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 3
It may be doubted whether the duration of any one great period ofsubsidence over the whole or part of the archipelago, together with acontemporaneous accumulation of sediment, would EXCEED the averageduration of the same specific forms; and these contingencies areindispensable for the preservation of all the transitional gradationsbetween any two or more species. If such gradations were not fullypreserved, transitional varieties would merely appear as so manydistinct species. It is, also, probable that each great period ofsubsidence would be interrupted by oscillations of level, and thatslight climatal changes would intervene during such lengthy periods;and in these cases the inhabitants of the archipelago would have tomigrate, and no closely consecutive record of their modificationscould be preserved in any one formation.
Very many of the marine inhabitants of the archipelago now rangethousands of miles beyond its confines; and analogy leads me tobelieve that it would be chiefly these far-ranging species which wouldoftenest produce new varieties; and the varieties would at firstgenerally be local or confined to one place, but if possessed of anydecided advantage, or when further modified and improved, they wouldslowly spread and supplant their parent-forms. When such varietiesreturned to their ancient homes, as they would differ from theirformer state, in a nearly uniform, though perhaps extremely slightdegree, they would, according to the principles followed by manypalaeontologists, be ranked as new and distinct species.
If then, there be some degree of truth in these remarks, we have noright to expect to find in our geological formations, an infinitenumber of those fine transitional forms, which on my theory assuredlyhave connected all the past and present species of the same group intoone long and branching chain of life. We ought only to look for a fewlinks, some more closely, some more distantly related to each other;and these links, let them be ever so close, if found in differentstages of the same formation, would, by most palaeontologists, beranked as distinct species. But I do not pretend that I should everhave suspected how poor a record of the mutations of life, the bestpreserved geological section presented, had not the difficulty of ournot discovering innumerable transitional links between the specieswhich appeared at the commencement and close of each formation,pressed so hardly on my theory.
ON THE SUDDEN APPEARANCE OF WHOLE GROUPS OF ALLIED SPECIES.
The abrupt manner in which whole groups of species suddenly appear incertain formations, has been urged by several palaeontologists, forinstance, by Agassiz, Pictet, and by none more forcibly than byProfessor Sedgwick, as a fatal objection to the belief in thetransmutation of species. If numerous species, belonging to the samegenera or families, have really started into life all at once, thefact would be fatal to the theory of descent with slow modificationthrough natural selection. For the development of a group of forms,all of which have descended from some one progenitor, must have beenan extremely slow process; and the progenitors must have lived longages before their modified descendants. But we continually over-ratethe perfection of the geological record, and falsely infer, becausecertain genera or families have not been found beneath a certainstage, that they did not exist before that stage. We continuallyforget how large the world is, compared with the area over which ourgeological formations have been carefully examined; we forget thatgroups of species may elsewhere have long existed and have slowlymultiplied before they invaded the ancient archipelagoes of Europe andof the United States. We do not make due allowance for the enormousintervals of time, which have probably elapsed between our consecutiveformations,--longer perhaps in some cases than the time required forthe accumulation of each formation. These intervals will have giventime for the multiplication of species from some one or some fewparent-forms; and in the succeeding formation such species will appearas if suddenly created.
I may here recall a remark formerly made, namely that it might requirea long succession of ages to adapt an organism to some new andpeculiar line of life, for instance to fly through the air; but thatwhen this had been effected, and a few species had thus acquired agreat advantage over other organisms, a comparatively short time wouldbe necessary to produce many divergent forms, which would be able tospread rapidly and widely throughout the world.
I will now give a few examples to illustrate these remarks; and toshow how liable we are to error in supposing that whole groups ofspecies have suddenly been produced. I may recall the well-known factthat in geological treatises, published not many years ago, the greatclass of mammals was always spoken of as having abruptly come in atthe commencement of the tertiary series. And now one of the richestknown accumulations of fossil mammals belongs to the middle of thesecondary series; and one true mammal has been discovered in the newred sandstone at nearly the commencement of this great series. Cuvierused to urge that no monkey occurred in any tertiary stratum; but nowextinct species have been discovered in India, South America, and inEurope even as far back as the eocene stage. The most striking case,however, is that of the Whale family; as these animals have hugebones, are marine, and range over the world, the fact of not a singlebone of a whale having been discovered in any secondary formation,seemed fully to justify the belief that this great and distinct orderhad been suddenly produced in the interval between the latestsecondary and earliest tertiary formation. But now we may read in theSupplement to Lyell's 'Manual,' published in 1858, clear evidence ofthe existence of whales in the upper greensand, some time before theclose of the secondary period.
I may give another instance, which from having passed under my owneyes has much struck me. In a memoir on Fossil Sessile Cirripedes, Ihave stated that, from the number of existing and extinct tertiaryspecies; from the extraordinary abundance of the individuals of manyspecies all over the world, from the Arctic regions to the equator,inhabiting various zones of depths from the upper tidal limits to 50fathoms; from the perfect manner in which specimens are preserved inthe oldest tertiary beds; from the ease with which even a fragment ofa valve can be recognised; from all these circumstances, I inferredthat had sessile cirripedes existed during the secondary periods, theywould certainly have been preserved and discovered; and as not onespecies had been discovered in beds of this age, I concluded that thisgreat group had been suddenly developed at the commencement of thetertiary series. This was a sore trouble to me, adding as I thoughtone more instance of the abrupt appearance of a great group ofspecies. But my work had hardly been published, when a skilfulpalaeontologist, M. Bosquet, sent me a drawing of a perfect specimenof an unmistakeable sessile cirripede, which he had himself extractedfrom the chalk of Belgium. And, as if to make the case as striking aspossible, this sessile cirripede was a Chthamalus, a very common,large, and ubiquitous genus, of which not one specimen has as yet beenfound even in any tertiary stratum. Hence we now positively know thatsessile cirripedes existed during the secondary period; and thesecirripedes might have been the progenitors of our many tertiary andexisting species.
with new and peculiar species. Traces of life havebeen detected in the Longmynd beds beneath Barrande's so-calledprimordial zone. The presence!
The case most frequently insisted on by palaeontologists of theapparently sudden appearance of a whole group of species, is that ofthe teleostean fishes, low down in the Chalk period. This groupincludes the large majority of existing species. Lately, ProfessorPictet has carried their existence one sub-stage further back; andsome palaeontologists believe that certain much older fishes, of whichthe affinities are as yet imperfectly known, are really teleostean.Assuming, however, that the whole of them did appear, as Agassizbelieves, at the commencement of the chalk formation, the fact wouldcertainly be highly remarkable; but I cannot see that it would be aninsuperable difficulty on my theory, unless it could likewise be shownthat the species of this group appeared suddenly and simultaneouslythroughout the world at this same period. It is almost superfluous toremark that hardly any fossil-fish are known from south of theequator; and by running through Pictet's Palaeontology it will be seenthat very few species are known from several formations in Europe.Some few families of fish now have a confined range; the teleosteanfish might formerly have had a similarly confined range, and afterhaving been largely developed in some one sea, might have spreadwidely. Nor have we any right to suppose that the seas of the worldhave always been so freely open from south to north as they are atpresent. Even at this day, if the Malay Archipelago were convertedinto land, the tropical parts of the Indian Ocean would form a largeand perfectly enclosed basin, in which any great group of marineanimals might be multiplied; and here they would remain confined,until some of the species became adapted to a cooler climate, and wereenabled to double the southern capes of Africa or Australia, and thusreach other and distant seas.
From these and similar considerations, but chiefly from our ignoranceof the geology of other countries beyond the confines of Europe andthe United States; and from the revolution in our palaeontologicalideas on many points, which the discoveries of even the last dozenyears have effected, it seems to me to be about as rash in us todogmatize on the succession of organic beings throughout the world, asit would be for a naturalist to land for five minutes on some onebarren point in Australia, and then to discuss the number and range ofits productions.
ON THE SUDDEN APPEARANCE OF GROUPS OF ALLIED SPECIES IN THE LOWESTKNOWN FOSSILIFEROUS STRATA.
There is another and allied difficulty, which is much graver. I alludeto the manner in which numbers of species of the same group, suddenlyappear in the lowest known fossiliferous rocks. Most of the argumentswhich have convinced me that all the existing species of the samegroup have descended from one progenitor, apply with nearly equalforce to the earliest known species. For instance, I cannot doubt thatall the Silurian trilobites have descended from some one crustacean,which must have lived long before the Silurian age, and which probablydiffered greatly from any known animal. Some of the most ancientSilurian animals, as the Nautilus, Lingula, etc., do not differ muchfrom living species; and it cannot on my theory be supposed, thatthese old species were the progenitors of all the species of theorders to which they belong, for they do not present characters in anydegree intermediate between them. If, moreover, they had been theprogenitors of these orders, they would almost certainly have beenlong ago supplanted and exterminated by their numerous and improveddescendants.
Consequently, if my theory be true, it is indisputable that before thelowest Silurian stratum was deposited, long periods elapsed, as longas, or probably far longer than, the whole interval from the Silurianage to the present day; and that during these vast, yet quite unknown,periods of time, the world swarmed with living creatures.
To the question why we do not find records of these vast primordialperiods, I can give no satisfactory answer. Several of the mosteminent geologists, with Sir R. Murchison at their head, are convincedthat we see in the organic remains of the lowest Silurian stratum thedawn of life on this planet. Other highly competent judges, as Lyelland the late E. Forbes, dispute this conclusion. We should not forgetthat only a small portion of the world is known with accuracy. M.Barrande has lately added another and lower stage to the Siluriansystem, abounding with new and peculiar species. Traces of life havebeen detected in the Longmynd beds beneath Barrande's so-calledprimordial zone. The presence of phosphatic nodules and bituminousmatter in some of the lowest azoic rocks, probably indicates theformer existence of life at these periods. But the difficulty ofunderstanding the absence of vast piles of fossiliferous strata, whichon my theory no doubt were somewhere accumulated before the Silurianepoch, is very great. If these most ancient beds had been wholly wornaway by denudation, or obliterated by metamorphic action, we ought tofind only small remnants of the formations next succeeding them inage, and these ought to be very generally in a metamorphosedcondition. But the descriptions which we now possess of the Siluriandeposits over immense territories in Russia and in North America, donot support the view, that the older a formation is, the more it hassuffered the extremity of denudation and metamorphism.
The case at present must remain inexplicable; and may be truly urgedas a valid argument against the views here entertained. To show thatit may hereafter receive some explanation, I will give the followinghypothesis. From the nature of the organic remains, which do notappear to have inhabited profound depths, in the several formations ofEurope and of the United States; and from the amount of sediment,miles in thickness, of which the formations are composed, we may inferthat from first to last large islands or tracts of land, whence thesediment was derived, occurred in the neighbourhood of the existingcontinents of Europe and North America. But we do not know what wasthe state of things in the intervals between the successiveformations; whether Europe and the United States during theseintervals existed as dry land, or as a submarine surface near land, onwhich sediment was not deposited, or again as the bed of an open andunfathomable sea.
Looking to the existing oceans, which are thrice as extensive as theland, we see them studded with many islands; but not one oceanicisland is as yet known to afford even a remnant of any palaeozoic orsecondary formation. Hence we may perhaps infer, that during thepalaeozoic and secondary periods, neither continents nor continentalislands existed where our oceans now extend; for had they existedthere, palaeozoic and secondary formations would in all probabilityhave been accumulated from sediment derived from their wear and tear;and would have been at least partially upheaved by the oscillations oflevel, which we may fairly conclude must have intervened during theseenormously long periods. If then we may infer anything from thesefacts, we may infer that where our oceans now extend, oceans haveextended from the remotest period of which we have any record; and onthe other hand, that where continents now exist, large tracts of landhave existed, subjected no doubt to great oscillations of level, sincethe earliest silurian period. The coloured map appended to my volumeon Coral Reefs, led me to conclude that the great oceans are stillmainly areas of subsidence, the great archipelagoes still areas ofoscillations of level, and the continents areas of elevation. But havewe any right to assume that things have thus remained from eternity?Our continents seem to have been formed by a preponderance, duringmany oscillations of level, of the force of elevation; but may not theareas of preponderant movement have changed in the lapse of ages? At aperiod immeasurably antecedent to the silurian epoch, continents mayhave existed where oceans are now spread out; and clear and openoceans may have existed where our continents now stand. Nor should webe justified in assuming that if, for instance, the bed of the PacificOcean were now converted into a continent, we should there findformations older than the silurian strata, supposing such to have beenformerly deposited; for it might well happen that strata which hadsubsided some miles nearer to the centre of the earth, and which hadbeen pressed on by an enormous weight of superincumbent water, mighthave undergone far more metamorphic action than strata which havealways remained nearer to the surface. The immense areas in some partsof the world, for instance in South America, of bare metamorphicrocks, which must have been heated under great pressure, have alwaysseemed to me to require some special explanation; and we may perhapsbelieve that we see in these large areas, the many formations longanterior to the silurian epoch in a completely metamorphosedcondition.
The several difficulties here discussed, namely our not finding in thesuccessive formations infinitely numerous transitional links betweenthe many species which now exist or have existed; the sudden manner inwhich whole groups of species appear in our European formations; thealmost entire absence, as at present known, of fossiliferousformations beneath the Silurian strata, are all undoubtedly of thegravest nature. We see this in the plainest manner by the fact thatall the most eminent palaeontologists, namely Cuvier, Owen, Agassiz,Barrande, Falconer, E. Forbes, etc., and all our greatest geologists,as Lyell, Murchison, Sedgwick, etc., have unanimously, oftenvehemently, maintained the immutability of species. But I have reasonto believe that one great authority, Sir Charles Lyell, from furtherreflexion entertains grave doubts on this subject. I feel how rash itis to differ from these great authorities, to whom, with others, weowe all our knowledge. Those who think the natural geological recordin any degree perfect, and who do not attach much weight to the factsand arguments of other kinds given in this volume, will undoubtedly atonce reject my theory. For my part, following out Lyell's metaphor, Ilook at the natural geological record, as a history of the worldimperfectly kept, and written in a changing dialect; of this historywe possess the last volume alone, relating only to two or threecountries. Of this volume, only here and there a short chapter hasbeen preserved; and of each page, only here and there a few lines.Each word of the slowly-changing language, in which the history issupposed to be written, being more or less different in theinterrupted succession of chapters, may represent the apparentlyabruptly changed forms of life, entombed in our consecutive, butwidely separated formations. On this view, the difficulties abovediscussed are greatly diminished, or even disappear.