Principles of Geology Part 54

Some species of the vulture tribe are said to be cosmopolites; and the common wild goose (_Anas anser_, Linn.), if we may believe some ornithologists, is a general inhabitant of the globe, being met with from Lapland to the Cape of Good Hope, frequent in Arabia, Persia, China, and j.a.pan, and in the American continent from Hudson's Bay to South Carolina.[900] An extraordinary range has also been attributed to the nightingale, which extends from western Europe to Persia, and still farther. In a work ent.i.tled Specchio Comparativo,[901] by Charles Bonaparte, many species of birds are enumerated as common to Rome and Philadelphia: the greater part of these are migratory, but some of them, such as the long-eared owl (_Strix otus_), are permanent in both countries. The correspondence of the ornithological fauna of the eastern and western hemispheres increases considerably, as might have been antic.i.p.ated, in high northern lat.i.tudes.[902]

_Their facilities of diffusion._--In parallel zones of the northern and southern hemispheres, a great general correspondence of form is observable, both in the aquatic and terrestrial birds; but there is rarely any specific ident.i.ty; and this phenomenon is truly remarkable, when we recollect the readiness with which some birds, not gifted with great powers of flight, shift their quarters to different regions, and the facility with which others, possessing great strength of wing, perform their aerial voyage. Some migrate periodically from high lat.i.tudes, to avoid the cold of winter, and the accompaniments of cold,--scarcity of insects and vegetable food; others, it is said, for some particular kinds of nutriment required for rearing their young: for this purpose they often traverse the ocean for thousands of miles, and recross it at other periods, with equal security.

Periodical migrations, no less regular, are mentioned by Humboldt, of many American water-fowl, from one part of the tropics to another, in a zone where there is the same temperature throughout the year. Immense flights of ducks leave the valley of the Orinoco, when the increasing depth of its waters and the flooding of its sh.o.r.es prevent them from catching fish, insects, and aquatic worms. They then betake themselves to the Rio Negro and Amazon, having pa.s.sed from the eighth and third degrees of north lat.i.tude to the first and fourth of south lat.i.tude, directing their course south-south-east. In September, when the Orinoco decreases and re-enters its channel, these birds return northwards.[903]

The insectivorous swallows which visit our island would perish during winter, if they did not annually repair to warmer climes. It is supposed that in these aerial excursions the average rapidity of their flight is not less than fifty miles an hour; so that, when aided by the wind, they soon reach warmer lat.i.tudes. Spallanzani calculated that the swallow can fly at the rate of ninety-two miles an hour, and conceived that the rapidity of the swift might be three times greater.[904] The rate of flight of the eider duck (_Anas mollissima_) is said to be ninety miles an hour; and Bachman says that the hawk, wild pigeon (_Columba migratoria_), and several species of wild ducks, in North America, fly at the rate of forty miles an hour, or nearly a thousand miles in twenty-four hours.[905]

When we reflect how easily different species, in a great lapse of ages, may be each overtaken by gales and hurricanes, and, abandoning themselves to the tempest, be scattered at random through various regions of the earth's surface, where the temperature of the atmosphere, the vegetation, and the animal productions, might be suited to their wants, we shall be prepared to find some species capriciously distributed, and to be sometimes unable to determine the native countries of each. Captain Smyth informs me, that, when engaged in his survey of the Mediterranean, he encountered a gale in the Gulf of Lyons, at the distance of between twenty and thirty leagues from the coast of France, which bore along many land birds of various species, some of which alighted on the ship, while others were thrown with violence against the sails. In this manner islands become tenanted by species of birds inhabiting the nearest mainland.

_Geographical Distribution and Dissemination of Reptiles._

A few facts respecting the third great cla.s.s of vertebrated animals will suffice to show that the plan of nature in regard to their location on the globe is perfectly a.n.a.logous to that already exemplified in other parts of the organic creation, and has probably been determined by similar causes.

_Habitations of reptiles._--Of the great saurians, the gavials which inhabit the Ganges differ from the cayman of America, or the crocodile of the Nile. The monitor of New Holland is specifically distinct from the Indian species; these latter, again, from the African, and all from their congeners in the new world. So in regard to snakes; we find the boa of America represented by the python, a different though nearly allied genus in India. America is the country of the rattlesnake; Africa, of the cerastes; and Asia, of the hooded snake, or cobra di capello. The amphibious genera Siren and Menopoma belong to North America, possessing both lungs and gills, and respiring at pleasure either air or water. The only a.n.a.logous animal of the old world is the _Proteus anguinus_ of the lakes of Lower Carniola, and the grotto of Adelsberg between Trieste and Vienna.[906]

There is a legend that St. Patrick expelled all reptiles from Ireland; and certain it is that none of the three species of snakes common in England, nor the toad, have been observed there by naturalists. They have our common frog, and our water-newt, and according to Ray (Quad.

264.), the green lizard (_Lacerta viridis_).

_Migrations of the larger reptiles._--The range of the large reptiles is, in general, quite as limited as that of some orders of the terrestrial mammalia. The great saurians sometimes cross a considerable tract in order to pa.s.s from one river to another; but their motions by land are generally slower than those of quadrupeds. By water, however, they may transport themselves to distant situations more easily. The larger alligator of the Ganges sometimes descends beyond the brackish water of the delta into the sea; and in such cases it might chance to be drifted away by a current, and survive till it reached a sh.o.r.e at some distance; but such casualties are probably very rare.

Turtles migrate in large droves from one part of the ocean to another during the ovipositing season; and they find their way annually to the island of Ascension, from which the nearest land is about 800 miles distant. Dr. Fleming mentions, that an individual of the hawk's bill turtle (_Chelonia imbricata_), so common in the American seas, has been taken at Papa Stour, one of the West Zetland Islands;[907] and, according to Sibbald, "the same animal came into Orkney." Another was taken, in 1774, in the Severn, according to Turton. Two instances, also, of the occurrence of the leathern tortoise (_C. coriacea_), on the coast of Cornwall, in 1756, are mentioned by Borlase. These animals of more southern seas can be considered only as stragglers, attracted to our sh.o.r.es during uncommonly warm seasons by an abundant supply of food, or carried by the Gulf stream, or driven by storms to high lat.i.tudes.

Some of the smaller reptiles lay their eggs on aquatic plants; and these must often be borne rapidly by rivers, and conveyed to distant regions in a manner similar to the dispersion of seeds before adverted to. But that the larger ophidians may be themselves transported across the seas, is evident from the following most interesting account of the arrival of one at the island of St. Vincent. It is worthy of being recorded, says Mr. Guilding, "that a n.o.ble specimen of the _Boa constrictor_ was lately conveyed to us by the currents, twisted round the trunk of a large sound cedar tree, which had probably been washed out of the bank by the floods of some great South American river, while its huge folds hung on the branches, as it waited for its prey. The monster was fortunately destroyed after killing a few sheep, and his skeleton now hangs before me in my study, putting me in mind how much reason I might have had to fear in my future rambles through the forests of St. Vincent, had this formidable reptile been a pregnant female, and escaped to a safe retreat."[908]

CHAPTER x.x.xIX.

LAWS WHICH REGULATE THE GEOGRAPHICAL DISTRIBUTION OF SPECIES--_continued_.

Geographical distribution and migration of Fish--of Testaoea--of Zoophytes--Distribution of Insects--Migratory instincts of some species--Certain types characterize particular countries--Their means of dissemination--Geographical distribution and diffusion of man--Speculations as to the birth-place of the human species--Progress of human population--Drifting of canoes to vast distances--On the involuntary influence of man in extending the range of many other species.

_Geographical Distribution and Migrations of Fish._

Although we are less acquainted with the habitations of marine animals than with the grouping of the terrestrial species before described, yet it is well ascertained that their distribution is governed by the same general laws. The testimony borne by MM. Peron and Lesueur to this important fact is remarkably strong. These eminent naturalists, after collecting and describing many thousand species of marine animals which they brought to Europe from the southern hemisphere, insist most emphatically on their distinctness from those north of the equator; and this remark they extend to animals of all cla.s.ses, from those of a more simple to those of a more complex organization--from the sponges and Medusae to the Cetacea. "Among all those which we have been able to examine," say they, "with our own eyes, or with regard to which it has appeared to us possible to p.r.o.nounce with certainty, there is not a single animal of the southern regions which is not distinguished by essential characters from the a.n.a.logous species in the northern seas."[909]

On comparing the freshwater fish of Europe and North America, Sir John Richardson remarks, that the only species which is unequivocally common to the two continents is the pike (_Esox lucius_); and it is curious that this fish is unknown to the westward of the Rocky Mountains, the very coast which approaches nearest to the old continent.[910] According to the same author the genera of freshwater fish in China agree closely with those of the peninsula of India, but the species are not the same.

"As in the distribution," he adds, "of marine fish, the interposition of a continent stretching from the tropics far into the temperate or colder parts of the ocean, separate different ichthyological groups; so with respect to the freshwater species, the intrusion of arms of the sea running far to the northwards, or the interposition of a lofty mountain-chain, effects the same thing. The freshwater fish of the Cape of Good Hope and the South American ones, are different from those of India and China, &c."[911]

Cuvier and Valenciennes, in their "Histoire des Poissons," observe, that very few species of fish cross the Atlantic. Although their statement is correct, it is found that a great many species are common to the opposite sides of the Indian Ocean, inhabiting alike the Red Sea, the eastern coast of Africa, Madagascar, the Mauritius, the Indian Ocean, the southern seas of China, the Malay archipelago, the northern coasts of Australia, and the whole of Polynesia![912] This very wide diffusion, says Sir J. Richardson, may have been promoted by chains of islands running east and west, which are wanting in the deep Atlantic. An archipelago extending far in longitude, favours the migration of fish by multiplying the places of deposit for sp.a.w.n along the sh.o.r.es of islands, and on intervening coral banks; and in such places, also, fish find their appropriate food.

The flying fish are found (some stragglers excepted) only between the tropics: in receding from the line, they never approach a higher lat.i.tude than the fortieth parallel. The course of the Gulf stream, however, and the warmth of its water, enable some tropical fish to extend their habitations far into the temperate zone; thus the chaetodons which abound in the seas of hot climates, are found among the Bermudas on the thirty-second parallel, where they are preserved in basins inclosed from the sea, as an important article of food for the garrison and inhabitants. Other fish, following the direction of the same great current, range from the coast of Brazil to the banks of Newfoundland.[913]

All are aware that there are certain fish of pa.s.sage which have their periodical migrations, like some tribes of birds. The salmon, towards the season of sp.a.w.ning, ascends the rivers for hundreds of miles, leaping up the cataracts which it meets in its course, and then retreats again into the depths of the ocean. The herring and the haddock, after frequenting certain sh.o.r.es, in vast shoals, for a series of years, desert them again, and resort to other stations, followed by the species which prey on them. Eels are said to descend into the sea for the purpose of producing their young, which are seen returning into the fresh water by myriads, extremely small in size, but possessing the power of surmounting every obstacle which occurs in the course of a river, by applying their slimy and glutinous bodies to the surface of rocks, or the gates of a lock, even when dry, and so climbing over it.[914] Before the year 1800 there were no eels in Lake Wener, the largest inland lake in Sweden, which discharges its waters by the celebrated cataracts of Trolhattan. But I am informed by Professor Nilsson, that since the ca.n.a.l was opened uniting the river Gotha with the lake by a series of nine locks, each of great height, eels have been observed in abundance in the lake. It appears, therefore, that though they were unable to ascend the falls, they have made their way by the locks, by which in a very short s.p.a.ce a difference of level of 114 feet is overcome.

Gmelin says, that the Anseres (wild geese, ducks, and others) subsist, in their migrations, on the sp.a.w.n of fish; and that oftentimes, when they void the sp.a.w.n, two or three days afterwards, the eggs retain their vitality unimpaired.[915] When there are many disconnected freshwater lakes in a mountainous region, at various elevations, each remote from the other, it has often been deemed inconceivable how they could all become stocked with fish from one common source; but it has been suggested, that the minute eggs of these animals may sometimes be entangled in the feathers of water-fowl. These, when they alight to wash and plume themselves in the water, may often unconsciously contribute to propagate swarms of fish, which, in due season, will supply them with food. Some of the water-beetles, also, as the Dyticidae, are amphibious, and in the evening quit their lakes and pools, and, flying in the air, transport the minute ova of fishes to distant waters. In this manner some naturalists account for the fry of fish appearing occasionally in small pools caused by heavy rains; but the showers of small fish, stated in so many accounts to have fallen from the atmosphere, require farther investigation.

_Geographical Distribution and Migrations of Testacea._

The Testacea, of which so great a variety of species occurs in the sea, are a cla.s.s of animals of peculiar importance to the geologist; because their remains are found in strata of all ages, and generally in a higher state of preservation than those of other organic beings. Climate has a decided influence on the geographical distribution of species in this cla.s.s; but as there is much greater uniformity of temperature in the waters of the ocean, than in the atmosphere which invests the land, the diffusion of marine mollusks is on the whole more extensive.

Some forms attain their fullest development in warm lat.i.tudes; and are often exclusively confined to the torrid zone, as _Nautilus_, _Harpa_, _Terebellum_, _Pyramidella_, _Delphinula_, _Aspergillum_, _Tridacna_, _Cucullaea_, _Cra.s.satella_, _Corbis_, _Perna_, and _Plicatula_. Other forms are limited to one region of the sea, as the _Trigonia_ to parts of Australia, and the _Concholepas_ to the western coast of South America. The marine species inhabiting the ocean on the opposite sides of the narrow isthmus of Panama, are found to differ almost entirely, as we might have antic.i.p.ated, since a West Indian mollusk cannot enter the Pacific without coasting round South America, and pa.s.sing through the inclement climate of Cape Horn. The continuity of the existing lines of continent from north to south, prevents any one species from belting the globe, or from following the direction of the isothermal lines.

Currents also flowing permanently in certain directions, and the influx at certain points of great bodies of fresh water, limit the extension of many species. Those which love deep water are arrested by shoals; others, fitted for shallow seas, cannot migrate across unfathomable abysses. The nature also of the ground has an important influence on the testaceous fauna, both on the land and beneath the waters. Certain species prefer a sandy, others a gravelly, and some a muddy sea-bottom.

On the land, limestone is of all rocks the most favourable to the number and propagation of species of the genera Helix, Clausilia, Bulimus, and others. Professor E. Forbes has shown as the result of his labours in dredging in the aegean Sea, that there are eight well-marked regions of depth, each characterized by its peculiar testaceous fauna. The first of these, called the littoral zone, extends to a depth of two fathoms only; but this narrow belt is inhabited by more than one hundred species. The second region, of which ten fathoms is the inferior limit, is almost equally populous; and a copious list of species is given as characteristic of each region down to the seventh, which lies between the depths of 80 and 105 fathoms, all the inhabited s.p.a.ce below this being included in the eighth province, where no less than 65 species of Testacea have been taken. The majority of the sh.e.l.ls in this lowest zone are white or transparent. Only two species of Mollusca are common to all the eight regions, namely, _Arca lactea_ and _Cerithium lima_.[916]

_Great range of some provinces and species._--In Europe conchologists distinguish between the arctic fauna, the southern boundary of which corresponds with the isothermal line of 32 F., and the Celtic, which, commencing with that limit as its northern frontier, extends southwards to the mouth of the English Channel and Cape Finisterre, in France. From that point begins the Lusitanian fauna, which, according to the recent observations of Mr. M'Andrew (1852), ranges to the Canary Islands. The Mediterranean province is distinct from all those above enumerated, although it has some species in common with each.

The Indo-Pacific region is by far the most extensive of all. It reaches from the Red Sea and the eastern coast of Africa, to the Indian Archipelago, and adjoining parts of the Pacific Ocean. To the geologist it furnishes a fact of no small interest, by teaching us that one group of living species of mollusca may prevail throughout an area exceeding in magnitude the utmost limits we can as yet a.s.sign to any a.s.semblage of contemporaneous fossil species. Mr. c.u.ming obtained more than a hundred species of sh.e.l.ls from the eastern coast of Africa identical with those collected by himself at the Philippines and in the eastern coral islands of the Pacific Ocean, a distance equal to that from pole to pole.[917]

Certain species of the genus _Ianthina_ have a very wide range, being common to seas north and south of the equator. They are all provided with a beautifully contrived float, which renders them buoyant, facilitating their dispersion, and enabling them to become active agents in disseminating other species. Captain King took a specimen of _Ianthina fragilis_, alive, a little north of the equator, so loaded with barnacles (_Pentelasmis_) and their ova that the upper part of its sh.e.l.l was invisible. The "Rock Whelk" (_Purpura lapillus_), a well-known British univalve, inhabits both the North Atlantic and North Pacific.

_Helix putris_ (_Succinea putris_, Lam.), so common in Europe, where it reaches from Norway to Italy, is also said to occur in the United States and in Newfoundland. As this animal inhabits constantly the borders of pools and streams where there is much moisture, it is not impossible that different water-fowl have been the agents of spreading some of its minute eggs, which may have been entangled in their feathers. The freshwater snail, _Lymneus pal.u.s.tris_, so abundant in English ponds, ranges uninterruptedly from Europe to Cashmere, and thence to the eastern parts of Asia. _Helix aspersa_, one of the commonest of our larger land-sh.e.l.ls, is found in St. Helena and other distant countries.

Some conchologists have conjectured that it was accidentally imported into St. Helena in some ship; for it is an eatable species, and these animals are capable of retaining life during long voyages, without air or nourishment.[918]

Perhaps no species has a better claim to be called cosmopolite than one of our British bivalves, _Saxicava rugosa_. It is spread over all the north-polar seas, and ranges in one direction through Europe to Senegal, occurring on both sides of the Atlantic; while in another it finds its way into the North Pacific, and thence to the Indian Ocean. Nor do its migrations cease till it reaches the Australian seas.

A British brachiopod, named _Terebratula caput-serpentis_, is common, according to Professor E. Forbes, to both sides of the North Atlantic, and to the South African and Chinese seas.

_Confined range of other species._--Mr. Lowe, in a memoir published in the Cambridge Transactions in 1834, enumerates seventy-one species of land Mollusca, collected by him in the islands of Madeira and Porto Santo, sixty of which belonged to the genus Helix alone, including as sub-genera Bulimus and Achatina, and excluding Vitrina and Clausilia; forty-four of these are new. It is remarkable that very few of the above-mentioned species are common to the neighbouring archipelago of the Canaries; but it is a still more striking fact, that of the sixty species of the three genera above mentioned, thirty-one are natives of Porto Santo; whereas, in Madeira, which contains ten times the superficies, were found but twenty-nine. Of these only four were common to the two islands, which are separated by a distance of only twelve leagues; and two even of these four (namely _Helix rhodostoma_ and _H.

ventrosa_) are species of general diffusion, common to Madeira, the Canaries, and the south of Europe.[919]

The confined range of these mollusks may easily be explained, if we admit that species have only one birth-place; and the only problem to be solved would relate to the exceptions--to account for the dissemination of some species throughout several islands, and the European continent.

May not the eggs, when washed into the sea by the undermining of cliffs, or blown by a storm from the land, float uninjured to a distant sh.o.r.e?

_Their mode of diffusion._--Notwithstanding the proverbially slow motion of snails and mollusks in general, and although many aquatic species adhere constantly to the same rock for their whole lives, they are by no means dest.i.tute of provision for disseminating themselves rapidly over a wide area. "Some Mollusca," says Professor E. Forbes, "migrate in their larva state, for all of them undergo a metamorphosis either in the egg or out of the egg. The gasteropoda commence life under the form of a small spiral sh.e.l.l, and an animal furnished with ciliated wings, or lobes, like a pteropod, by means of which it can swim freely, and in this form can migrate with ease through the sea."[920]

We are accustomed to a.s.sociate in our minds the idea of the greatest locomotive powers with the most mature and perfect state of each species of invertebrate animal, especially when they undergo a series of transformations; but in all the Mollusca the reverse is true. The young fry of the c.o.c.kle, for example (_Cardium_), possess, when young or in the larva state, an apparatus which enables them both to swim and to be carried along easily by a marine current. (See fig. 99.)

[Ill.u.s.tration: Fig. 99.

Tne young fry of a c.o.c.kle (Cardium pygmaeum,) from Loven's Kongl.

Vetenskaps. Akadem. Handling, 1848.

A, The young just hatched, magnified 100 diameters.

B, the same farther advanced.

_a_, The ciliated organ of locomotion with its filamentous appendage _b_.

_c_, The rudimentary intestine.

_d_, The rudimentary sh.e.l.l.