物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 11. GEOGRAPHICAL DISTRIBUTION. Page 3
As the warmth returned, the arctic forms would retreat northward,closely followed up in their retreat by the productions of the moretemperate regions. And as the snow melted from the bases of themountains, the arctic forms would seize on the cleared and thawedground, always ascending higher and higher, as the warmth increased,whilst their brethren were pursuing their northern journey. Hence,when the warmth had fully returned, the same arctic species, which hadlately lived in a body together on the lowlands of the Old and NewWorlds, would be left isolated on distant mountain-summits (havingbeen exterminated on all lesser heights) and in the arctic regions ofboth hemispheres.
Thus we can understand the identity of many plants at points soimmensely remote as on the mountains of the United States and ofEurope. We can thus also understand the fact that the Alpine plants ofeach mountain-range are more especially related to the arctic formsliving due north or nearly due north of them: for the migration as thecold came on, and the re-migration on the returning warmth, willgenerally have been due south and north. The Alpine plants, forexample, of Scotland, as remarked by Mr. H. C. Watson, and those ofthe Pyrenees, as remarked by Ramond, are more especially allied to theplants of northern Scandinavia; those of the United States toLabrador; those of the mountains of Siberia to the arctic regions ofthat country. These views, grounded as they are on the perfectlywell-ascertained occurrence of a former Glacial period, seem to me toexplain in so satisfactory a manner the present distribution of theAlpine and Arctic productions of Europe and America, that when inother regions we find the same species on distant mountain-summits, wemay almost conclude without other evidence, that a colder climatepermitted their former migration across the low intervening tracts,since become too warm for their existence.
If the climate, since the Glacial period, has ever been in any degreewarmer than at present (as some geologists in the United Statesbelieve to have been the case, chiefly from the distribution of thefossil Gnathodon), then the arctic and temperate productions will at avery late period have marched a little further north, and subsequentlyhave retreated to their present homes; but I have met with nosatisfactory evidence with respect to this intercalated slightlywarmer period, since the Glacial period.
The arctic forms, during their long southern migration andre-migration northward, will have been exposed to nearly the sameclimate, and, as is especially to be noticed, they will have kept in abody together; consequently their mutual relations will not have beenmuch disturbed, and, in accordance with the principles inculcated inthis volume, they will not have been liable to much modification. Butwith our Alpine productions, left isolated from the moment of thereturning warmth, first at the bases and ultimately on the summits ofthe mountains, the case will have been somewhat different; for it isnot likely that all the same arctic species will have been left onmountain ranges distant from each other, and have survived there eversince; they will, also, in all probability have become mingled withancient Alpine species, which must have existed on the mountainsbefore the commencement of the Glacial epoch, and which during itscoldest period will have been temporarily driven down to the plains;they will, also, have been exposed to somewhat different climatalinfluences. Their mutual relations will thus have been in some degreedisturbed; consequently they will have been liable to modification;and this we find has been the case; for if we compare the presentAlpine plants and animals of the several great Europeanmountain-ranges, though very many of the species are identically thesame, some present varieties, some are ranked as doubtful forms, andsome few are distinct yet closely allied or representative species.
In illustrating what, as I believe, actually took place during theGlacial period, I assumed that at its commencement the arcticproductions were as uniform round the polar regions as they are at thepresent day. But the foregoing remarks on distribution apply not onlyto strictly arctic forms, but also to many sub-arctic and to some fewnorthern temperate forms, for some of these are the same on the lowermountains and on the plains of North America and Europe; and it may bereasonably asked how I account for the necessary degree of uniformityof the sub-arctic and northern temperate forms round the world, at thecommencement of the Glacial period. At the present day, the sub-arcticand northern temperate productions of the Old and New Worlds areseparated from each other by the Atlantic Ocean and by the extremenorthern part of the Pacific. During the Glacial period, when theinhabitants of the Old and New Worlds lived further southwards than atpresent, they must have been still more completely separated by widerspaces of ocean. I believe the above difficulty may be surmounted bylooking to still earlier changes of climate of an opposite nature. Wehave good reason to believe that during the newer Pliocene period,before the Glacial epoch, and whilst the majority of the inhabitantsof the world were specifically the same as now, the climate was warmerthan at the present day. Hence we may suppose that the organisms nowliving under the climate of latitude 60 deg, during the Plioceneperiod lived further north under the Polar Circle, in latitude 66deg-67 deg; and that the strictly arctic productions then lived on thebroken land still nearer to the pole. Now if we look at a globe, weshall see that under the Polar Circle there is almost continuous landfrom western Europe, through Siberia, to eastern America. And to thiscontinuity of the circumpolar land, and to the consequent freedom forintermigration under a more favourable climate, I attribute thenecessary amount of uniformity in the sub-arctic and northerntemperate productions of the Old and New Worlds, at a period anteriorto the Glacial epoch.
Believing, from reasons before alluded to, that our continents havelong remained in nearly the same relative position, though subjectedto large, but partial oscillations of level, I am strongly inclined toextend the above view, and to infer that during some earlier and stillwarmer period, such as the older Pliocene period, a large number ofthe same plants and animals inhabited the almost continuouscircumpolar land; and that these plants and animals, both in the Oldand New Worlds, began slowly to migrate southwards as the climatebecame less warm, long before the commencement of the Glacial period.We now see, as I believe, their descendants, mostly in a modifiedcondition, in the central parts of Europe and the United States. Onthis view we can understand the relationship, with very littleidentity, between the productions of North America and Europe,--arelationship which is most remarkable, considering the distance of thetwo areas, and their separation by the Atlantic Ocean. We can furtherunderstand the singular fact remarked on by several observers, thatthe productions of Europe and America during the later tertiary stageswere more closely related to each other than they are at the presenttime; for during these warmer periods the northern parts of the Oldand New Worlds will have been almost continuously united by land,serving as a bridge, since rendered impassable by cold, for theinter-migration of their inhabitants.
During the slowly decreasing warmth of the Pliocene period, as soon asthe species in common, which inhabited the New and Old Worlds,migrated south of the Polar Circle, they must have been completely cutoff from each other. This separation, as far as the more temperateproductions are concerned, took place long ages ago. And as the plantsand animals migrated southward, they will have become mingled in theone great region with the native American productions, and have had tocompete with them; and in the other great region, with those of theOld World. Consequently we have here everything favourable for muchmodification,--for far more modification than with the Alpineproductions, left isolated, within a much more recent period, on theseveral mountain-ranges and on the arctic lands of the two Worlds.Hence it has come, that when we compare the now living productions ofthe temperate regions of the New and Old Worlds, we find very fewidentical species (though Asa Gray has lately shown that more plantsare identical than was formerly supposed), but we find in every greatclass many forms, which some naturalists rank as geographical races,and others as distinct species; and a host of closely allied orrepresentative forms which are ranked by all naturalists asspecifically distinct.
As on the land, so in the waters of the sea, a slow southern migrationof a marine fauna, which during the Pliocene or even a somewhatearlier period, was nearly uniform along the continuous shores of thePolar Circle, will account, on the theory of modification, for manyclosely allied forms now living in areas completely sundered. Thus, Ithink, we can understand the presence of many existing and tertiaryrepresentative forms on the eastern and western shores of temperateNorth America; and the still more striking case of many closely alliedcrustaceans (as described in Dana's admirable work), of some fish andother marine animals, in the Mediterranean and in the seas ofJapan,--areas now separated by a continent and by nearly a hemisphereof equatorial ocean.
These cases of relationship, without identity, of the inhabitants ofseas now disjoined, and likewise of the past and present inhabitantsof the temperate lands of North America and Europe, are inexplicableon the theory of creation. We cannot say that they have been createdalike, in correspondence with the nearly similar physical conditionsof the areas; for if we compare, for instance, certain parts of SouthAmerica with the southern continents of the Old World, we seecountries closely corresponding in all their physical conditions, butwith their inhabitants utterly dissimilar.
But we must return to our more immediate subject, the Glacial period.I am convinced that Forbes's view may be largely extended. In Europewe have the plainest evidence of the cold period, from the westernshores of Britain to the Oural range, and southward to the Pyrenees.We may infer, from the frozen mammals and nature of the mountainvegetation, that Siberia was similarly affected. Along the Himalaya,at points 900 miles apart, glaciers have left the marks of theirformer low descent; and in Sikkim, Dr. Hooker saw maize growing ongigantic ancient moraines. South of the equator, we have some directevidence of former glacial action in New Zealand; and the same plants,found on widely separated mountains in this island, tell the samestory. If one account which has been published can be trusted, we havedirect evidence of glacial action in the south-eastern corner ofAustralia.
Looking to America; in the northern half, ice-borne fragments of rockhave been observed on the eastern side as far south as lat. 36 deg-37deg, and on the shores of the Pacific, where the climate is now sodifferent, as far south as lat. 46 deg; erratic boulders have, also,been noticed on the Rocky Mountains. In the Cordillera of EquatorialSouth America, glaciers once extended far below their present level.In central Chile I was astonished at the structure of a vast mound ofdetritus, about 800 feet in height, crossing a valley of the Andes;and this I now feel convinced was a gigantic moraine, left far belowany existing glacier. Further south on both sides of the continent,from lat. 41 deg to the southernmost extremity, we have the clearestevidence of former glacial action, in huge boulders transported farfrom their parent source.
We do not know that the Glacial epoch was strictly simultaneous atthese several far distant points on opposite sides of the world. Butwe have good evidence in almost every case, that the epoch wasincluded within the latest geological period. We have, also, excellentevidence, that it endured for an enormous time, as measured by years,at each point. The cold may have come on, or have ceased, earlier atone point of the globe than at another, but seeing that it endured forlong at each, and that it was contemporaneous in a geological sense,it seems to me probable that it was, during a part at least of theperiod, actually simultaneous throughout the world. Without somedistinct evidence to the contrary, we may at least admit as probablethat the glacial action was simultaneous on the eastern and westernsides of North America, in the Cordillera under the equator and underthe warmer temperate zones, and on both sides of the southernextremity of the continent. If this be admitted, it is difficult toavoid believing that the temperature of the whole world was at thisperiod simultaneously cooler. But it would suffice for my purpose, ifthe temperature was at the same time lower along certain broad beltsof longitude.
On this view of the whole world, or at least of broad longitudinalbelts, having been simultaneously colder from pole to pole, much lightcan be thrown on the present distribution of identical and alliedspecies. In America, Dr. Hooker has shown that between forty and fiftyof the flowering plants of Tierra del Fuego, forming no inconsiderablepart of its scanty flora, are common to Europe, enormously remote asthese two points are; and there are many closely allied species. Onthe lofty mountains of equatorial America a host of peculiar speciesbelonging to European genera occur. On the highest mountains ofBrazil, some few European genera were found by Gardner, which do notexist in the wide intervening hot countries. So on the Silla ofCaraccas the illustrious Humboldt long ago found species belonging togenera characteristic of the Cordillera. On the mountains ofAbyssinia, several European forms and some few representatives of thepeculiar flora of the Cape of Good Hope occur. At the Cape of GoodHope a very few European species, believed not to have been introducedby man, and on the mountains, some few representative European formsare found, which have not been discovered in the intertropical partsof Africa. On the Himalaya, and on the isolated mountain-ranges of thepeninsula of India, on the heights of Ceylon, and on the volcaniccones of Java, many plants occur, either identically the same orrepresenting each other, and at the same time representing plants ofEurope, not found in the intervening hot lowlands. A list of thegenera collected on the loftier peaks of Java raises a picture of acollection made on a hill in Europe! Still more striking is the factthat southern Australian forms are clearly represented by plantsgrowing on the summits of the mountains of Borneo. Some of theseAustralian forms, as I hear from Dr. Hooker, extend along the heightsof the peninsula of Malacca, and are thinly scattered, on the one handover India and on the other as far north as Japan.
On the southern mountains of Australia, Dr. F. Muller has discoveredseveral European species; other species, not introduced by man, occuron the lowlands; and a long list can be given, as I am informed by Dr.Hooker, of European genera, found in Australia, but not in theintermediate torrid regions. In the admirable 'Introduction to theFlora of New Zealand,' by Dr. Hooker, analogous and striking facts aregiven in regard to the plants of that large island. Hence we see thatthroughout the world, the plants growing on the more lofty mountains,and on the temperate lowlands of the northern and southernhemispheres, are sometimes identically the same; but they are muchoftener specifically distinct, though related to each other in a mostremarkable manner.
This brief abstract applies to plants alone: some strictly analogousfacts could be given on the distribution of terrestrial animals. Inmarine productions, similar cases occur; as an example, I may quote aremark by the highest authority, Professor Dana, that "it is certainlya wonderful fact that New Zealand should have a closer resemblance inits crustacea to Great Britain, its antipode, than to any other partof the world." Sir J. Richardson, also, speaks of the reappearance onthe shores of New Zealand, Tasmania, etc., of northern forms of fish.Dr. Hooker informs me that twenty-five species of Algae are common toNew Zealand and to Europe, but have not been found in the intermediatetropical seas.
It should be observed that the northern species and forms found in thesouthern parts of the southern hemisphere, and on the mountain-rangesof the intertropical regions, are not arctic, but belong to thenorthern temperate zones. As Mr. H. C. Watson has recently remarked,"In receding from polar towards equatorial latitudes, the Alpine ormountain floras really become less and less arctic." Many of the formsliving on the mountains of the warmer regions of the earth and in thesouthern hemisphere are of doubtful value, being ranked by somenaturalists as specifically distinct, by others as varieties; but someare certainly identical, and many, though closely related to northernforms, must be ranked as distinct species.
Now let us see what light can be thrown on the foregoing facts, on thebelief, supported as it is by a large body of geological evidence,that the whole world, or a large part of it, was during the Glacialperiod simultaneously much colder than at present. The Glacial period,as measured by years, must have been very long; and when we rememberover what vast spaces some naturalised plants and animals have spreadwithin a few centuries, this period will have been ample for anyamount of migration. As the cold came slowly on, all the tropicalplants and other productions will have retreated from both sidestowards the equator, followed in the rear by the temperateproductions, and these by the arctic; but with the latter we are notnow concerned. The tropical plants probably suffered much extinction;how much no one can say; perhaps formerly the tropics supported asmany species as we see at the present day crowded together at the Capeof Good Hope, and in parts of temperate Australia. As we know thatmany tropical plants and animals can withstand a considerable amountof cold, many might have escaped extermination during a moderate fallof temperature, more especially by escaping into the warmest spots.But the great fact to bear in mind is, that all tropical productionswill have suffered to a certain extent. On the other hand, thetemperate productions, after migrating nearer to the equator, thoughthey will have been placed under somewhat new conditions, will havesuffered less. And it is certain that many temperate plants, ifprotected from the inroads of competitors, can withstand a much warmerclimate than their own. Hence, it seems to me possible, bearing inmind that the tropical productions were in a suffering state and couldnot have presented a firm front against intruders, that a certainnumber of the more vigorous and dominant temperate forms might havepenetrated the native ranks and have reached or even crossed theequator. The invasion would, of course, have been greatly favoured byhigh land, and perhaps by a dry climate; for Dr. Falconer informs methat it is the damp with the heat of the tropics which is sodestructive to perennial plants from a temperate climate. On the otherhand, the most humid and hottest districts will have afforded anasylum to the tropical natives. The mountain-ranges north-west of theHimalaya, and the long line of the Cordillera, seem to have affordedtwo great lines of invasion: and it is a striking fact, latelycommunicated to me by Dr. Hooker, that all the flowering plants, aboutforty-six in number, common to Tierra del Fuego and to Europe stillexist in North America, which must have lain on the line of march. ButI do not doubt that some temperate productions entered and crossedeven the LOWLANDS of the tropics at the period when the cold was mostintense,--when arctic forms had migrated some twenty-five degrees oflatitude from their native country and covered the land at the foot ofthe Pyrenees. At this period of extreme cold, I believe that theclimate under the equator at the level of the sea was about the samewith that now felt there at the height of six or seven thousand feet.During this the coldest period, I suppose that large spaces of thetropical lowlands were clothed with a mingled tropical and temperatevegetation, like that now growing with strange luxuriance at the baseof the Himalaya, as graphically described by Hooker.