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 1
On the absence of intermediate varieties at the present day.On the nature of extinct intermediate varieties; on their number.On the vast lapse of time, as inferred from the rate of deposition andof denudation.On the poorness of our palaeontological collections.On the intermittence of geological formations.On the absence of intermediate varieties in any one formation.On the sudden appearance of groups of species.On their sudden appearance in the lowest known fossiliferous strata.
In the sixth chapter I enumerated the chief objections which might bejustly urged against the views maintained in this volume. Most of themhave now been discussed. One, namely the distinctness of specificforms, and their not being blended together by innumerabletransitional links, is a very obvious difficulty. I assigned reasonswhy such links do not commonly occur at the present day, under thecircumstances apparently most favourable for their presence, namely onan extensive and continuous area with graduated physical conditions. Iendeavoured to show, that the life of each species depends in a moreimportant manner on the presence of other already defined organicforms, than on climate; and, therefore, that the really governingconditions of life do not graduate away quite insensibly like heat ormoisture. I endeavoured, also, to show that intermediate varieties,from existing in lesser numbers than the forms which they connect,will generally be beaten out and exterminated during the course offurther modification and improvement. The main cause, however, ofinnumerable intermediate links not now occurring everywhere throughoutnature depends on the very process of natural selection, through whichnew varieties continually take the places of and exterminate theirparent-forms. But just in proportion as this process of exterminationhas acted on an enormous scale, so must the number of intermediatevarieties, which have formerly existed on the earth, be trulyenormous. Why then is not every geological formation and every stratumfull of such intermediate links? Geology assuredly does not reveal anysuch finely graduated organic chain; and this, perhaps, is the mostobvious and gravest objection which can be urged against my theory.The explanation lies, as I believe, in the extreme imperfection of thegeological record.
In the first place it should always be borne in mind what sort ofintermediate forms must, on my theory, have formerly existed. I havefound it difficult, when looking at any two species, to avoidpicturing to myself, forms DIRECTLY intermediate between them. Butthis is a wholly false view; we should always look for formsintermediate between each species and a common but unknown progenitor;and the progenitor will generally have differed in some respects fromall its modified descendants. To give a simple illustration: thefantail and pouter pigeons have both descended from the rock-pigeon;if we possessed all the intermediate varieties which have everexisted, we should have an extremely close series between both and therock-pigeon; but we should have no varieties directly intermediatebetween the fantail and pouter; none, for instance, combining a tailsomewhat expanded with a crop somewhat enlarged, the characteristicfeatures of these two breeds. These two breeds, moreover, have becomeso much modified, that if we had no historical or indirect evidenceregarding their origin, it would not have been possible to havedetermined from a mere comparison of their structure with that of therock-pigeon, whether they had descended from this species or from someother allied species, such as C. oenas.
So with natural species, if we look to forms very distinct, forinstance to the horse and tapir, we have no reason to suppose thatlinks ever existed directly intermediate between them, but betweeneach and an unknown common parent. The common parent will have had inits whole organisation much general resemblance to the tapir and tothe horse; but in some points of structure may have differedconsiderably from both, even perhaps more than they differ from eachother. Hence in all such cases, we should be unable to recognise theparent-form of any two or more species, even if we closely comparedthe structure of the parent with that of its modified descendants,unless at the same time we had a nearly perfect chain of theintermediate links.
It is just possible by my theory, that one of two living forms mighthave descended from the other; for instance, a horse from a tapir; andin this case DIRECT intermediate links will have existed between them.But such a case would imply that one form had remained for a very longperiod unaltered, whilst its descendants had undergone a vast amountof change; and the principle of competition between organism andorganism, between child and parent, will render this a very rareevent; for in all cases the new and improved forms of life will tendto supplant the old and unimproved forms.
By the theory of natural selection all living species have beenconnected with the parent-species of each genus, by differences notgreater than we see between the varieties of the same species at thepresent day; and these parent-species, now generally extinct, have intheir turn been similarly connected with more ancient species; and soon backwards, always converging to the common ancestor of each greatclass. So that the number of intermediate and transitional links,between all living and extinct species, must have been inconceivablygreat. But assuredly, if this theory be true, such have lived uponthis earth.
ON THE LAPSE OF TIME.
Independently of our not finding fossil remains of such infinitelynumerous connecting links, it may be objected, that time will not havesufficed for so great an amount of organic change, all changes havingbeen effected very slowly through natural selection. It is hardlypossible for me even to recall to the reader, who may not be apractical geologist, the facts leading the mind feebly to comprehendthe lapse of time. He who can read Sir Charles Lyell's grand work onthe Principles of Geology, which the future historian will recogniseas having produced a revolution in natural science, yet does not admithow incomprehensibly vast have been the past periods of time, may atonce close this volume. Not that it suffices to study the Principlesof Geology, or to read special treatises by different observers onseparate formations, and to mark how each author attempts to give aninadequate idea of the duration of each formation or even eachstratum. A man must for years examine for himself great piles ofsuperimposed strata, and watch the sea at work grinding down old rocksand making fresh sediment, before he can hope to comprehend anythingof the lapse of time, the monuments of which we see around us.
It is good to wander along lines of sea-coast, when formed ofmoderately hard rocks, and mark the process of degradation. The tidesin most cases reach the cliffs only for a short time twice a day, andthe waves eat into them only when they are charged with sand orpebbles; for there is reason to believe that pure water can effectlittle or nothing in wearing away rock. At last the base of the cliffis undermined, huge fragments fall down, and these remaining fixed,have to be worn away, atom by atom, until reduced in size they can berolled about by the waves, and then are more quickly ground intopebbles, sand, or mud. But how often do we see along the bases ofretreating cliffs rounded boulders, all thickly clothed by marineproductions, showing how little they are abraded and how seldom theyare rolled about! Moreover, if we follow for a few miles any line ofrocky cliff, which is undergoing degradation, we find that it is onlyhere and there, along a short length or round a promontory, that thecliffs are at the present time suffering. The appearance of thesurface and the vegetation show that elsewhere years have elapsedsince the waters washed their base.
He who most closely studies the action of the sea on our shores, will,I believe, be most deeply impressed with the slowness with which rockycoasts are worn away. The observations on this head by Hugh Miller,and by that excellent observer Mr. Smith of Jordan Hill, are mostimpressive. With the mind thus impressed, let any one examine beds ofconglomerate many thousand feet in thickness, which, though probablyformed at a quicker rate than many other deposits, yet, from beingformed of worn and rounded pebbles, each of which bears the stamp oftime, are good to show how slowly the mass has been accumulated. Lethim remember Lyell's profound remark, that the thickness and extent ofsedimentary formations are the result and measure of the degradationwhich the earth's crust has elsewhere suffered. And what an amount ofdegradation is implied by the sedimentary deposits of many countries!Professor Ramsay has given me the maximum thickness, in most casesfrom actual measurement, in a few cases from estimate, of eachformation in different parts of Great Britain; and this is theresult:--
FeetPalaeozoic strata (not including igneous beds)..57,154.Secondary strata................................13,190.Tertiary strata..................................2,240.
--making altogether 72,584 feet; that is, very nearly thirteen andthree-quarters British miles. Some of these formations, which arerepresented in England by thin beds, are thousands of feet inthickness on the Continent. Moreover, between each successiveformation, we have, in the opinion of most geologists, enormously longblank periods. So that the lofty pile of sedimentary rocks in Britain,gives but an inadequate idea of the time which has elapsed duringtheir accumulation; yet what time this must have consumed! Goodobservers have estimated that sediment is deposited by the greatMississippi river at the rate of only 600 feet in a hundred thousandyears. This estimate may be quite erroneous; yet, considering overwhat wide spaces very fine sediment is transported by the currents ofthe sea, the process of accumulation in any one area must be extremelyslow.
structure may have differedconsiderably from both, even perhaps more than they differ from eachother. Hence in all such cases, we should be unable to recognise theparent-form of any two or more species, even if we closely comparedthe structure of the parent with that of .
But the amount of denudation which the strata have in many placessuffered, independently of the rate of accumulation of the degradedmatter, probably offers the best evidence of the lapse of time. Iremember having been much struck with the evidence of denudation, whenviewing volcanic islands, which have been worn by the waves and paredall round into perpendicular cliffs of one or two thousand feet inheight; for the gentle slope of the lava-streams, due to theirformerly liquid state, showed at a glance how far the hard, rocky bedshad once extended into the open ocean. The same story is still moreplainly told by faults,--those great cracks along which the stratahave been upheaved on one side, or thrown down on the other, to theheight or depth of thousands of feet; for since the crust cracked, thesurface of the land has been so completely planed down by the actionof the sea, that no trace of these vast dislocations is externallyvisible.
The Craven fault, for instance, extends for upwards of 30 miles, andalong this line the vertical displacement of the strata has variedfrom 600 to 3000 feet. Professor Ramsay has published an account of adownthrow in Anglesea of 2300 feet; and he informs me that he fullybelieves there is one in Merionethshire of 12,000 feet; yet in thesecases there is nothing on the surface to show such prodigiousmovements; the pile of rocks on the one or other side having beensmoothly swept away. The consideration of these facts impresses mymind almost in the same manner as does the vain endeavour to grapplewith the idea of eternity.
I am tempted to give one other case, the well-known one of thedenudation of the Weald. Though it must be admitted that thedenudation of the Weald has been a mere trifle, in comparison withthat which has removed masses of our palaeozoic strata, in parts tenthousand feet in thickness, as shown in Professor Ramsay's masterlymemoir on this subject. Yet it is an admirable lesson to stand on theNorth Downs and to look at the distant South Downs; for, rememberingthat at no great distance to the west the northern and southernescarpments meet and close, one can safely picture to oneself thegreat dome of rocks which must have covered up the Weald within solimited a period as since the latter part of the Chalk formation. Thedistance from the northern to the southern Downs is about 22 miles,and the thickness of the several formations is on an average about1100 feet, as I am informed by Professor Ramsay. But if, as somegeologists suppose, a range of older rocks underlies the Weald, on theflanks of which the overlying sedimentary deposits might haveaccumulated in thinner masses than elsewhere, the above estimate wouldbe erroneous; but this source of doubt probably would not greatlyaffect the estimate as applied to the western extremity of thedistrict. If, then, we knew the rate at which the sea commonly wearsaway a line of cliff of any given height, we could measure the timerequisite to have denuded the Weald. This, of course, cannot be done;but we may, in order to form some crude notion on the subject, assumethat the sea would eat into cliffs 500 feet in height at the rate ofone inch in a century. This will at first appear much too small anallowance; but it is the same as if we were to assume a cliff one yardin height to be eaten back along a whole line of coast at the rate ofone yard in nearly every twenty-two years. I doubt whether any rock,even as soft as chalk, would yield at this rate excepting on the mostexposed coasts; though no doubt the degradation of a lofty cliff wouldbe more rapid from the breakage of the fallen fragments. On the otherhand, I do not believe that any line of coast, ten or twenty miles inlength, ever suffers degradation at the same time along its wholeindented length; and we must remember that almost all strata containharder layers or nodules, which from long resisting attrition form abreakwater at the base. Hence, under ordinary circumstances, Iconclude that for a cliff 500 feet in height, a denudation of one inchper century for the whole length would be an ample allowance. At thisrate, on the above data, the denudation of the Weald must haverequired 306,662,400 years; or say three hundred million years.
The action of fresh water on the gently inclined Wealden district,when upraised, could hardly have been great, but it would somewhatreduce the above estimate. On the other hand, during oscillations oflevel, which we know this area has undergone, the surface may haveexisted for millions of years as land, and thus have escaped theaction of the sea: when deeply submerged for perhaps equally longperiods, it would, likewise, have escaped the action of thecoast-waves. So that in all probability a far longer period than 300million years has elapsed since the latter part of the Secondaryperiod.
I have made these few remarks because it is highly important for us togain some notion, however imperfect, of the lapse of years. Duringeach of these years, over the whole world, the land and the water hasbeen peopled by hosts of living forms. What an infinite number ofgenerations, which the mind cannot grasp, must have succeeded eachother in the long roll of years! Now turn to our richest geologicalmuseums, and what a paltry display we behold!
ON THE POORNESS OF OUR PALAEONTOLOGICAL COLLECTIONS.
That our palaeontological collections are very imperfect, is admittedby every one. The remark of that admirable palaeontologist, the lateEdward Forbes, should not be forgotten, namely, that numbers of ourfossil species are known and named from single and often brokenspecimens, or from a few specimens collected on some one spot. Only asmall portion of the surface of the earth has been geologicallyexplored, and no part with sufficient care, as the importantdiscoveries made every year in Europe prove. No organism wholly softcan be preserved. Shells and bones will decay and disappear when lefton the bottom of the sea, where sediment is not accumulating. Ibelieve we are continually taking a most erroneous view, when wetacitly admit to ourselves that sediment is being deposited overnearly the whole bed of the sea, at a rate sufficiently quick to embedand preserve fossil remains. Throughout an enormously large proportionof the ocean, the bright blue tint of the water bespeaks its purity.The many cases on record of a formation conformably covered, after anenormous interval of time, by another and later formation, without theunderlying bed having suffered in the interval any wear and tear, seemexplicable only on the view of the bottom of the sea not rarely lyingfor ages in an unaltered condition. The remains which do becomeembedded, if in sand or gravel, will when the beds are upraisedgenerally be dissolved by the percolation of rain-water. I suspectthat but few of the very many animals which live on the beach betweenhigh and low watermark are preserved. For instance, the severalspecies of the Chthamalinae (a sub-family of sessile cirripedes) coatthe rocks all over the world in infinite numbers: they are allstrictly littoral, with the exception of a single Mediterraneanspecies, which inhabits deep water and has been found fossil inSicily, whereas not one other species has hitherto been found in anytertiary formation: yet it is now known that the genus Chthamalusexisted during the chalk period. The molluscan genus Chiton offers apartially analogous case.
With respect to the terrestrial productions which lived during theSecondary and Palaeozoic periods, it is superfluous to state that ourevidence from fossil remains is fragmentary in an extreme degree. Forinstance, not a land shell is known belonging to either of these vastperiods, with one exception discovered by Sir C. Lyell in thecarboniferous strata of North America. In regard to mammiferousremains, a single glance at the historical table published in theSupplement to Lyell's Manual, will bring home the truth, howaccidental and rare is their preservation, far better than pages ofdetail. Nor is their rarity surprising, when we remember how large aproportion of the bones of tertiary mammals have been discoveredeither in caves or in lacustrine deposits; and that not a cave or truelacustrine bed is known belonging to the age of our secondary orpalaeozoic formations.
But the imperfection in the geological record mainly results fromanother and more important cause than any of the foregoing; namely,from the several formations being separated from each other by wideintervals of time. When we see the formations tabulated in writtenworks, or when we follow them in nature, it is difficult to avoidbelieving that they are closely consecutive. But we know, forinstance, from Sir R. Murchison's great work on Russia, what wide gapsthere are in that country between the superimposed formations; so itis in North America, and in many other parts of the world. The mostskilful geologist, if his attention had been exclusively confined tothese large territories, would never have suspected that during theperiods which were blank and barren in his own country, great piles ofsediment, charged with new and peculiar forms of life, had elsewherebeen accumulated. And if in each separate territory, hardly any ideacan be formed of the length of time which has elapsed between theconsecutive formations, we may infer that this could nowhere beascertained. The frequent and great changes in the mineralogicalcomposition of consecutive formations, generally implying greatchanges in the geography of the surrounding lands, whence the sedimenthas been derived, accords with the belief of vast intervals of timehaving elapsed between each formation.