物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 12. GEOGRAPHICAL DISTRIBUTION--continued. Page 1
Distribution of fresh-water productions.On the inhabitants of oceanic islands.Absence of Batrachians and of terrestrial Mammals.On the relation of the inhabitants of islands to those of the nearestmainland.On colonisation from the nearest source with subsequent modification.Summary of the last and present chapters.
As lakes and river-systems are separated from each other by barriersof land, it might have been thought that fresh-water productions wouldnot have ranged widely within the same country, and as the sea isapparently a still more impassable barrier, that they never would haveextended to distant countries. But the case is exactly the reverse.Not only have many fresh-water species, belonging to quite differentclasses, an enormous range, but allied species prevail in a remarkablemanner throughout the world. I well remember, when first collecting inthe fresh waters of Brazil, feeling much surprise at the similarity ofthe fresh-water insects, shells, etc., and at the dissimilarity of thesurrounding terrestrial beings, compared with those of Britain.
But this power in fresh-water productions of ranging widely, though sounexpected, can, I think, in most cases be explained by their havingbecome fitted, in a manner highly useful to them, for short andfrequent migrations from pond to pond, or from stream to stream; andliability to wide dispersal would follow from this capacity as analmost necessary consequence. We can here consider only a few cases.In regard to fish, I believe that the same species never occur in thefresh waters of distant continents. But on the same continent thespecies often range widely and almost capriciously; for tworiver-systems will have some fish in common and some different. A fewfacts seem to favour the possibility of their occasional transport byaccidental means; like that of the live fish not rarely dropped bywhirlwinds in India, and the vitality of their ova when removed fromthe water. But I am inclined to attribute the dispersal of fresh-waterfish mainly to slight changes within the recent period in the level ofthe land, having caused rivers to flow into each other. Instances,also, could be given of this having occurred during floods, withoutany change of level. We have evidence in the loess of the Rhine ofconsiderable changes of level in the land within a very recentgeological period, and when the surface was peopled by existing landand fresh-water shells. The wide difference of the fish on oppositesides of continuous mountain-ranges, which from an early period musthave parted river-systems and completely prevented their inosculation,seems to lead to this same conclusion. With respect to alliedfresh-water fish occurring at very distant points of the world, nodoubt there are many cases which cannot at present be explained: butsome fresh-water fish belong to very ancient forms, and in such casesthere will have been ample time for great geographical changes, andconsequently time and means for much migration. In the second place,salt-water fish can with care be slowly accustomed to live in freshwater; and, according to Valenciennes, there is hardly a single groupof fishes confined exclusively to fresh water, so that we may imaginethat a marine member of a fresh-water group might travel far along theshores of the sea, and subsequently become modified and adapted to thefresh waters of a distant land.
Some species of fresh-water shells have a very wide range, and alliedspecies, which, on my theory, are descended from a common parent andmust have proceeded from a single source, prevail throughout theworld. Their distribution at first perplexed me much, as their ova arenot likely to be transported by birds, and they are immediately killedby sea water, as are the adults. I could not even understand how somenaturalised species have rapidly spread throughout the same country.But two facts, which I have observed--and no doubt many others remainto be observed--throw some light on this subject. When a duck suddenlyemerges from a pond covered with duck-weed, I have twice seen theselittle plants adhering to its back; and it has happened to me, inremoving a little duck-weed from one aquarium to another, that I havequite unintentionally stocked the one with fresh-water shells from theother. But another agency is perhaps more effectual: I suspended aduck's feet, which might represent those of a bird sleeping in anatural pond, in an aquarium, where many ova of fresh-water shellswere hatching; and I found that numbers of the extremely minute andjust hatched shells crawled on the feet, and clung to them so firmlythat when taken out of the water they could not be jarred off, thoughat a somewhat more advanced age they would voluntarily drop off. Thesejust hatched molluscs, though aquatic in their nature, survived on theduck's feet, in damp air, from twelve to twenty hours; and in thislength of time a duck or heron might fly at least six or seven hundredmiles, and would be sure to alight on a pool or rivulet, if blownacross sea to an oceanic island or to any other distant point. SirCharles Lyell also informs me that a Dyticus has been caught with anAncylus (a fresh-water shell like a limpet) firmly adhering to it; anda water-beetle of the same family, a Colymbetes, once flew on boardthe 'Beagle,' when forty-five miles distant from the nearest land: howmuch farther it might have flown with a favouring gale no one cantell.
With respect to plants, it has long been known what enormous rangesmany fresh-water and even marsh-species have, both over continents andto the most remote oceanic islands. This is strikingly shown, asremarked by Alph. de Candolle, in large groups of terrestrial plants,which have only a very few aquatic members; for these latter seemimmediately to acquire, as if in consequence, a very wide range. Ithink favourable means of dispersal explain this fact. I have beforementioned that earth occasionally, though rarely, adheres in somequantity to the feet and beaks of birds. Wading birds, which frequentthe muddy edges of ponds, if suddenly flushed, would be the mostlikely to have muddy feet. Birds of this order I can show are thegreatest wanderers, and are occasionally found on the most remote andbarren islands in the open ocean; they would not be likely to alighton the surface of the sea, so that the dirt would not be washed offtheir feet; when making land, they would be sure to fly to theirnatural fresh-water haunts. I do not believe that botanists are awarehow charged the mud of ponds is with seeds: I have tried severallittle experiments, but will here give only the most striking case: Itook in February three table-spoonfuls of mud from three differentpoints, beneath water, on the edge of a little pond; this mud when dryweighed only 6 3/4 ounces; I kept it covered up in my study for sixmonths, pulling up and counting each plant as it grew; the plants wereof many kinds, and were altogether 537 in number; and yet the viscidmud was all contained in a breakfast cup! Considering these facts, Ithink it would be an inexplicable circumstance if water-birds did nottransport the seeds of fresh-water plants to vast distances, and ifconsequently the range of these plants was not very great. The sameagency may have come into play with the eggs of some of the smallerfresh-water animals.
Other and unknown agencies probably have also played a part. I havestated that fresh-water fish eat some kinds of seeds, though theyreject many other kinds after having swallowed them; even small fishswallow seeds of moderate size, as of the yellow water-lily andPotamogeton. Herons and other birds, century after century, have goneon daily devouring fish; they then take flight and go to other waters,or are blown across the sea; and we have seen that seeds retain theirpower of germination, when rejected in pellets or in excrement, manyhours afterwards. When I saw the great size of the seeds of that finewater-lily, the Nelumbium, and remembered Alph. de Candolle's remarkson this plant, I thought that its distribution must remain quiteinexplicable; but Audubon states that he found the seeds of the greatsouthern water-lily (probably, according to Dr. Hooker, the Nelumbiumluteum) in a heron's stomach; although I do not know the fact, yetanalogy makes me believe that a heron flying to another pond andgetting a hearty meal of fish, would probably reject from its stomacha pellet containing the seeds of the Nelumbium undigested; or theseeds might be dropped by the bird whilst feeding its young, in thesame way as fish are known sometimes to be dropped.
deficient in certain classes, and theirplaces are apparently occupied by the other inhabitants; in theGalapagos Islands reptiles, and in New Zealand gigantic winglessbirds, take the place of mammals. In the plants of the GalapagosIslands, Dr. Hooker has.
In considering these several means of distribution, it should beremembered that when a pond or stream is first formed, for instance,on a rising islet, it will be unoccupied; and a single seed or eggwill have a good chance of succeeding. Although there will always be astruggle for life between the individuals of the species, however few,already occupying any pond, yet as the number of kinds is small,compared with those on the land, the competition will probably be lesssevere between aquatic than between terrestrial species; consequentlyan intruder from the waters of a foreign country, would have a betterchance of seizing on a place, than in the case of terrestrialcolonists. We should, also, remember that some, perhaps many,fresh-water productions are low in the scale of nature, and that wehave reason to believe that such low beings change or become modifiedless quickly than the high; and this will give longer time than theaverage for the migration of the same aquatic species. We should notforget the probability of many species having formerly ranged ascontinuously as fresh-water productions ever can range, over immenseareas, and having subsequently become extinct in intermediate regions.But the wide distribution of fresh-water plants and of the loweranimals, whether retaining the same identical form or in some degreemodified, I believe mainly depends on the wide dispersal of theirseeds and eggs by animals, more especially by fresh-water birds, whichhave large powers of flight, and naturally travel from one to anotherand often distant piece of water. Nature, like a careful gardener,thus takes her seeds from a bed of a particular nature, and drops themin another equally well fitted for them.
ON THE INHABITANTS OF OCEANIC ISLANDS.
We now come to the last of the three classes of facts, which I haveselected as presenting the greatest amount of difficulty, on the viewthat all the individuals both of the same and of allied species havedescended from a single parent; and therefore have all proceeded froma common birthplace, notwithstanding that in the course of time theyhave come to inhabit distant points of the globe. I have alreadystated that I cannot honestly admit Forbes's view on continentalextensions, which, if legitimately followed out, would lead to thebelief that within the recent period all existing islands have beennearly or quite joined to some continent. This view would remove manydifficulties, but it would not, I think, explain all the facts inregard to insular productions. In the following remarks I shall notconfine myself to the mere question of dispersal; but shall considersome other facts, which bear on the truth of the two theories ofindependent creation and of descent with modification.
The species of all kinds which inhabit oceanic islands are few innumber compared with those on equal continental areas: Alph. deCandolle admits this for plants, and Wollaston for insects. If we lookto the large size and varied stations of New Zealand, extending over780 miles of latitude, and compare its flowering plants, only 750 innumber, with those on an equal area at the Cape of Good Hope or inAustralia, we must, I think, admit that something quite independentlyof any difference in physical conditions has caused so great adifference in number. Even the uniform county of Cambridge has 847plants, and the little island of Anglesea 764, but a few ferns and afew introduced plants are included in these numbers, and thecomparison in some other respects is not quite fair. We have evidencethat the barren island of Ascension aboriginally possessed underhalf-a-dozen flowering plants; yet many have become naturalised on it,as they have on New Zealand and on every other oceanic island whichcan be named. In St. Helena there is reason to believe that thenaturalised plants and animals have nearly or quite exterminated manynative productions. He who admits the doctrine of the creation of eachseparate species, will have to admit, that a sufficient number of thebest adapted plants and animals have not been created on oceanicislands; for man has unintentionally stocked them from various sourcesfar more fully and perfectly than has nature.
Although in oceanic islands the number of kinds of inhabitants isscanty, the proportion of endemic species (i.e. those found nowhereelse in the world) is often extremely large. If we compare, forinstance, the number of the endemic land-shells in Madeira, or of theendemic birds in the Galapagos Archipelago, with the number found onany continent, and then compare the area of the islands with that ofthe continent, we shall see that this is true. This fact might havebeen expected on my theory, for, as already explained, speciesoccasionally arriving after long intervals in a new and isolateddistrict, and having to compete with new associates, will be eminentlyliable to modification, and will often produce groups of modifieddescendants. But it by no means follows, that, because in an islandnearly all the species of one class are peculiar, those of anotherclass, or of another section of the same class, are peculiar; and thisdifference seems to depend on the species which do not become modifiedhaving immigrated with facility and in a body, so that their mutualrelations have not been much disturbed. Thus in the Galapagos Islandsnearly every land-bird, but only two out of the eleven marine birds,are peculiar; and it is obvious that marine birds could arrive atthese islands more easily than land-birds. Bermuda, on the other hand,which lies at about the same distance from North America as theGalapagos Islands do from South America, and which has a very peculiarsoil, does not possess one endemic land bird; and we know from Mr. J.M. Jones's admirable account of Bermuda, that very many North Americanbirds, during their great annual migrations, visit either periodicallyor occasionally this island. Madeira does not possess one peculiarbird, and many European and African birds are almost every year blownthere, as I am informed by Mr. E. V. Harcourt. So that these twoislands of Bermuda and Madeira have been stocked by birds, which forlong ages have struggled together in their former homes, and havebecome mutually adapted to each other; and when settled in their newhomes, each kind will have been kept by the others to their properplaces and habits, and will consequently have been little liable tomodification. Madeira, again, is inhabited by a wonderful number ofpeculiar land-shells, whereas not one species of sea-shell is confinedto its shores: now, though we do not know how seashells are dispersed,yet we can see that their eggs or larvae, perhaps attached to seaweedor floating timber, or to the feet of wading-birds, might betransported far more easily than land-shells, across three or fourhundred miles of open sea. The different orders of insects in Madeiraapparently present analogous facts.
Oceanic islands are sometimes deficient in certain classes, and theirplaces are apparently occupied by the other inhabitants; in theGalapagos Islands reptiles, and in New Zealand gigantic winglessbirds, take the place of mammals. In the plants of the GalapagosIslands, Dr. Hooker has shown that the proportional numbers of thedifferent orders are very different from what they are elsewhere. Suchcases are generally accounted for by the physical conditions of theislands; but this explanation seems to me not a little doubtful.Facility of immigration, I believe, has been at least as important asthe nature of the conditions.
Many remarkable little facts could be given with respect to theinhabitants of remote islands. For instance, in certain islands nottenanted by mammals, some of the endemic plants have beautifullyhooked seeds; yet few relations are more striking than the adaptationof hooked seeds for transportal by the wool and fur of quadrupeds.This case presents no difficulty on my view, for a hooked seed mightbe transported to an island by some other means; and the plant thenbecoming slightly modified, but still retaining its hooked seeds,would form an endemic species, having as useless an appendage as anyrudimentary organ,--for instance, as the shrivelled wings under thesoldered elytra of many insular beetles. Again, islands often possesstrees or bushes belonging to orders which elsewhere include onlyherbaceous species; now trees, as Alph. de Candolle has shown,generally have, whatever the cause may be, confined ranges. Hencetrees would be little likely to reach distant oceanic islands; and anherbaceous plant, though it would have no chance of successfullycompeting in stature with a fully developed tree, when established onan island and having to compete with herbaceous plants alone, mightreadily gain an advantage by growing taller and taller and overtoppingthe other plants. If so, natural selection would often tend to add tothe stature of herbaceous plants when growing on an island, towhatever order they belonged, and thus convert them first into bushesand ultimately into trees.
With respect to the absence of whole orders on oceanic islands, BorySt. Vincent long ago remarked that Batrachians (frogs, toads, newts)have never been found on any of the many islands with which the greatoceans are studded. I have taken pains to verify this assertion, and Ihave found it strictly true. I have, however, been assured that a frogexists on the mountains of the great island of New Zealand; but Isuspect that this exception (if the information be correct) may beexplained through glacial agency. This general absence of frogs,toads, and newts on so many oceanic islands cannot be accounted for bytheir physical conditions; indeed it seems that islands are peculiarlywell fitted for these animals; for frogs have been introduced intoMadeira, the Azores, and Mauritius, and have multiplied so as tobecome a nuisance. But as these animals and their spawn are known tobe immediately killed by sea-water, on my view we can see that therewould be great difficulty in their transportal across the sea, andtherefore why they do not exist on any oceanic island. But why, on thetheory of creation, they should not have been created there, it wouldbe very difficult to explain.
Mammals offer another and similar case. I have carefully searched theoldest voyages, but have not finished my search; as yet I have notfound a single instance, free from doubt, of a terrestrial mammal(excluding domesticated animals kept by the natives) inhabiting anisland situated above 300 miles from a continent or great continentalisland; and many islands situated at a much less distance are equallybarren. The Falkland Islands, which are inhabited by a wolf-like fox,come nearest to an exception; but this group cannot be considered asoceanic, as it lies on a bank connected with the mainland; moreover,icebergs formerly brought boulders to its western shores, and they mayhave formerly transported foxes, as so frequently now happens in thearctic regions. Yet it cannot be said that small islands will notsupport small mammals, for they occur in many parts of the world onvery small islands, if close to a continent; and hardly an island canbe named on which our smaller quadrupeds have not become naturalisedand greatly multiplied. It cannot be said, on the ordinary view ofcreation, that there has not been time for the creation of mammals;many volcanic islands are sufficiently ancient, as shown by thestupendous degradation which they have suffered and by their tertiarystrata: there has also been time for the production of endemic speciesbelonging to other classes; and on continents it is thought thatmammals appear and disappear at a quicker rate than other and loweranimals. Though terrestrial mammals do not occur on oceanic islands,aerial mammals do occur on almost every island. New Zealand possessestwo bats found nowhere else in the world: Norfolk Island, the VitiArchipelago, the Bonin Islands, the Caroline and MarianneArchipelagoes, and Mauritius, all possess their peculiar bats. Why, itmay be asked, has the supposed creative force produced bats and noother mammals on remote islands? On my view this question can easilybe answered; for no terrestrial mammal can be transported across awide space of sea, but bats can fly across. Bats have been seenwandering by day far over the Atlantic Ocean; and two North Americanspecies either regularly or occasionally visit Bermuda, at thedistance of 600 miles from the mainland. I hear from Mr. Tomes, whohas specially studied this family, that many of the same species haveenormous ranges, and are found on continents and on far distantislands. Hence we have only to suppose that such wandering specieshave been modified through natural selection in their new homes inrelation to their new position, and we can understand the presence ofendemic bats on islands, with the absence of all terrestrial mammals.