enumerated, as natives of our own island.* In other parts of the world there have been now collected (1846), upwards of 100,000 species, specimens of which are preserved in European herbariums. It was not to be supposed, therefore, that the ancients should have acquired any correct notions respecting what may be called the geography of plants, although the * Barton's Lectures on the Geography of Plants, p. 2. 1827. influence of climate on the character of the vegetation could hardly have escaped their observation. Antecedently to investigation, there was no reason for presuming that the vegetable productions, growing wild in the eastern hemisphere, should be unlike those of the western, in the same latitude; nor that the plants of the Cape of Good Hope should be unlike those of the south of Europe; situations where the climate is little dissimilar. The contrary supposition would have seemed more probable, and we might have anticipated an almost perfect identity in the animals and plants which inhabit corresponding parallels of latitude. The discovery, therefore, that each separate region of the globe, both of the land and water, is occupied by distinct groups of species, and that most of the exceptions to this general rule may be referred to disseminating causes now in operation, is eminently calculated to excite curiosity, and to stimulate us to seek some hypothesis respecting the first introduction of species which may be reconcileable with such phenomena. Botanical geography. A comparison of the plants of different regions of the globe affords results more to be depended upon in the present state of our knowledge than those relating to the animal kingdom, because the science of botany is more advanced, and probably comprehends a great proportion of the total number of the vegetable productions of the whole earth, Humboldt, in several eloquent passages of his Personal Narrative, was among the first to promulgate philosophical views on this subject. Every hemisphere, says this traveller, produces plants of different species; and it is not by the diversity of climates that we can attempt to explain why equinoctial Africa has no Laurineæ, and the New World no Heaths; why the Calceolaria are found only in the southern hemisphere; why the birds of the continent of India glow with colours less splendid than the birds of the hot parts of America: finally, why the tiger is peculiar to Asia, and the ornithorhynchus to New Holland.* "We can conceive," he adds, "that a small number of the families of plants, for instance, the Musacea and the Palms, cannot belong to very cold regions, on account of their internal structure and the importance of certain organs; but we cannot explain why no one of the family of Melastomas vegetates north of the parallel of thirty degrees; or why no rose-tree belongs to the southern hemisphere. Analogy of climates is often found in the two continents without identity of productions."† The luminous essay of De Candolle on "Botanical Geography" presents us with the fruits of his own researches and those of Humboldt, Brown, and other eminent botanists, so arranged, that the principal phenomena of the distribution of plants are exhibited in connexion with the causes to which they are chiefly referrible.‡ "It might not, perhaps, be difficult," observes this writer, "to find *Pers. Nar., vol. v. p. 180. Botanique. Extrait du 18me vol. du † Ibid. Dict. des Sci. Nat. Essai Elémentaire de Géographie 592 STATIONS AND HABITATIONS OF PLANTS. [CH. XXXVIII. two points, in the United States and in Europe, or in equinoctial America and Africa, which present all the same circumstances: as, for example, the same temperature, the same height above the sea, a similar soil, an equal dose of humidity; yet nearly all, perhaps all, the plants in these two similar localities shall be distinct. A certain degree of analogy, indeed, of aspect, and even of structure, might very possibly be discoverable between the plants of the two localities in question; but the species would in general be different. Circumstances, therefore, different from those which now determine the stations, have had an influence on the habitations of plants." Stations and habitations of plants. As I shall frequently have occasion to speak of the stations and habitations of plants in the technical sense in which the terms are used in the above passage, I may remind the geologist that station indicates the peculiar nature of the locality where each species is accustomed to grow, and has reference to climate, soil, humidity, light, elevation above the sea, and other analogous circumstances; whereas, by habitation is meant a general indication of the country where a plant grows wild. Thus the station of a plant may be a salt-marsh, a hill-side, the bed of the sea, or a stagnant pool. Its habitation may be Europe, North America, or New Holland, between the tropics. The study of stations has been styled the topography, that of habitations the geography, of botany. The terms thus defined, express each a distinct class of ideas, which have been often confounded together, and which are equally applicable in zoology. In farther illustration of the principle above alluded to, that difference of longitude, independently of any influence of temperature, is accompanied by a great, and sometimes a complete, diversity in the species of plants, De Candolle observes, that, out of 2891 species of phænogamous plants described by Pursh, in the United States, there are only 385 which are found in northern or temperate Europe. MM. Humboldt and Bonpland, in all their travels through equinoctial America, found only twenty-four species (these being all Cyperaceae and Graminea) common to America and any part of the Old World. They collected, it is true, chiefly on the mountains, or the proportion would have been larger; for Dr. J. Hooker informs me that many tropical plants of the New World are identical with African species. Nevertheless, the general discordance of these Floras is very striking. On comparing New Holland with Europe, Mr. Brown ascertained that, out of 4100 species, discovered in Australia, there were only 166 common to Europe, and of this small number there were some few which may have been transported thither by man. Almost all of the 166 species were cryptogamic, and the rest consist, in nearly every case, of phænogamous plants which also inhabit intervening regions. But what is still more remarkable, in the more widely separated parts of the ancient continent, notwithstanding the existence of an uninterrupted land-communication, the diversity in the specific character of the respective vegetations is almost as striking. Thus there is found one assemblage of species in China, another in the countries bordering the Black Sea and the Caspian, a third in those surrounding the Mediterranean, a fourth in the great platforms of Siberia and Tartary, and so forth. The distinctness of the groups of indigenous plants, in the same parallel of latitude, is greatest where continents are disjoined by a wide expanse of ocean. In the northern hemisphere, near the pole, where the extremities of Europe, Asia, and America unite or approach near to one another, a considerable number of the same species of plants are found, common to the three continents. But it has been remarked, that these plants, which are thus so widely diffused in the arctic regions, are also found in the chain of the Aleutian islands, which stretch almost across from America to Asia, and which may probably have served as the channel of communication for the partial blending of the Floras of the adjoining regions. It has, indeed, been observed to be a general rule, that plants found at two points very remote from each other occur also in places intermediate. Dr. J. Hooker informs me that in high latitudes in the southern ocean, in spite of the great extent of the sea, Floras of widely disconnected islands contain many species in common. Perhaps icebergs, transporting to vast distances not only stones, but soil with the seeds of plants, may explain this unusually wide diffusion of insular plants. In islands very distant from continents the total number of plants is comparatively small; but a large proportion of the species are such as occur nowhere else. In so far as the Flora of such islands is not peculiar to them, it contains, in general, species common to the nearest main lands.* The islands of the great southern ocean exemplify these rules; the easternmost containing more American, and the western more Indian plants.† Madeira and Teneriffe contain many species, and even entire genera, peculiar to them; but they have also plants in common with Portugal, Spain, the Azores, and the north-west coast of Africa.‡ In the Canaries, out of 533 species of phænogamous plants, it is said that 310 are peculiar to these islands, and the rest identical with those of the African continent; but in the Flora of St. Helena, which is so far distant even from the western shores of Africa, there have been found, out of thirty native species of the phænogamous class, only one or two which are to be found in any other part of the globe. On the other hand, of sixty cryptogamic plants, collected by Dr. J. Hooker in the same island, twelve only were peculiar. The natural history of the Galapagos archipelago, described by Mr. Darwin, affords another very instructive illustration of the laws governing the geographical distribution of plants and animals in islands. This group consists of ten principal islands, situated in the Pacific Ocean, under the equator, about 600 miles westward of the coast of South America. As they are all formed of volcanic rocks, many of the craters, of which there are about 2000 in number, having a very fresh aspect, we may regard the whole as much more modern in origin than the mass of the adjoining continent; yet neither has the Flora or Fauna been derived from South America, but consists of species, for the most part, indigenous, yet stamped with a character decidedly South American. What is still more singular, there is a difference between the species inhabiting the different islands. Of flowering plants, for example, there are 185 species at present known, and forty cryptogamic, making together 225. One hundred of the former class are new species, probably confined to this archipelago; and of the rest, ten at least have been introduced by man. Of twenty-one species of Composite, all but one are peculiar, and they belong to twelve genera, no less than ten of which genera are confined to the Galapagos. Dr. Hooker observes, that the type of this Flora has an undoubted relation to that of the western side of South America, and he detects in it no affinity with that of the numerous islands scattered over other parts of the Pacific. So in regard to the birds, reptiles, land-shells, and insects, this archipelago, standing as it does in the Pacific Ocean, is zoologically part of America. Although each small island is not more than fifty or sixty miles apart, and most of them are in sight of each other, formed of precisely the same rocks, rising nearly to an equal height, and placed under a similar climate, they are tenanted each by a different set of beings, the tortoises, mockingthrushes, finches, beetles, scarcely any of them ever ranging over the whole, and often not even common to any two of the islands. "The archipelago," says Mr. Darwin, "is a little world within itself, or rather a satellite attached to America ; whence it has derived a few stray colonists, and has received the general character of its indigenous productions. One is astonished," he adds "at the amount of creative force displayed on so many small, barren, and rocky islands, and still more so, at its diverse, yet analogous action on points so near each other. I have said that the Galapagos archipelago might be called a satellite attached to America, but it should rather be called a group of satellites physically similar, organically distinct, yet intimately related to each other, and all related in a marked, though much lesser degree, to the great American continent."* Number of botanical provinces.-De Candolle has enumerated twenty great botanical provinces inhabited by indigenous or aboriginal plants; and although many of these contain a variety of species which are common to several others, and sometimes to places very remote, yet the lines of demarcation are, upon the whole, astonishingly well defined.† Nor is it likely that the bearing of the evidence Voyage of the Beagle, 2d edition, 1845, p. 377. See a farther subdivision, by which twenty-seven provinces are made, by M. Alph. De Candolle, son of De Candolle. Monogr. des Campanulées. Paris, 1830. |