物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 10. ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS. Page 1
On the slow and successive appearance of new species.On their different rates of change.Species once lost do not reappear.Groups of species follow the same general rules in their appearanceand disappearance as do single species.On Extinction.On simultaneous changes in the forms of life throughout the world.On the affinities of extinct species to each other and to livingspecies.On the state of development of ancient forms.On the succession of the same types within the same areas.Summary of preceding and present chapters.
Let us now see whether the several facts and rules relating to thegeological succession of organic beings, better accord with the commonview of the immutability of species, or with that of their slow andgradual modification, through descent and natural selection.
New species have appeared very slowly, one after another, both on theland and in the waters. Lyell has shown that it is hardly possible toresist the evidence on this head in the case of the several tertiarystages; and every year tends to fill up the blanks between them, andto make the percentage system of lost and new forms more gradual. Insome of the most recent beds, though undoubtedly of high antiquity ifmeasured by years, only one or two species are lost forms, and onlyone or two are new forms, having here appeared for the first time,either locally, or, as far as we know, on the face of the earth. If wemay trust the observations of Philippi in Sicily, the successivechanges in the marine inhabitants of that island have been many andmost gradual. The secondary formations are more broken; but, as Bronnhas remarked, neither the appearance nor disappearance of their manynow extinct species has been simultaneous in each separate formation.
Species of different genera and classes have not changed at the samerate, or in the same degree. In the oldest tertiary beds a few livingshells may still be found in the midst of a multitude of extinctforms. Falconer has given a striking instance of a similar fact, in anexisting crocodile associated with many strange and lost mammals andreptiles in the sub-Himalayan deposits. The Silurian Lingula differsbut little from the living species of this genus; whereas most of theother Silurian Molluscs and all the Crustaceans have changed greatly.The productions of the land seem to change at a quicker rate thanthose of the sea, of which a striking instance has lately beenobserved in Switzerland. There is some reason to believe thatorganisms, considered high in the scale of nature, change more quicklythan those that are low: though there are exceptions to this rule. Theamount of organic change, as Pictet has remarked, does not strictlycorrespond with the succession of our geological formations; so thatbetween each two consecutive formations, the forms of life have seldomchanged in exactly the same degree. Yet if we compare any but the mostclosely related formations, all the species will be found to haveundergone some change. When a species has once disappeared from theface of the earth, we have reason to believe that the same identicalform never reappears. The strongest apparent exception to this latterrule, is that of the so-called "colonies" of M. Barrande, whichintrude for a period in the midst of an older formation, and thenallow the pre-existing fauna to reappear; but Lyell's explanation,namely, that it is a case of temporary migration from a distinctgeographical province, seems to me satisfactory.
These several facts accord well with my theory. I believe in no fixedlaw of development, causing all the inhabitants of a country to changeabruptly, or simultaneously, or to an equal degree. The process ofmodification must be extremely slow. The variability of each speciesis quite independent of that of all others. Whether such variabilitybe taken advantage of by natural selection, and whether the variationsbe accumulated to a greater or lesser amount, thus causing a greateror lesser amount of modification in the varying species, depends onmany complex contingencies,--on the variability being of a beneficialnature, on the power of intercrossing, on the rate of breeding, on theslowly changing physical conditions of the country, and moreespecially on the nature of the other inhabitants with which thevarying species comes into competition. Hence it is by no meanssurprising that one species should retain the same identical form muchlonger than others; or, if changing, that it should change less. Wesee the same fact in geographical distribution; for instance, in theland-shells and coleopterous insects of Madeira having come to differconsiderably from their nearest allies on the continent of Europe,whereas the marine shells and birds have remained unaltered. We canperhaps understand the apparently quicker rate of change interrestrial and in more highly organised productions compared withmarine and lower productions, by the more complex relations of thehigher beings to their organic and inorganic conditions of life, asexplained in a former chapter. When many of the inhabitants of acountry have become modified and improved, we can understand, on theprinciple of competition, and on that of the many all-importantrelations of organism to organism, that any form which does not becomein some degree modified and improved, will be liable to beexterminated. Hence we can see why all the species in the same regiondo at last, if we look to wide enough intervals of time, becomemodified; for those which do not change will become extinct.
In members of the same class the average amount of change, during longand equal periods of time, may, perhaps, be nearly the same; but asthe accumulation of long-enduring fossiliferous formations depends ongreat masses of sediment having been deposited on areas whilstsubsiding, our formations have been almost necessarily accumulated atwide and irregularly intermittent intervals; consequently the amountof organic change exhibited by the fossils embedded in consecutiveformations is not equal. Each formation, on this view, does not mark anew and complete act of creation, but only an occasional scene, takenalmost at hazard, in a slowly changing drama.
We can clearly understand why a species when once lost should neverreappear, even if the very same conditions of life, organic andinorganic, should recur. For though the offspring of one species mightbe adapted (and no doubt this has occurred in innumerable instances)to fill the exact place of another species in the economy of nature,and thus supplant it; yet the two forms--the old and the new--wouldnot be identically the same; for both would almost certainly inheritdifferent characters from their distinct progenitors. For instance, itis just possible, if our fantail-pigeons were all destroyed, thatfanciers, by striving during long ages for the same object, might makea new breed hardly distinguishable from our present fantail; but ifthe parent rock-pigeon were also destroyed, and in nature we haveevery reason to believe that the parent-form will generally besupplanted and exterminated by its improved offspring, it is quiteincredible that a fantail, identical with the existing breed, could beraised from any other species of pigeon, or even from the otherwell-established races of the domestic pigeon, for the newly-formedfantail would be almost sure to inherit from its new progenitor someslight characteristic differences.
Groups of species, that is, genera and families, follow the samegeneral rules in their appearance and disappearance as do singlespecies, changing more or less quickly, and in a greater or lesserdegree. A group does not reappear after it has once disappeared; orits existence, as long as it lasts, is continuous. I am aware thatthere are some apparent exceptions to this rule, but the exceptionsare surprisingly few, so few, that E. Forbes, Pictet, and Woodward(though all strongly opposed to such views as I maintain) admit itstruth; and the rule strictly accords with my theory. For as all thespecies of the same group have descended from some one species, it isclear that as long as any species of the group have appeared in thelong succession of ages, so long must its members have continuouslyexisted, in order to have generated either new and modified or thesame old and unmodified forms. Species of the genus Lingula, forinstance, must have continuously existed by an unbroken succession ofgenerations, from the lowest Silurian stratum to the present day.
We have seen in the last chapter that the species of a group sometimesfalsely appear to have come in abruptly; and I have attempted to givean explanation of this fact, which if true would have been fatal to myviews. But such cases are certainly exceptional; the general rulebeing a gradual increase in number, till the group reaches itsmaximum, and then, sooner or later, it gradually decreases. If thenumber of the species of a genus, or the number of the genera of afamily, be represented by a vertical line of varying thickness,crossing the successive geological formations in which the species arefound, the line will sometimes falsely appear to begin at its lowerend, not in a sharp point, but abruptly; it then gradually thickensupwards, sometimes keeping for a space of equal thickness, andultimately thins out in the upper beds, marking the decrease and finalextinction of the species. This gradual increase in number of thespecies of a group is strictly conformable with my theory; as thespecies of the same genus, and the genera of the same family, canincrease only slowly and progressively; for the process ofmodification and the production of a number of allied forms must beslow and gradual,--one species giving rise first to two or threevarieties, these being slowly converted into species, which in theirturn produce by equally slow steps other species, and so on, like thebranching of a great tree from a single stem, till the group becomeslarge.
ON EXTINCTION.
We have as yet spoken only incidentally of the disappearance ofspecies and of groups of species. On the theory of natural selectionthe extinction of old forms and the production of new and improvedforms are intimately connected together. The old notion of all theinhabitants of the earth having been swept away at successive periodsby catastrophes, is very generally given up, even by those geologists,as Elie de Beaumont, Murchison, Barrande, etc., whose general viewswould naturally lead them to this conclusion. On the contrary, we haveevery reason to believe, from the study of the tertiary formations,that species and groups of species gradually disappear, one afteranother, first from one spot, then from another, and finally from theworld. Both single species and whole groups of species last for veryunequal periods; some groups, as we have seen, having endured from theearliest known dawn of life to the present day; some havingdisappeared before the close of the palaeozoic period. No fixed lawseems to determine the length of time during which any single speciesor any single genus endures. There is reason to believe that thecomplete extinction of the species of a group is generally a slowerprocess than their production: if the appearance and disappearance ofa group of species be represented, as before, by a vertical line ofvarying thickness, the line is found to taper more gradually at itsupper end, which marks the progress of extermination, than at itslower end, which marks the first appearance and increase in numbers ofthe species. In some cases, however, the extermination of whole groupsof beings, as of ammonites towards the close of the secondary period,has been wonderfully sudden.
species generally become rare before they becomeextinct--to feel no surprise at the rarity of a species, and yet tomarvel greatly when it ceases to exist, is much the same as to admitthat sickness in the individual is the forerunner of death--to feel nosurprise !
The whole subject of the extinction of species has been involved inthe most gratuitous mystery. Some authors have even supposed that asthe individual has a definite length of life, so have species adefinite duration. No one I think can have marvelled more at theextinction of species, than I have done. When I found in La Plata thetooth of a horse embedded with the remains of Mastodon, Megatherium,Toxodon, and other extinct monsters, which all co-existed with stillliving shells at a very late geological period, I was filled withastonishment; for seeing that the horse, since its introduction by theSpaniards into South America, has run wild over the whole country andhas increased in numbers at an unparalleled rate, I asked myself whatcould so recently have exterminated the former horse under conditionsof life apparently so favourable. But how utterly groundless was myastonishment! Professor Owen soon perceived that the tooth, though solike that of the existing horse, belonged to an extinct species. Hadthis horse been still living, but in some degree rare, no naturalistwould have felt the least surprise at its rarity; for rarity is theattribute of a vast number of species of all classes, in allcountries. If we ask ourselves why this or that species is rare, weanswer that something is unfavourable in its conditions of life; butwhat that something is, we can hardly ever tell. On the supposition ofthe fossil horse still existing as a rare species, we might have feltcertain from the analogy of all other mammals, even of theslow-breeding elephant, and from the history of the naturalisation ofthe domestic horse in South America, that under more favourableconditions it would in a very few years have stocked the wholecontinent. But we could not have told what the unfavourable conditionswere which checked its increase, whether some one or severalcontingencies, and at what period of the horse's life, and in whatdegree, they severally acted. If the conditions had gone on, howeverslowly, becoming less and less favourable, we assuredly should nothave perceived the fact, yet the fossil horse would certainly havebecome rarer and rarer, and finally extinct;--its place being seizedon by some more successful competitor.
It is most difficult always to remember that the increase of everyliving being is constantly being checked by unperceived injuriousagencies; and that these same unperceived agencies are amplysufficient to cause rarity, and finally extinction. We see in manycases in the more recent tertiary formations, that rarity precedesextinction; and we know that this has been the progress of events withthose animals which have been exterminated, either locally or wholly,through man's agency. I may repeat what I published in 1845, namely,that to admit that species generally become rare before they becomeextinct--to feel no surprise at the rarity of a species, and yet tomarvel greatly when it ceases to exist, is much the same as to admitthat sickness in the individual is the forerunner of death--to feel nosurprise at sickness, but when the sick man dies, to wonder and tosuspect that he died by some unknown deed of violence.
The theory of natural selection is grounded on the belief that eachnew variety, and ultimately each new species, is produced andmaintained by having some advantage over those with which it comesinto competition; and the consequent extinction of less-favoured formsalmost inevitably follows. It is the same with our domesticproductions: when a new and slightly improved variety has been raised,it at first supplants the less improved varieties in the sameneighbourhood; when much improved it is transported far and near, likeour short-horn cattle, and takes the place of other breeds in othercountries. Thus the appearance of new forms and the disappearance ofold forms, both natural and artificial, are bound together. In certainflourishing groups, the number of new specific forms which have beenproduced within a given time is probably greater than that of the oldforms which have been exterminated; but we know that the number ofspecies has not gone on indefinitely increasing, at least during thelater geological periods, so that looking to later times we maybelieve that the production of new forms has caused the extinction ofabout the same number of old forms.
The competition will generally be most severe, as formerly explainedand illustrated by examples, between the forms which are most likeeach other in all respects. Hence the improved and modifieddescendants of a species will generally cause the extermination of theparent-species; and if many new forms have been developed from any onespecies, the nearest allies of that species, i.e. the species of thesame genus, will be the most liable to extermination. Thus, as Ibelieve, a number of new species descended from one species, that is anew genus, comes to supplant an old genus, belonging to the samefamily. But it must often have happened that a new species belongingto some one group will have seized on the place occupied by a speciesbelonging to a distinct group, and thus caused its extermination; andif many allied forms be developed from the successful intruder, manywill have to yield their places; and it will generally be alliedforms, which will suffer from some inherited inferiority in common.But whether it be species belonging to the same or to a distinctclass, which yield their places to other species which have beenmodified and improved, a few of the sufferers may often long bepreserved, from being fitted to some peculiar line of life, or frominhabiting some distant and isolated station, where they have escapedsevere competition. For instance, a single species of Trigonia, agreat genus of shells in the secondary formations, survives in theAustralian seas; and a few members of the great and almost extinctgroup of Ganoid fishes still inhabit our fresh waters. Therefore theutter extinction of a group is generally, as we have seen, a slowerprocess than its production.
With respect to the apparently sudden extermination of whole familiesor orders, as of Trilobites at the close of the palaeozoic period andof Ammonites at the close of the secondary period, we must rememberwhat has been already said on the probable wide intervals of timebetween our consecutive formations; and in these intervals there mayhave been much slow extermination. Moreover, when by suddenimmigration or by unusually rapid development, many species of a newgroup have taken possession of a new area, they will have exterminatedin a correspondingly rapid manner many of the old inhabitants; and theforms which thus yield their places will commonly be allied, for theywill partake of some inferiority in common.
Thus, as it seems to me, the manner in which single species and wholegroups of species become extinct, accords well with the theory ofnatural selection. We need not marvel at extinction; if we mustmarvel, let it be at our presumption in imagining for a moment that weunderstand the many complex contingencies, on which the existence ofeach species depends. If we forget for an instant, that each speciestends to increase inordinately, and that some check is always inaction, yet seldom perceived by us, the whole economy of nature willbe utterly obscured. Whenever we can precisely say why this species ismore abundant in individuals than that; why this species and notanother can be naturalised in a given country; then, and not tillthen, we may justly feel surprise why we cannot account for theextinction of this particular species or group of species.