物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 9. ON THE IMPERFECTION OF THE GEOLOGICAL RECORD. Page 2
But we can, I think, see why the geological formations of each regionare almost invariably intermittent; that is, have not followed eachother in close sequence. Scarcely any fact struck me more whenexamining many hundred miles of the South American coasts, which havebeen upraised several hundred feet within the recent period, than theabsence of any recent deposits sufficiently extensive to last for evena short geological period. Along the whole west coast, which isinhabited by a peculiar marine fauna, tertiary beds are so scantilydeveloped, that no record of several successive and peculiar marinefaunas will probably be preserved to a distant age. A littlereflection will explain why along the rising coast of the western sideof South America, no extensive formations with recent or tertiaryremains can anywhere be found, though the supply of sediment must forages have been great, from the enormous degradation of the coast-rocksand from muddy streams entering the sea. The explanation, no doubt,is, that the littoral and sub-littoral deposits are continually wornaway, as soon as they are brought up by the slow and gradual rising ofthe land within the grinding action of the coast-waves.
We may, I think, safely conclude that sediment must be accumulated inextremely thick, solid, or extensive masses, in order to withstand theincessant action of the waves, when first upraised and duringsubsequent oscillations of level. Such thick and extensiveaccumulations of sediment may be formed in two ways; either, inprofound depths of the sea, in which case, judging from the researchesof E. Forbes, we may conclude that the bottom will be inhabited byextremely few animals, and the mass when upraised will give a mostimperfect record of the forms of life which then existed; or, sedimentmay be accumulated to any thickness and extent over a shallow bottom,if it continue slowly to subside. In this latter case, as long as therate of subsidence and supply of sediment nearly balance each other,the sea will remain shallow and favourable for life, and thus afossiliferous formation thick enough, when upraised, to resist anyamount of degradation, may be formed.
I am convinced that all our ancient formations, which are rich infossils, have thus been formed during subsidence. Since publishing myviews on this subject in 1845, I have watched the progress of Geology,and have been surprised to note how author after author, in treatingof this or that great formation, has come to the conclusion that itwas accumulated during subsidence. I may add, that the only ancienttertiary formation on the west coast of South America, which has beenbulky enough to resist such degradation as it has as yet suffered, butwhich will hardly last to a distant geological age, was certainlydeposited during a downward oscillation of level, and thus gainedconsiderable thickness.
All geological facts tell us plainly that each area has undergonenumerous slow oscillations of level, and apparently these oscillationshave affected wide spaces. Consequently formations rich in fossils andsufficiently thick and extensive to resist subsequent degradation, mayhave been formed over wide spaces during periods of subsidence, butonly where the supply of sediment was sufficient to keep the seashallow and to embed and preserve the remains before they had time todecay. On the other hand, as long as the bed of the sea remainedstationary, THICK deposits could not have been accumulated in theshallow parts, which are the most favourable to life. Still less couldthis have happened during the alternate periods of elevation; or, tospeak more accurately, the beds which were then accumulated will havebeen destroyed by being upraised and brought within the limits of thecoast-action.
Thus the geological record will almost necessarily be renderedintermittent. I feel much confidence in the truth of these views, forthey are in strict accordance with the general principles inculcatedby Sir C. Lyell; and E. Forbes independently arrived at a similarconclusion.
One remark is here worth a passing notice. During periods of elevationthe area of the land and of the adjoining shoal parts of the sea willbe increased, and new stations will often be formed;--allcircumstances most favourable, as previously explained, for theformation of new varieties and species; but during such periods therewill generally be a blank in the geological record. On the other hand,during subsidence, the inhabited area and number of inhabitants willdecrease (excepting the productions on the shores of a continent whenfirst broken up into an archipelago), and consequently duringsubsidence, though there will be much extinction, fewer new varietiesor species will be formed; and it is during these very periods ofsubsidence, that our great deposits rich in fossils have beenaccumulated. Nature may almost be said to have guarded against thefrequent discovery of her transitional or linking forms.
From the foregoing considerations it cannot be doubted that thegeological record, viewed as a whole, is extremely imperfect; but ifwe confine our attention to any one formation, it becomes moredifficult to understand, why we do not therein find closely graduatedvarieties between the allied species which lived at its commencementand at its close. Some cases are on record of the same speciespresenting distinct varieties in the upper and lower parts of the sameformation, but, as they are rare, they may be here passed over.Although each formation has indisputably required a vast number ofyears for its deposition, I can see several reasons why each shouldnot include a graduated series of links between the species which thenlived; but I can by no means pretend to assign due proportional weightto the following considerations.
Although each formation may mark a very long lapse of years, eachperhaps is short compared with the period requisite to change onespecies into another. I am aware that two palaeontologists, whoseopinions are worthy of much deference, namely Bronn and Woodward, haveconcluded that the average duration of each formation is twice orthrice as long as the average duration of specific forms. Butinsuperable difficulties, as it seems to me, prevent us coming to anyjust conclusion on this head. When we see a species first appearing inthe middle of any formation, it would be rash in the extreme to inferthat it had not elsewhere previously existed. So again when we find aspecies disappearing before the uppermost layers have been deposited,it would be equally rash to suppose that it then became whollyextinct. We forget how small the area of Europe is compared with therest of the world; nor have the several stages of the same formationthroughout Europe been correlated with perfect accuracy.
With marine animals of all kinds, we may safely infer a large amountof migration during climatal and other changes; and when we see aspecies first appearing in any formation, the probability is that itonly then first immigrated into that area. It is well known, forinstance, that several species appeared somewhat earlier in thepalaeozoic beds of North America than in those of Europe; time havingapparently been required for their migration from the American to theEuropean seas. In examining the latest deposits of various quarters ofthe world, it has everywhere been noted, that some few still existingspecies are common in the deposit, but have become extinct in theimmediately surrounding sea; or, conversely, that some are nowabundant in the neighbouring sea, but are rare or absent in thisparticular deposit. It is an excellent lesson to reflect on theascertained amount of migration of the inhabitants of Europe duringthe Glacial period, which forms only a part of one whole geologicalperiod; and likewise to reflect on the great changes of level, on theinordinately great change of climate, on the prodigious lapse of time,all included within this same glacial period. Yet it may be doubtedwhether in any quarter of the world, sedimentary deposits, INCLUDINGFOSSIL REMAINS, have gone on accumulating within the same area duringthe whole of this period. It is not, for instance, probable thatsediment was deposited during the whole of the glacial period near themouth of the Mississippi, within that limit of depth at which marineanimals can flourish; for we know what vast geographical changesoccurred in other parts of America during this space of time. Whensuch beds as were deposited in shallow water near the mouth of theMississippi during some part of the glacial period shall have beenupraised, organic remains will probably first appear and disappear atdifferent levels, owing to the migration of species and togeographical changes. And in the distant future, a geologist examiningthese beds, might be tempted to conclude that the average duration oflife of the embedded fossils had been less than that of the glacialperiod, instead of having been really far greater, that is extendingfrom before the glacial epoch to the present day.
In order to get a perfect gradation between two forms in the upper andlower parts of the same formation, the deposit must have gone onaccumulating for a very long period, in order to have given sufficienttime for the slow process of variation; hence the deposit willgenerally have to be a very thick one; and the species undergoingmodification will have had to live on the same area throughout thiswhole time. But we have seen that a thick fossiliferous formation canonly be accumulated during a period of subsidence; and to keep thedepth approximately the same, which is necessary in order to enablethe same species to live on the same space, the supply of sedimentmust nearly have counterbalanced the amount of subsidence. But thissame movement of subsidence will often tend to sink the area whencethe sediment is derived, and thus diminish the supply whilst thedownward movement continues. In fact, this nearly exact balancingbetween the supply of sediment and the amount of subsidence isprobably a rare contingency; for it has been observed by more than onepalaeontologist, that very thick deposits are usually barren oforganic remains, except near their upper or lower limits.
It would seem that each separate formation, like the whole pile offormations in any country, has generally been intermittent in itsaccumulation. When we see, as is so often the case, a formationcomposed of beds of different mineralogical composition, we mayreasonably suspect that the process of deposition has been muchinterrupted, as a change in the currents of the sea and a supply ofsediment of a different nature will generally have been due togeographical changes requiring much time. Nor will the closestinspection of a formation give any idea of the time which itsdeposition has consumed. Many instances could be given of beds only afew feet in thickness, representing formations, elsewhere thousands offeet in thickness, and which must have required an enormous period fortheir accumulation; yet no one ignorant of this fact would havesuspected the vast lapse of time represented by the thinner formation.Many cases could be given of the lower beds of a formation having beenupraised, denuded, submerged, and then re-covered by the upper beds ofthe same formation,--facts, showing what wide, yet easily overlooked,intervals have occurred in its accumulation. In other cases we havethe plainest evidence in great fossilised trees, still standingupright as they grew, of many long intervals of time and changes oflevel during the process of deposition, which would never even havebeen suspected, had not the trees chanced to have been preserved:thus, Messrs. Lyell and Dawson found carboniferous beds 1400 feetthick in Nova Scotia, with ancient root-bearing strata, one above theother, at no less than sixty-eight different levels. Hence, when thesame species occur at the bottom, middle, and top of a formation, theprobability is that they have not lived on the same spot during thewhole period of deposition, but have disappeared and reappeared,perhaps many times, during the same geological period. So that if suchspecies were to undergo a considerable amount of modification duringany one geological period, a section would not probably include allthe fine intermediate gradations which must on my theory have existedbetween them, but abrupt, though perhaps very slight, changes of form.
It is all-important to remember that naturalists have no golden ruleby which to distinguish species and varieties; they grant some littlevariability to each species, but when they meet with a somewhatgreater amount of difference between any two forms, they rank both asspecies, unless they are enabled to connect them together by closeintermediate gradations. And this from the reasons just assigned wecan seldom hope to effect in any one geological section. Supposing Band C to be two species, and a third, A, to be found in an underlyingbed; even if A were strictly intermediate between B and C, it wouldsimply be ranked as a third and distinct species, unless at the sametime it could be most closely connected with either one or both formsby intermediate varieties. Nor should it be forgotten, as beforeexplained, that A might be the actual progenitor of B and C, and yetmight not at all necessarily be strictly intermediate between them inall points of structure. So that we might obtain the parent-speciesand its several modified descendants from the lower and upper beds ofa formation, and unless we obtained numerous transitional gradations,we should not recognise their relationship, and should consequently becompelled to rank them all as distinct species.
It is notorious on what excessively slight differences manypalaeontologists have founded their species; and they do this the morereadily if the specimens come from different sub-stages of the sameformation. Some experienced conchologists are now sinking many of thevery fine species of D'Orbigny and others into the rank of varieties;and on this view we do find the kind of evidence of change which on mytheory we ought to find. Moreover, if we look to rather widerintervals, namely, to distinct but consecutive stages of the samegreat formation, we find that the embedded fossils, though almostuniversally ranked as specifically different, yet are far more closelyallied to each other than are the species found in more widelyseparated formations; but to this subject I shall have to return inthe following chapter.
One other consideration is worth notice: with animals and plants thatcan propagate rapidly and are not highly locomotive, there is reasonto suspect, as we have formerly seen, that their varieties aregenerally at first local; and that such local varieties do not spreadwidely and supplant their parent-forms until they have been modifiedand perfected in some considerable degree. According to this view, thechance of discovering in a formation in any one country all the earlystages of transition between any two forms, is small, for thesuccessive changes are supposed to have been local or confined to someone spot. Most marine animals have a wide range; and we have seen thatwith plants it is those which have the widest range, that oftenestpresent varieties; so that with shells and other marine animals, it isprobably those which have had the widest range, far exceeding thelimits of the known geological formations of Europe, which haveoftenest given rise, first to local varieties and ultimately to newspecies; and this again would greatly lessen the chance of our beingable to trace the stages of transition in any one geologicalformation.
It should not be forgotten, that at the present day, with perfectspecimens for examination, two forms can seldom be connected byintermediate varieties and thus proved to be the same species, untilmany specimens have been collected from many places; and in the caseof fossil species this could rarely be effected by palaeontologists.We shall, perhaps, best perceive the improbability of our beingenabled to connect species by numerous, fine, intermediate, fossillinks, by asking ourselves whether, for instance, geologists at somefuture period will be able to prove, that our different breeds ofcattle, sheep, horses, and dogs have descended from a single stock orfrom several aboriginal stocks; or, again, whether certain sea-shellsinhabiting the shores of North America, which are ranked by someconchologists as distinct species from their European representatives,and by other conchologists as only varieties, are really varieties orare, as it is called, specifically distinct. This could be effectedonly by the future geologist discovering in a fossil state numerousintermediate gradations; and such success seems to me improbable inthe highest degree.
Geological research, though it has added numerous species to existingand extinct genera, and has made the intervals between some few groupsless wide than they otherwise would have been, yet has done scarcelyanything in breaking down the distinction between species, byconnecting them together by numerous, fine, intermediate varieties;and this not having been effected, is probably the gravest and mostobvious of all the many objections which may be urged against myviews. Hence it will be worth while to sum up the foregoing remarks,under an imaginary illustration. The Malay Archipelago is of about thesize of Europe from the North Cape to the Mediterranean, and fromBritain to Russia; and therefore equals all the geological formationswhich have been examined with any accuracy, excepting those of theUnited States of America. I fully agree with Mr. Godwin-Austen, thatthe present condition of the Malay Archipelago, with its numerouslarge islands separated by wide and shallow seas, probably representsthe former state of Europe, when most of our formations wereaccumulating. The Malay Archipelago is one of the richest regions ofthe whole world in organic beings; yet if all the species were to becollected which have ever lived there, how imperfectly would theyrepresent the natural history of the world!
But we have every reason to believe that the terrestrial productionsof the archipelago would be preserved in an excessively imperfectmanner in the formations which we suppose to be there accumulating. Isuspect that not many of the strictly littoral animals, or of thosewhich lived on naked submarine rocks, would be embedded; and thoseembedded in gravel or sand, would not endure to a distant epoch.Wherever sediment did not accumulate on the bed of the sea, or whereit did not accumulate at a sufficient rate to protect organic bodiesfrom decay, no remains could be preserved.
In our archipelago, I believe that fossiliferous formations could beformed of sufficient thickness to last to an age, as distant infuturity as the secondary formations lie in the past, only duringperiods of subsidence. These periods of subsidence would be separatedfrom each other by enormous intervals, during which the area would beeither stationary or rising; whilst rising, each fossiliferousformation would be destroyed, almost as soon as accumulated, by theincessant coast-action, as we now see on the shores of South America.During the periods of subsidence there would probably be muchextinction of life; during the periods of elevation, there would bemuch variation, but the geological record would then be least perfect.