物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 12. GEOGRAPHICAL DISTRIBUTION--continued. Page 2
Besides the absence of terrestrial mammals in relation to theremoteness of islands from continents, there is also a relation, to acertain extent independent of distance, between the depth of the seaseparating an island from the neighbouring mainland, and the presencein both of the same mammiferous species or of allied species in a moreor less modified condition. Mr. Windsor Earl has made some strikingobservations on this head in regard to the great Malay Archipelago,which is traversed near Celebes by a space of deep ocean; and thisspace separates two widely distinct mammalian faunas. On either sidethe islands are situated on moderately deep submarine banks, and theyare inhabited by closely allied or identical quadrupeds. No doubt somefew anomalies occur in this great archipelago, and there is muchdifficulty in forming a judgment in some cases owing to the probablenaturalisation of certain mammals through man's agency; but we shallsoon have much light thrown on the natural history of this archipelagoby the admirable zeal and researches of Mr. Wallace. I have not as yethad time to follow up this subject in all other quarters of the world;but as far as I have gone, the relation generally holds good. We seeBritain separated by a shallow channel from Europe, and the mammalsare the same on both sides; we meet with analogous facts on manyislands separated by similar channels from Australia. The West IndianIslands stand on a deeply submerged bank, nearly 1000 fathoms indepth, and here we find American forms, but the species and even thegenera are distinct. As the amount of modification in all casesdepends to a certain degree on the lapse of time, and as duringchanges of level it is obvious that islands separated by shallowchannels are more likely to have been continuously united within arecent period to the mainland than islands separated by deeperchannels, we can understand the frequent relation between the depth ofthe sea and the degree of affinity of the mammalian inhabitants ofislands with those of a neighbouring continent,--an inexplicablerelation on the view of independent acts of creation.
All the foregoing remarks on the inhabitants of oceanicislands,--namely, the scarcity of kinds--the richness in endemic formsin particular classes or sections of classes,--the absence of wholegroups, as of batrachians, and of terrestrial mammals notwithstandingthe presence of aerial bats,--the singular proportions of certainorders of plants,--herbaceous forms having been developed into trees,etc.,--seem to me to accord better with the view of occasional meansof transport having been largely efficient in the long course of time,than with the view of all our oceanic islands having been formerlyconnected by continuous land with the nearest continent; for on thislatter view the migration would probably have been more complete; andif modification be admitted, all the forms of life would have beenmore equally modified, in accordance with the paramount importance ofthe relation of organism to organism.
I do not deny that there are many and grave difficulties inunderstanding how several of the inhabitants of the more remoteislands, whether still retaining the same specific form or modifiedsince their arrival, could have reached their present homes. But theprobability of many islands having existed as halting-places, of whichnot a wreck now remains, must not be overlooked. I will here give asingle instance of one of the cases of difficulty. Almost all oceanicislands, even the most isolated and smallest, are inhabited byland-shells, generally by endemic species, but sometimes by speciesfound elsewhere. Dr. Aug. A. Gould has given several interesting casesin regard to the land-shells of the islands of the Pacific. Now it isnotorious that land-shells are very easily killed by salt; their eggs,at least such as I have tried, sink in sea-water and are killed by it.Yet there must be, on my view, some unknown, but highly efficientmeans for their transportal. Would the just-hatched young occasionallycrawl on and adhere to the feet of birds roosting on the ground, andthus get transported? It occurred to me that land-shells, whenhybernating and having a membranous diaphragm over the mouth of theshell, might be floated in chinks of drifted timber across moderatelywide arms of the sea. And I found that several species did in thisstate withstand uninjured an immersion in sea-water during seven days:one of these shells was the Helix pomatia, and after it had againhybernated I put it in sea-water for twenty days, and it perfectlyrecovered. As this species has a thick calcareous operculum, I removedit, and when it had formed a new membranous one, I immersed it forfourteen days in sea-water, and it recovered and crawled away: butmore experiments are wanted on this head. The most striking andimportant fact for us in regard to the inhabitants of islands, istheir affinity to those of the nearest mainland, without beingactually the same species. Numerous instances could be given of thisfact. I will give only one, that of the Galapagos Archipelago,situated under the equator, between 500 and 600 miles from the shoresof South America. Here almost every product of the land and waterbears the unmistakeable stamp of the American continent. There aretwenty-six land birds, and twenty-five of these are ranked by Mr.Gould as distinct species, supposed to have been created here; yet theclose affinity of most of these birds to American species in everycharacter, in their habits, gestures, and tones of voice, wasmanifest. So it is with the other animals, and with nearly all theplants, as shown by Dr. Hooker in his admirable memoir on the Flora ofthis archipelago. The naturalist, looking at the inhabitants of thesevolcanic islands in the Pacific, distant several hundred miles fromthe continent, yet feels that he is standing on American land. Whyshould this be so? why should the species which are supposed to havebeen created in the Galapagos Archipelago, and nowhere else, bear soplain a stamp of affinity to those created in America? There isnothing in the conditions of life, in the geological nature of theislands, in their height or climate, or in the proportions in whichthe several classes are associated together, which resembles closelythe conditions of the South American coast: in fact there is aconsiderable dissimilarity in all these respects. On the other hand,there is a considerable degree of resemblance in the volcanic natureof the soil, in climate, height, and size of the islands, between theGalapagos and Cape de Verde Archipelagos: but what an entire andabsolute difference in their inhabitants! The inhabitants of the Capede Verde Islands are related to those of Africa, like those of theGalapagos to America. I believe this grand fact can receive no sort ofexplanation on the ordinary view of independent creation; whereas onthe view here maintained, it is obvious that the Galapagos Islandswould be likely to receive colonists, whether by occasional means oftransport or by formerly continuous land, from America; and the Capede Verde Islands from Africa; and that such colonists would be liableto modification;--the principle of inheritance still betraying theiroriginal birthplace.
Many analogous facts could be given: indeed it is an almost universalrule that the endemic productions of islands are related to those ofthe nearest continent, or of other near islands. The exceptions arefew, and most of them can be explained. Thus the plants of KerguelenLand, though standing nearer to Africa than to America, are related,and that very closely, as we know from Dr. Hooker's account, to thoseof America: but on the view that this island has been mainly stockedby seeds brought with earth and stones on icebergs, drifted by theprevailing currents, this anomaly disappears. New Zealand in itsendemic plants is much more closely related to Australia, the nearestmainland, than to any other region: and this is what might have beenexpected; but it is also plainly related to South America, which,although the next nearest continent, is so enormously remote, that thefact becomes an anomaly. But this difficulty almost disappears on theview that both New Zealand, South America, and other southern landswere long ago partially stocked from a nearly intermediate thoughdistant point, namely from the antarctic islands, when they wereclothed with vegetation, before the commencement of the Glacialperiod. The affinity, which, though feeble, I am assured by Dr. Hookeris real, between the flora of the south-western corner of Australiaand of the Cape of Good Hope, is a far more remarkable case, and is atpresent inexplicable: but this affinity is confined to the plants, andwill, I do not doubt, be some day explained.
The law which causes the inhabitants of an archipelago, thoughspecifically distinct, to be closely allied to those of the nearestcontinent, we sometimes see displayed on a small scale, yet in a mostinteresting manner, within the limits of the same archipelago. Thusthe several islands of the Galapagos Archipelago are tenanted, as Ihave elsewhere shown, in a quite marvellous manner, by very closelyrelated species; so that the inhabitants of each separate island,though mostly distinct, are related in an incomparably closer degreeto each other than to the inhabitants of any other part of the world.And this is just what might have been expected on my view, for theislands are situated so near each other that they would almostcertainly receive immigrants from the same original source, or fromeach other. But this dissimilarity between the endemic inhabitants ofthe islands may be used as an argument against my views; for it may beasked, how has it happened in the several islands situated withinsight of each other, having the same geological nature, the sameheight, climate, etc., that many of the immigrants should have beendifferently modified, though only in a small degree. This longappeared to me a great difficulty: but it arises in chief part fromthe deeply-seated error of considering the physical conditions of acountry as the most important for its inhabitants; whereas it cannot,I think, be disputed that the nature of the other inhabitants, withwhich each has to compete, is at least as important, and generally afar more important element of success. Now if we look to thoseinhabitants of the Galapagos Archipelago which are found in otherparts of the world (laying on one side for the moment the endemicspecies, which cannot be here fairly included, as we are consideringhow they have come to be modified since their arrival), we find aconsiderable amount of difference in the several islands. Thisdifference might indeed have been expected on the view of the islandshaving been stocked by occasional means of transport--a seed, forinstance, of one plant having been brought to one island, and that ofanother plant to another island. Hence when in former times animmigrant settled on any one or more of the islands, or when itsubsequently spread from one island to another, it would undoubtedlybe exposed to different conditions of life in the different islands,for it would have to compete with different sets of organisms: aplant, for instance, would find the best-fitted ground more perfectlyoccupied by distinct plants in one island than in another, and itwould be exposed to the attacks of somewhat different enemies. If thenit varied, natural selection would probably favour different varietiesin the different islands. Some species, however, might spread and yetretain the same character throughout the group, just as we see oncontinents some species spreading widely and remaining the same.
The really surprising fact in this case of the Galapagos Archipelago,and in a lesser degree in some analogous instances, is that the newspecies formed in the separate islands have not quickly spread to theother islands. But the islands, though in sight of each other, areseparated by deep arms of the sea, in most cases wider than theBritish Channel, and there is no reason to suppose that they have atany former period been continuously united. The currents of the seaare rapid and sweep across the archipelago, and gales of wind areextraordinarily rare; so that the islands are far more effectuallyseparated from each other than they appear to be on a map.Nevertheless a good many species, both those found in other parts ofthe world and those confined to the archipelago, are common to theseveral islands, and we may infer from certain facts that these haveprobably spread from some one island to the others. But we often take,I think, an erroneous view of the probability of closely alliedspecies invading each other's territory, when put into freeintercommunication. Undoubtedly if one species has any advantagewhatever over another, it will in a very brief time wholly or in partsupplant it; but if both are equally well fitted for their own placesin nature, both probably will hold their own places and keep separatefor almost any length of time. Being familiar with the fact that manyspecies, naturalised through man's agency, have spread withastonishing rapidity over new countries, we are apt to infer that mostspecies would thus spread; but we should remember that the forms whichbecome naturalised in new countries are not generally closely alliedto the aboriginal inhabitants, but are very distinct species,belonging in a large proportion of cases, as shown by Alph. deCandolle, to distinct genera. In the Galapagos Archipelago, many evenof the birds, though so well adapted for flying from island to island,are distinct on each; thus there are three closely-allied species ofmocking-thrush, each confined to its own island. Now let us supposethe mocking-thrush of Chatham Island to be blown to Charles Island,which has its own mocking-thrush: why should it succeed inestablishing itself there? We may safely infer that Charles Island iswell stocked with its own species, for annually more eggs are laidthere than can possibly be reared; and we may infer that themocking-thrush peculiar to Charles Island is at least as well fittedfor its home as is the species peculiar to Chatham Island. Sir C.Lyell and Mr. Wollaston have communicated to me a remarkable factbearing on this subject; namely, that Madeira and the adjoining isletof Porto Santo possess many distinct but representative land-shells,some of which live in crevices of stone; and although large quantitiesof stone are annually transported from Porto Santo to Madeira, yetthis latter island has not become colonised by the Porto Santospecies: nevertheless both islands have been colonised by someEuropean land-shells, which no doubt had some advantage over theindigenous species. From these considerations I think we need notgreatly marvel at the endemic and representative species, whichinhabit the several islands of the Galapagos Archipelago, not havinguniversally spread from island to island. In many other instances, asin the several districts of the same continent, pre-occupation hasprobably played an important part in checking the commingling ofspecies under the same conditions of life. Thus, the south-east andsouth-west corners of Australia have nearly the same physicalconditions, and are united by continuous land, yet they are inhabitedby a vast number of distinct mammals, birds, and plants.
The principle which determines the general character of the fauna andflora of oceanic islands, namely, that the inhabitants, when notidentically the same, yet are plainly related to the inhabitants ofthat region whence colonists could most readily have beenderived,--the colonists having been subsequently modified and betterfitted to their new homes,--is of the widest application throughoutnature. We see this on every mountain, in every lake and marsh. ForAlpine species, excepting in so far as the same forms, chiefly ofplants, have spread widely throughout the world during the recentGlacial epoch, are related to those of the surrounding lowlands;--thuswe have in South America, Alpine humming-birds, Alpine rodents, Alpineplants, etc., all of strictly American forms, and it is obvious that amountain, as it became slowly upheaved, would naturally be colonisedfrom the surrounding lowlands. So it is with the inhabitants of lakesand marshes, excepting in so far as great facility of transport hasgiven the same general forms to the whole world. We see this sameprinciple in the blind animals inhabiting the caves of America and ofEurope. Other analogous facts could be given. And it will, I believe,be universally found to be true, that wherever in two regions, letthem be ever so distant, many closely allied or representative speciesoccur, there will likewise be found some identical species, showing,in accordance with the foregoing view, that at some former periodthere has been intercommunication or migration between the tworegions. And wherever many closely-allied species occur, there will befound many forms which some naturalists rank as distinct species, andsome as varieties; these doubtful forms showing us the steps in theprocess of modification.
This relation between the power and extent of migration of a species,either at the present time or at some former period under differentphysical conditions, and the existence at remote points of the worldof other species allied to it, is shown in another and more generalway. Mr. Gould remarked to me long ago, that in those genera of birdswhich range over the world, many of the species have very wide ranges.I can hardly doubt that this rule is generally true, though it wouldbe difficult to prove it. Amongst mammals, we see it strikinglydisplayed in Bats, and in a lesser degree in the Felidae and Canidae.We see it, if we compare the distribution of butterflies and beetles.So it is with most fresh-water productions, in which so many generarange over the world, and many individual species have enormousranges. It is not meant that in world-ranging genera all the specieshave a wide range, or even that they have on an AVERAGE a wide range;but only that some of the species range very widely; for the facilitywith which widely-ranging species vary and give rise to new forms willlargely determine their average range. For instance, two varieties ofthe same species inhabit America and Europe, and the species thus hasan immense range; but, if the variation had been a little greater, thetwo varieties would have been ranked as distinct species, and thecommon range would have been greatly reduced. Still less is it meant,that a species which apparently has the capacity of crossing barriersand ranging widely, as in the case of certain powerfully-winged birds,will necessarily range widely; for we should never forget that torange widely implies not only the power of crossing barriers, but themore important power of being victorious in distant lands in thestruggle for life with foreign associates. But on the view of all thespecies of a genus having descended from a single parent, though nowdistributed to the most remote points of the world, we ought to find,and I believe as a general rule we do find, that some at least of thespecies range very widely; for it is necessary that the unmodifiedparent should range widely, undergoing modification during itsdiffusion, and should place itself under diverse conditions favourablefor the conversion of its offspring, firstly into new varieties andultimately into new species.