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 3
In nature the case will be far more complicated than is represented inthe diagram; for the groups will have been more numerous, they willhave endured for extremely unequal lengths of time, and will have beenmodified in various degrees. As we possess only the last volume of thegeological record, and that in a very broken condition, we have noright to expect, except in very rare cases, to fill up wide intervalsin the natural system, and thus unite distinct families or orders. Allthat we have a right to expect, is that those groups, which havewithin known geological periods undergone much modification, should inthe older formations make some slight approach to each other; so thatthe older members should differ less from each other in some of theircharacters than do the existing members of the same groups; and thisby the concurrent evidence of our best palaeontologists seemsfrequently to be the case.
Thus, on the theory of descent with modification, the main facts withrespect to the mutual affinities of the extinct forms of life to eachother and to living forms, seem to me explained in a satisfactorymanner. And they are wholly inexplicable on any other view.
On this same theory, it is evident that the fauna of any great periodin the earth's history will be intermediate in general characterbetween that which preceded and that which succeeded it. Thus, thespecies which lived at the sixth great stage of descent in the diagramare the modified offspring of those which lived at the fifth stage,and are the parents of those which became still more modified at theseventh stage; hence they could hardly fail to be nearly intermediatein character between the forms of life above and below. We must,however, allow for the entire extinction of some preceding forms, andfor the coming in of quite new forms by immigration, and for a largeamount of modification, during the long and blank intervals betweenthe successive formations. Subject to these allowances, the fauna ofeach geological period undoubtedly is intermediate in character,between the preceding and succeeding faunas. I need give only oneinstance, namely, the manner in which the fossils of the Devoniansystem, when this system was first discovered, were at once recognisedby palaeontologists as intermediate in character between those of theoverlying carboniferous, and underlying Silurian system. But eachfauna is not necessarily exactly intermediate, as unequal intervals oftime have elapsed between consecutive formations.
It is no real objection to the truth of the statement, that the faunaof each period as a whole is nearly intermediate in character betweenthe preceding and succeeding faunas, that certain genera offerexceptions to the rule. For instance, mastodons and elephants, whenarranged by Dr. Falconer in two series, first according to theirmutual affinities and then according to their periods of existence, donot accord in arrangement. The species extreme in character are notthe oldest, or the most recent; nor are those which are intermediatein character, intermediate in age. But supposing for an instant, inthis and other such cases, that the record of the first appearance anddisappearance of the species was perfect, we have no reason to believethat forms successively produced necessarily endure for correspondinglengths of time: a very ancient form might occasionally last muchlonger than a form elsewhere subsequently produced, especially in thecase of terrestrial productions inhabiting separated districts. Tocompare small things with great: if the principal living and extinctraces of the domestic pigeon were arranged as well as they could be inserial affinity, this arrangement would not closely accord with theorder in time of their production, and still less with the order oftheir disappearance; for the parent rock-pigeon now lives; and manyvarieties between the rock-pigeon and the carrier have become extinct;and carriers which are extreme in the important character of length ofbeak originated earlier than short-beaked tumblers, which are at theopposite end of the series in this same respect.
Closely connected with the statement, that the organic remains from anintermediate formation are in some degree intermediate in character,is the fact, insisted on by all palaeontologists, that fossils fromtwo consecutive formations are far more closely related to each other,than are the fossils from two remote formations. Pictet gives as awell-known instance, the general resemblance of the organic remainsfrom the several stages of the chalk formation, though the species aredistinct in each stage. This fact alone, from its generality, seems tohave shaken Professor Pictet in his firm belief in the immutability ofspecies. He who is acquainted with the distribution of existingspecies over the globe, will not attempt to account for the closeresemblance of the distinct species in closely consecutive formations,by the physical conditions of the ancient areas having remained nearlythe same. Let it be remembered that the forms of life, at least thoseinhabiting the sea, have changed almost simultaneously throughout theworld, and therefore under the most different climates and conditions.Consider the prodigious vicissitudes of climate during the pleistoceneperiod, which includes the whole glacial period, and note how littlethe specific forms of the inhabitants of the sea have been affected.
On the theory of descent, the full meaning of the fact of fossilremains from closely consecutive formations, though ranked as distinctspecies, being closely related, is obvious. As the accumulation ofeach formation has often been interrupted, and as long blank intervalshave intervened between successive formations, we ought not to expectto find, as I attempted to show in the last chapter, in any one or twoformations all the intermediate varieties between the species whichappeared at the commencement and close of these periods; but we oughtto find after intervals, very long as measured by years, but onlymoderately long as measured geologically, closely allied forms, or, asthey have been called by some authors, representative species; andthese we assuredly do find. We find, in short, such evidence of theslow and scarcely sensible mutation of specific forms, as we have ajust right to expect to find.
ON THE STATE OF DEVELOPMENT OF ANCIENT FORMS.
There has been much discussion whether recent forms are more highlydeveloped than ancient. I will not here enter on this subject, fornaturalists have not as yet defined to each other's satisfaction whatis meant by high and low forms. But in one particular sense the morerecent forms must, on my theory, be higher than the more ancient; foreach new species is formed by having had some advantage in thestruggle for life over other and preceding forms. If under a nearlysimilar climate, the eocene inhabitants of one quarter of the worldwere put into competition with the existing inhabitants of the same orsome other quarter, the eocene fauna or flora would certainly bebeaten and exterminated; as would a secondary fauna by an eocene, anda palaeozoic fauna by a secondary fauna. I do not doubt that thisprocess of improvement has affected in a marked and sensible mannerthe organisation of the more recent and victorious forms of life, incomparison with the ancient and beaten forms; but I can see no way oftesting this sort of progress. Crustaceans, for instance, not thehighest in their own class, may have beaten the highest molluscs. Fromthe extraordinary manner in which European productions have recentlyspread over New Zealand, and have seized on places which must havebeen previously occupied, we may believe, if all the animals andplants of Great Britain were set free in New Zealand, that in thecourse of time a multitude of British forms would become thoroughlynaturalized there, and would exterminate many of the natives. On theother hand, from what we see now occurring in New Zealand, and fromhardly a single inhabitant of the southern hemisphere having becomewild in any part of Europe, we may doubt, if all the productions ofNew Zealand were set free in Great Britain, whether any considerablenumber would be enabled to seize on places now occupied by our nativeplants and animals. Under this point of view, the productions of GreatBritain may be said to be higher than those of New Zealand. Yet themost skilful naturalist from an examination of the species of the twocountries could not have foreseen this result.
Agassiz insists that ancient animals resemble to a certain extent theembryos of recent animals of the same classes; or that the geologicalsuccession of extinct forms is in some degree parallel to theembryological development of recent forms. I must follow Pictet andHuxley in thinking that the truth of this doctrine is very far fromproved. Yet I fully expect to see it hereafter confirmed, at least inregard to subordinate groups, which have branched off from each otherwithin comparatively recent times. For this doctrine of Agassizaccords well with the theory of natural selection. In a future chapterI shall attempt to show that the adult differs from its embryo, owingto variations supervening at a not early age, and being inherited at acorresponding age. This process, whilst it leaves the embryo almostunaltered, continually adds, in the course of successive generations,more and more difference to the adult.
TYPES WITHIN THE SAME AREAS, DURING THELATER TERTIARY PERIODS.groups; and thisby the concurrent evidence of our best palaeontologists seemsfrequently?
Thus the embryo comes to be left as a sort of picture, preserved bynature, of the ancient and less modified condition of each animal.This view may be true, and yet it may never be capable of full proof.Seeing, for instance, that the oldest known mammals, reptiles, andfish strictly belong to their own proper classes, though some of theseold forms are in a slight degree less distinct from each other thanare the typical members of the same groups at the present day, itwould be vain to look for animals having the common embryologicalcharacter of the Vertebrata, until beds far beneath the lowestSilurian strata are discovered--a discovery of which the chance isvery small.
ON THE SUCCESSION OF THE SAME TYPES WITHIN THE SAME AREAS, DURING THELATER TERTIARY PERIODS.
Now what does this remarkable law of the succession of the same typeswithin the same areas mean? He would be a bold man, who aftercomparing the present climate of Australia and of parts of SouthAmerica under the same latitude, would attempt to account, on the onehand, by dissimilar physical conditions for the dissimilarity of theinhabitants of these two continents, and, on the other hand, bysimilarity of conditions, for the uniformity of the same types in eachduring the later tertiary periods. Nor can it be pretended that it isan immutable law that marsupials should have been chiefly or solelyproduced in Australia; or that Edentata and other American typesshould have been solely produced in South America. For we know thatEurope in ancient times was peopled by numerous marsupials; and I haveshown in the publications above alluded to, that in America the law ofdistribution of terrestrial mammals was formerly different from whatit now is. North America formerly partook strongly of the presentcharacter of the southern half of the continent; and the southern halfwas formerly more closely allied, than it is at present, to thenorthern half. In a similar manner we know from Falconer and Cautley'sdiscoveries, that northern India was formerly more closely related inits mammals to Africa than it is at the present time. Analogous factscould be given in relation to the distribution of marine animals.
On the theory of descent with modification, the great law of the longenduring, but not immutable, succession of the same types within thesame areas, is at once explained; for the inhabitants of each quarterof the world will obviously tend to leave in that quarter, during thenext succeeding period of time, closely allied though in some degreemodified descendants. If the inhabitants of one continent formerlydiffered greatly from those of another continent, so will theirmodified descendants still differ in nearly the same manner anddegree. But after very long intervals of time and after greatgeographical changes, permitting much inter-migration, the feeblerwill yield to the more dominant forms, and there will be nothingimmutable in the laws of past and present distribution.
It may be asked in ridicule, whether I suppose that the megatheriumand other allied huge monsters have left behind them in South Americathe sloth, armadillo, and anteater, as their degenerate descendants.This cannot for an instant be admitted. These huge animals have becomewholly extinct, and have left no progeny. But in the caves of Brazil,there are many extinct species which are closely allied in size and inother characters to the species still living in South America; andsome of these fossils may be the actual progenitors of living species.It must not be forgotten that, on my theory, all the species of thesame genus have descended from some one species; so that if sixgenera, each having eight species, be found in one geologicalformation, and in the next succeeding formation there be six otherallied or representative genera with the same number of species, thenwe may conclude that only one species of each of the six older generahas left modified descendants, constituting the six new genera. Theother seven species of the old genera have all died out and have leftno progeny. Or, which would probably be a far commoner case, two orthree species of two or three alone of the six older genera will havebeen the parents of the six new genera; the other old species and theother whole genera having become utterly extinct. In failing orders,with the genera and species decreasing in numbers, as apparently isthe case of the Edentata of South America, still fewer genera andspecies will have left modified blood-descendants.
SUMMARY OF THE PRECEDING AND PRESENT CHAPTERS.
I have attempted to show that the geological record is extremelyimperfect; that only a small portion of the globe has beengeologically explored with care; that only certain classes of organicbeings have been largely preserved in a fossil state; that the numberboth of specimens and of species, preserved in our museums, isabsolutely as nothing compared with the incalculable number ofgenerations which must have passed away even during a singleformation; that, owing to subsidence being necessary for theaccumulation of fossiliferous deposits thick enough to resist futuredegradation, enormous intervals of time have elapsed between thesuccessive formations; that there has probably been more extinctionduring the periods of subsidence, and more variation during theperiods of elevation, and during the latter the record will have beenleast perfectly kept; that each single formation has not beencontinuously deposited; that the duration of each formation is,perhaps, short compared with the average duration of specific forms;that migration has played an important part in the first appearance ofnew forms in any one area and formation; that widely ranging speciesare those which have varied most, and have oftenest given rise to newspecies; and that varieties have at first often been local. All thesecauses taken conjointly, must have tended to make the geologicalrecord extremely imperfect, and will to a large extent explain why wedo not find interminable varieties, connecting together all theextinct and existing forms of life by the finest graduated steps.
He who rejects these views on the nature of the geological record,will rightly reject my whole theory. For he may ask in vain where arethe numberless transitional links which must formerly have connectedthe closely allied or representative species, found in the severalstages of the same great formation. He may disbelieve in the enormousintervals of time which have elapsed between our consecutiveformations; he may overlook how important a part migration must haveplayed, when the formations of any one great region alone, as that ofEurope, are considered; he may urge the apparent, but often falselyapparent, sudden coming in of whole groups of species. He may askwhere are the remains of those infinitely numerous organisms whichmust have existed long before the first bed of the Silurian system wasdeposited: I can answer this latter question only hypothetically, bysaying that as far as we can see, where our oceans now extend theyhave for an enormous period extended, and where our oscillatingcontinents now stand they have stood ever since the Silurian epoch;but that long before that period, the world may have presented awholly different aspect; and that the older continents, formed offormations older than any known to us, may now all be in ametamorphosed condition, or may lie buried under the ocean.
Passing from these difficulties, all the other great leading facts inpalaeontology seem to me simply to follow on the theory of descentwith modification through natural selection. We can thus understandhow it is that new species come in slowly and successively; howspecies of different classes do not necessarily change together, or atthe same rate, or in the same degree; yet in the long run that allundergo modification to some extent. The extinction of old forms isthe almost inevitable consequence of the production of new forms. Wecan understand why when a species has once disappeared it neverreappears. Groups of species increase in numbers slowly, and endurefor unequal periods of time; for the process of modification isnecessarily slow, and depends on many complex contingencies. Thedominant species of the larger dominant groups tend to leave manymodified descendants, and thus new sub-groups and groups are formed.As these are formed, the species of the less vigorous groups, fromtheir inferiority inherited from a common progenitor, tend to becomeextinct together, and to leave no modified offspring on the face ofthe earth. But the utter extinction of a whole group of species mayoften be a very slow process, from the survival of a few descendants,lingering in protected and isolated situations. When a group has oncewholly disappeared, it does not reappear; for the link of generationhas been broken.
We can understand how the spreading of the dominant forms of life,which are those that oftenest vary, will in the long run tend topeople the world with allied, but modified, descendants; and thesewill generally succeed in taking the places of those groups of specieswhich are their inferiors in the struggle for existence. Hence, afterlong intervals of time, the productions of the world will appear tohave changed simultaneously.
We can understand how it is that all the forms of life, ancient andrecent, make together one grand system; for all are connected bygeneration. We can understand, from the continued tendency todivergence of character, why the more ancient a form is, the more itgenerally differs from those now living. Why ancient and extinct formsoften tend to fill up gaps between existing forms, sometimes blendingtwo groups previously classed as distinct into one; but more commonlyonly bringing them a little closer together. The more ancient a formis, the more often, apparently, it displays characters in some degreeintermediate between groups now distinct; for the more ancient a formis, the more nearly it will be related to, and consequently resemble,the common progenitor of groups, since become widely divergent.Extinct forms are seldom directly intermediate between existing forms;but are intermediate only by a long and circuitous course through manyextinct and very different forms. We can clearly see why the organicremains of closely consecutive formations are more closely allied toeach other, than are those of remote formations; for the forms aremore closely linked together by generation: we can clearly see why theremains of an intermediate formation are intermediate in character.
The inhabitants of each successive period in the world's history havebeaten their predecessors in the race for life, and are, in so far,higher in the scale of nature; and this may account for that vague yetill-defined sentiment, felt by many palaeontologists, thatorganisation on the whole has progressed. If it should hereafter beproved that ancient animals resemble to a certain extent the embryosof more recent animals of the same class, the fact will beintelligible. The succession of the same types of structure within thesame areas during the later geological periods ceases to bemysterious, and is simply explained by inheritance.
continued tendency todivergence of character, why the more ancient a form is, the more itgenerally differs from those now living. Why ancient and extinct formsoften tend to fill up gaps between existing forms, sometimes!
If then the geological record be as imperfect as I believe it to be,and it may at least be asserted that the record cannot be proved to bemuch more perfect, the main objections to the theory of naturalselection are greatly diminished or disappear. On the other hand, allthe chief laws of palaeontology plainly proclaim, as it seems to me,that species have been produced by ordinary generation: old formshaving been supplanted by new and improved forms of life, produced bythe laws of variation still acting round us, and preserved by NaturalSelection.