物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 11. GEOGRAPHICAL DISTRIBUTION. Page 1
Present distribution cannot be accounted for by differences inphysical conditions.Importance of barriers.Affinity of the productions of the same continent.Centres of creation.Means of dispersal, by changes of climate and of the level of theland, and by occasional means.Dispersal during the Glacial period co-extensive with the world.
In considering the distribution of organic beings over the face of theglobe, the first great fact which strikes us is, that neither thesimilarity nor the dissimilarity of the inhabitants of various regionscan be accounted for by their climatal and other physical conditions.Of late, almost every author who has studied the subject has come tothis conclusion. The case of America alone would almost suffice toprove its truth: for if we exclude the northern parts where thecircumpolar land is almost continuous, all authors agree that one ofthe most fundamental divisions in geographical distribution is thatbetween the New and Old Worlds; yet if we travel over the vastAmerican continent, from the central parts of the United States to itsextreme southern point, we meet with the most diversified conditions;the most humid districts, arid deserts, lofty mountains, grassyplains, forests, marshes, lakes, and great rivers, under almost everytemperature. There is hardly a climate or condition in the Old Worldwhich cannot be paralleled in the New--at least as closely as the samespecies generally require; for it is a most rare case to find a groupof organisms confined to any small spot, having conditions peculiar inonly a slight degree; for instance, small areas in the Old World couldbe pointed out hotter than any in the New World, yet these are notinhabited by a peculiar fauna or flora. Notwithstanding thisparallelism in the conditions of the Old and New Worlds, how widelydifferent are their living productions!
In the southern hemisphere, if we compare large tracts of land inAustralia, South Africa, and western South America, between latitudes25 deg and 35 deg, we shall find parts extremely similar in all theirconditions, yet it would not be possible to point out three faunas andfloras more utterly dissimilar. Or again we may compare theproductions of South America south of lat. 35 deg with those north of25 deg, which consequently inhabit a considerably different climate,and they will be found incomparably more closely related to eachother, than they are to the productions of Australia or Africa undernearly the same climate. Analogous facts could be given with respectto the inhabitants of the sea.
A second great fact which strikes us in our general review is, thatbarriers of any kind, or obstacles to free migration, are related in aclose and important manner to the differences between the productionsof various regions. We see this in the great difference of nearly allthe terrestrial productions of the New and Old Worlds, excepting inthe northern parts, where the land almost joins, and where, under aslightly different climate, there might have been free migration forthe northern temperate forms, as there now is for the strictly arcticproductions. We see the same fact in the great difference between theinhabitants of Australia, Africa, and South America under the samelatitude: for these countries are almost as much isolated from eachother as is possible. On each continent, also, we see the same fact;for on the opposite sides of lofty and continuous mountain-ranges, andof great deserts, and sometimes even of large rivers, we finddifferent productions; though as mountain chains, deserts, etc., arenot as impassable, or likely to have endured so long as the oceansseparating continents, the differences are very inferior in degree tothose characteristic of distinct continents.
Turning to the sea, we find the same law. No two marine faunas aremore distinct, with hardly a fish, shell, or crab in common, thanthose of the eastern and western shores of South and Central America;yet these great faunas are separated only by the narrow, butimpassable, isthmus of Panama. Westward of the shores of America, awide space of open ocean extends, with not an island as ahalting-place for emigrants; here we have a barrier of another kind,and as soon as this is passed we meet in the eastern islands of thePacific, with another and totally distinct fauna. So that here threemarine faunas range far northward and southward, in parallel lines notfar from each other, under corresponding climates; but from beingseparated from each other by impassable barriers, either of land oropen sea, they are wholly distinct. On the other hand, proceedingstill further westward from the eastern islands of the tropical partsof the Pacific, we encounter no impassable barriers, and we haveinnumerable islands as halting-places, until after travelling over ahemisphere we come to the shores of Africa; and over this vast spacewe meet with no well-defined and distinct marine faunas. Althoughhardly one shell, crab or fish is common to the above-named threeapproximate faunas of Eastern and Western America and the easternPacific islands, yet many fish range from the Pacific into the IndianOcean, and many shells are common to the eastern islands of thePacific and the eastern shores of Africa, on almost exactly oppositemeridians of longitude.
A third great fact, partly included in the foregoing statements, isthe affinity of the productions of the same continent or sea, thoughthe species themselves are distinct at different points and stations.It is a law of the widest generality, and every continent offersinnumerable instances. Nevertheless the naturalist in travelling, forinstance, from north to south never fails to be struck by the mannerin which successive groups of beings, specifically distinct, yetclearly related, replace each other. He hears from closely allied, yetdistinct kinds of birds, notes nearly similar, and sees their nestssimilarly constructed, but not quite alike, with eggs coloured innearly the same manner. The plains near the Straits of Magellan areinhabited by one species of Rhea (American ostrich), and northward theplains of La Plata by another species of the same genus; and not by atrue ostrich or emeu, like those found in Africa and Australia underthe same latitude. On these same plains of La Plata, we see the agoutiand bizcacha, animals having nearly the same habits as our hares andrabbits and belonging to the same order of Rodents, but they plainlydisplay an American type of structure. We ascend the lofty peaks ofthe Cordillera and we find an alpine species of bizcacha; we look tothe waters, and we do not find the beaver or musk-rat, but the coypuand capybara, rodents of the American type. Innumerable otherinstances could be given. If we look to the islands off the Americanshore, however much they may differ in geological structure, theinhabitants, though they may be all peculiar species, are essentiallyAmerican. We may look back to past ages, as shown in the last chapter,and we find American types then prevalent on the American continentand in the American seas. We see in these facts some deep organicbond, prevailing throughout space and time, over the same areas ofland and water, and independent of their physical conditions. Thenaturalist must feel little curiosity, who is not led to inquire whatthis bond is.
This bond, on my theory, is simply inheritance, that cause whichalone, as far as we positively know, produces organisms quite like,or, as we see in the case of varieties nearly like each other. Thedissimilarity of the inhabitants of different regions may beattributed to modification through natural selection, and in a quitesubordinate degree to the direct influence of different physicalconditions. The degree of dissimilarity will depend on the migrationof the more dominant forms of life from one region into another havingbeen effected with more or less ease, at periods more or lessremote;--on the nature and number of the former immigrants;--and ontheir action and reaction, in their mutual struggles for life;--therelation of organism to organism being, as I have already oftenremarked, the most important of all relations. Thus the highimportance of barriers comes into play by checking migration; as doestime for the slow process of modification through natural selection.Widely-ranging species, abounding in individuals, which have alreadytriumphed over many competitors in their own widely-extended homeswill have the best chance of seizing on new places, when they spreadinto new countries. In their new homes they will be exposed to newconditions, and will frequently undergo further modification andimprovement; and thus they will become still further victorious, andwill produce groups of modified descendants. On this principle ofinheritance with modification, we can understand how it is thatsections of genera, whole genera, and even families are confined tothe same areas, as is so commonly and notoriously the case.
I believe, as was remarked in the last chapter, in no law of necessarydevelopment. As the variability of each species is an independentproperty, and will be taken advantage of by natural selection, only sofar as it profits the individual in its complex struggle for life, sothe degree of modification in different species will be no uniformquantity. If, for instance, a number of species, which stand in directcompetition with each other, migrate in a body into a new andafterwards isolated country, they will be little liable tomodification; for neither migration nor isolation in themselves can doanything. These principles come into play only by bringing organismsinto new relations with each other, and in a lesser degree with thesurrounding physical conditions. As we have seen in the last chapterthat some forms have retained nearly the same character from anenormously remote geological period, so certain species have migratedover vast spaces, and have not become greatly modified.
On these views, it is obvious, that the several species of the samegenus, though inhabiting the most distant quarters of the world, mustoriginally have proceeded from the same source, as they have descendedfrom the same progenitor. In the case of those species, which haveundergone during whole geological periods but little modification,there is not much difficulty in believing that they may have migratedfrom the same region; for during the vast geographical and climatalchanges which will have supervened since ancient times, almost anyamount of migration is possible. But in many other cases, in which wehave reason to believe that the species of a genus have been producedwithin comparatively recent times, there is great difficulty on thishead. It is also obvious that the individuals of the same species,though now inhabiting distant and isolated regions, must haveproceeded from one spot, where their parents were first produced: for,as explained in the last chapter, it is incredible that individualsidentically the same should ever have been produced through naturalselection from parents specifically distinct.
We are thus brought to the question which has been largely discussedby naturalists, namely, whether species have been created at one ormore points of the earth's surface. Undoubtedly there are very manycases of extreme difficulty, in understanding how the same speciescould possibly have migrated from some one point to the severaldistant and isolated points, where now found. Nevertheless thesimplicity of the view that each species was first produced within asingle region captivates the mind. He who rejects it, rejects the veracausa of ordinary generation with subsequent migration, and calls inthe agency of a miracle. It is universally admitted, that in mostcases the area inhabited by a species is continuous; and when a plantor animal inhabits two points so distant from each other, or with aninterval of such a nature, that the space could not be easily passedover by migration, the fact is given as something remarkable andexceptional. The capacity of migrating across the sea is moredistinctly limited in terrestrial mammals, than perhaps in any otherorganic beings; and, accordingly, we find no inexplicable cases of thesame mammal inhabiting distant points of the world. No geologist willfeel any difficulty in such cases as Great Britain having beenformerly united to Europe, and consequently possessing the samequadrupeds. But if the same species can be produced at two separatepoints, why do we not find a single mammal common to Europe andAustralia or South America? The conditions of life are nearly thesame, so that a multitude of European animals and plants have becomenaturalised in America and Australia; and some of the aboriginalplants are identically the same at these distant points of thenorthern and southern hemispheres? The answer, as I believe, is, thatmammals have not been able to migrate, whereas some plants, from theirvaried means of dispersal, have migrated across the vast and brokeninterspace. The great and striking influence which barriers of everykind have had on distribution, is intelligible only on the view thatthe great majority of species have been produced on one side alone,and have not been able to migrate to the other side. Some fewfamilies, many sub-families, very many genera, and a still greaternumber of sections of genera are confined to a single region; and ithas been observed by several naturalists, that the most naturalgenera, or those genera in which the species are most closely relatedto each other, are generally local, or confined to one area. What astrange anomaly it would be, if, when coming one step lower in theseries, to the individuals of the same species, a directly oppositerule prevailed; and species were not local, but had been produced intwo or more distinct areas!
Hence it seems to me, as it has to many other naturalists, that theview of each species having been produced in one area alone, andhaving subsequently migrated from that area as far as its powers ofmigration and subsistence under past and present conditions permitted,is the most probable. Undoubtedly many cases occur, in which we cannotexplain how the same species could have passed from one point to theother. But the geographical and climatal changes, which have certainlyoccurred within recent geological times, must have interrupted orrendered discontinuous the formerly continuous range of many species.So that we are reduced to consider whether the exceptions tocontinuity of range are so numerous and of so grave a nature, that weought to give up the belief, rendered probable by generalconsiderations, that each species has been produced within one area,and has migrated thence as far as it could. It would be hopelesslytedious to discuss all the exceptional cases of the same species, nowliving at distant and separated points; nor do I for a moment pretendthat any explanation could be offered of many such cases. But aftersome preliminary remarks, I will discuss a few of the most strikingclasses of facts; namely, the existence of the same species on thesummits of distant mountain-ranges, and at distant points in thearctic and antarctic regions; and secondly (in the following chapter),the wide distribution of freshwater productions; and thirdly, theoccurrence of the same terrestrial species on islands and on themainland, though separated by hundreds of miles of open sea. If theexistence of the same species at distant and isolated points of theearth's surface, can in many instances be explained on the view ofeach species having migrated from a single birthplace; then,considering our ignorance with respect to former climatal andgeographical changes and various occasional means of transport, thebelief that this has been the universal law, seems to me incomparablythe safest.
In discussing this subject, we shall be enabled at the same time toconsider a point equally important for us, namely, whether the severaldistinct species of a genus, which on my theory have all descendedfrom a common progenitor, can have migrated (undergoing modificationduring some part of their migration) from the area inhabited by theirprogenitor. If it can be shown to be almost invariably the case, thata region, of which most of its inhabitants are closely related to, orbelong to the same genera with the species of a second region, hasprobably received at some former period immigrants from this otherregion, my theory will be strengthened; for we can clearly understand,on the principle of modification, why the inhabitants of a regionshould be related to those of another region, whence it has beenstocked. A volcanic island, for instance, upheaved and formed at thedistance of a few hundreds of miles from a continent, would probablyreceive from it in the course of time a few colonists, and theirdescendants, though modified, would still be plainly related byinheritance to the inhabitants of the continent. Cases of this natureare common, and are, as we shall hereafter more fully see,inexplicable on the theory of independent creation. This view of therelation of species in one region to those in another, does not differmuch (by substituting the word variety for species) from that latelyadvanced in an ingenious paper by Mr. Wallace, in which he concludes,that "every species has come into existence coincident both in spaceand time with a pre-existing closely allied species." And I now knowfrom correspondence, that this coincidence he attributes to generationwith modification.
The previous remarks on "single and multiple centres of creation" donot directly bear on another allied question,--namely whether all theindividuals of the same species have descended from a single pair, orsingle hermaphrodite, or whether, as some authors suppose, from manyindividuals simultaneously created. With those organic beings whichnever intercross (if such exist), the species, on my theory, must havedescended from a succession of improved varieties, which will neverhave blended with other individuals or varieties, but will havesupplanted each other; so that, at each successive stage ofmodification and improvement, all the individuals of each variety willhave descended from a single parent. But in the majority of cases,namely, with all organisms which habitually unite for each birth, orwhich often intercross, I believe that during the slow process ofmodification the individuals of the species will have been kept nearlyuniform by intercrossing; so that many individuals will have gone onsimultaneously changing, and the whole amount of modification will nothave been due, at each stage, to descent from a single parent. Toillustrate what I mean: our English racehorses differ slightly fromthe horses of every other breed; but they do not owe their differenceand superiority to descent from any single pair, but to continued carein selecting and training many individuals during many generations.
Before discussing the three classes of facts, which I have selected aspresenting the greatest amount of difficulty on the theory of "singlecentres of creation," I must say a few words on the means ofdispersal.