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 2
ON THE FORMS OF LIFE CHANGING ALMOST SIMULTANEOUSLY THROUGHOUT THEWORLD.
Scarcely any palaeontological discovery is more striking than thefact, that the forms of life change almost simultaneously throughoutthe world. Thus our European Chalk formation can be recognised in manydistant parts of the world, under the most different climates, wherenot a fragment of the mineral chalk itself can be found; namely, inNorth America, in equatorial South America, in Tierra del Fuego, atthe Cape of Good Hope, and in the peninsula of India. For at thesedistant points, the organic remains in certain beds present anunmistakeable degree of resemblance to those of the Chalk. It is notthat the same species are met with; for in some cases not one speciesis identically the same, but they belong to the same families, genera,and sections of genera, and sometimes are similarly characterised insuch trifling points as mere superficial sculpture. Moreover otherforms, which are not found in the Chalk of Europe, but which occur inthe formations either above or below, are similarly absent at thesedistant points of the world. In the several successive palaeozoicformations of Russia, Western Europe and North America, a similarparallelism in the forms of life has been observed by several authors:so it is, according to Lyell, with the several European and NorthAmerican tertiary deposits. Even if the few fossil species which arecommon to the Old and New Worlds be kept wholly out of view, thegeneral parallelism in the successive forms of life, in the stages ofthe widely separated palaeozoic and tertiary periods, would still bemanifest, and the several formations could be easily correlated.
These observations, however, relate to the marine inhabitants ofdistant parts of the world: we have not sufficient data to judgewhether the productions of the land and of fresh water change atdistant points in the same parallel manner. We may doubt whether theyhave thus changed: if the Megatherium, Mylodon, Macrauchenia, andToxodon had been brought to Europe from La Plata, without anyinformation in regard to their geological position, no one would havesuspected that they had coexisted with still living sea-shells; but asthese anomalous monsters coexisted with the Mastodon and Horse, itmight at least have been inferred that they had lived during one ofthe latter tertiary stages.
When the marine forms of life are spoken of as having changedsimultaneously throughout the world, it must not be supposed that thisexpression relates to the same thousandth or hundred-thousandth year,or even that it has a very strict geological sense; for if all themarine animals which live at the present day in Europe, and all thosethat lived in Europe during the pleistocene period (an enormouslyremote period as measured by years, including the whole glacialepoch), were to be compared with those now living in South America orin Australia, the most skilful naturalist would hardly be able to saywhether the existing or the pleistocene inhabitants of Europeresembled most closely those of the southern hemisphere. So, again,several highly competent observers believe that the existingproductions of the United States are more closely related to thosewhich lived in Europe during certain later tertiary stages, than tothose which now live here; and if this be so, it is evident thatfossiliferous beds deposited at the present day on the shores of NorthAmerica would hereafter be liable to be classed with somewhat olderEuropean beds. Nevertheless, looking to a remotely future epoch, therecan, I think, be little doubt that all the more modern MARINEformations, namely, the upper pliocene, the pleistocene and strictlymodern beds, of Europe, North and South America, and Australia, fromcontaining fossil remains in some degree allied, and from notincluding those forms which are only found in the older underlyingdeposits, would be correctly ranked as simultaneous in a geologicalsense.
The fact of the forms of life changing simultaneously, in the abovelarge sense, at distant parts of the world, has greatly struck thoseadmirable observers, MM. de Verneuil and d'Archiac. After referring tothe parallelism of the palaeozoic forms of life in various parts ofEurope, they add, "If struck by this strange sequence, we turn ourattention to North America, and there discover a series of analogousphenomena, it will appear certain that all these modifications ofspecies, their extinction, and the introduction of new ones, cannot beowing to mere changes in marine currents or other causes more or lesslocal and temporary, but depend on general laws which govern the wholeanimal kingdom." M. Barrande has made forcible remarks to preciselythe same effect. It is, indeed, quite futile to look to changes ofcurrents, climate, or other physical conditions, as the cause of thesegreat mutations in the forms of life throughout the world, under themost different climates. We must, as Barrande has remarked, look tosome special law. We shall see this more clearly when we treat of thepresent distribution of organic beings, and find how slight is therelation between the physical conditions of various countries, and thenature of their inhabitants.
This great fact of the parallel succession of the forms of lifethroughout the world, is explicable on the theory of naturalselection. New species are formed by new varieties arising, which havesome advantage over older forms; and those forms, which are alreadydominant, or have some advantage over the other forms in their owncountry, would naturally oftenest give rise to new varieties orincipient species; for these latter must be victorious in a stillhigher degree in order to be preserved and to survive. We havedistinct evidence on this head, in the plants which are dominant, thatis, which are commonest in their own homes, and are most widelydiffused, having produced the greatest number of new varieties. It isalso natural that the dominant, varying, and far-spreading species,which already have invaded to a certain extent the territories ofother species, should be those which would have the best chance ofspreading still further, and of giving rise in new countries to newvarieties and species. The process of diffusion may often be veryslow, being dependent on climatal and geographical changes, or onstrange accidents, but in the long run the dominant forms willgenerally succeed in spreading. The diffusion would, it is probable,be slower with the terrestrial inhabitants of distinct continents thanwith the marine inhabitants of the continuous sea. We might thereforeexpect to find, as we apparently do find, a less strict degree ofparallel succession in the productions of the land than of the sea.
Dominant species spreading from any region might encounter still moredominant species, and then their triumphant course, or even theirexistence, would cease. We know not at all precisely what are all theconditions most favourable for the multiplication of new and dominantspecies; but we can, I think, clearly see that a number ofindividuals, from giving a better chance of the appearance offavourable variations, and that severe competition with many alreadyexisting forms, would be highly favourable, as would be the power ofspreading into new territories. A certain amount of isolation,recurring at long intervals of time, would probably be alsofavourable, as before explained. One quarter of the world may havebeen most favourable for the production of new and dominant species onthe land, and another for those in the waters of the sea. If two greatregions had been for a long period favourably circumstanced in anequal degree, whenever their inhabitants met, the battle would beprolonged and severe; and some from one birthplace and some from theother might be victorious. But in the course of time, the formsdominant in the highest degree, wherever produced, would tendeverywhere to prevail. As they prevailed, they would cause theextinction of other and inferior forms; and as these inferior formswould be allied in groups by inheritance, whole groups would tendslowly to disappear; though here and there a single member might longbe enabled to survive.
Thus, as it seems to me, the parallel, and, taken in a large sense,simultaneous, succession of the same forms of life throughout theworld, accords well with the principle of new species having beenformed by dominant species spreading widely and varying; the newspecies thus produced being themselves dominant owing to inheritance,and to having already had some advantage over their parents or overother species; these again spreading, varying, and producing newspecies. The forms which are beaten and which yield their places tothe new and victorious forms, will generally be allied in groups, frominheriting some inferiority in common; and therefore as new andimproved groups spread throughout the world, old groups will disappearfrom the world; and the succession of forms in both ways willeverywhere tend to correspond.
There is one other remark connected with this subject worth making. Ihave given my reasons for believing that all our greater fossiliferousformations were deposited during periods of subsidence; and that blankintervals of vast duration occurred during the periods when the bed ofthe sea was either stationary or rising, and likewise when sedimentwas not thrown down quickly enough to embed and preserve organicremains. During these long and blank intervals I suppose that theinhabitants of each region underwent a considerable amount ofmodification and extinction, and that there was much migration fromother parts of the world. As we have reason to believe that largeareas are affected by the same movement, it is probable that strictlycontemporaneous formations have often been accumulated over very widespaces in the same quarter of the world; but we are far from havingany right to conclude that this has invariably been the case, and thatlarge areas have invariably been affected by the same movements. Whentwo formations have been deposited in two regions during nearly, butnot exactly the same period, we should find in both, from the causesexplained in the foregoing paragraphs, the same general succession inthe forms of life; but the species would not exactly correspond; forthere will have been a little more time in the one region than in theother for modification, extinction, and immigration.
I suspect that cases of this nature have occurred in Europe. Mr.Prestwich, in his admirable Memoirs on the eocene deposits of Englandand France, is able to draw a close general parallelism between thesuccessive stages in the two countries; but when he compares certainstages in England with those in France, although he finds in both acurious accordance in the numbers of the species belonging to the samegenera, yet the species themselves differ in a manner very difficultto account for, considering the proximity of the two areas,--unless,indeed, it be assumed that an isthmus separated two seas inhabited bydistinct, but contemporaneous, faunas. Lyell has made similarobservations on some of the later tertiary formations. Barrande, also,shows that there is a striking general parallelism in the successiveSilurian deposits of Bohemia and Scandinavia; nevertheless he finds asurprising amount of difference in the species. If the severalformations in these regions have not been deposited during the sameexact periods,--a formation in one region often corresponding with ablank interval in the other,--and if in both regions the species havegone on slowly changing during the accumulation of the severalformations and during the long intervals of time between them; in thiscase, the several formations in the two regions could be arranged inthe same order, in accordance with the general succession of the formof life, and the order would falsely appear to be strictly parallel;nevertheless the species would not all be the same in the apparentlycorresponding stages in the two regions.$ON THE AFFINITIES OF EXTINCT SPECIES TO EACH OTHER, AND TO LIVINGFORMS.
Let us now look to the mutual affinities of extinct and livingspecies. They all fall into one grand natural system; and this fact isat once explained on the principle of descent. The more ancient anyform is, the more, as a general rule, it differs from living forms.But, as Buckland long ago remarked, all fossils can be classed eitherin still existing groups, or between them. That the extinct forms oflife help to fill up the wide intervals between existing genera,families, and orders, cannot be disputed. For if we confine ourattention either to the living or to the extinct alone, the series isfar less perfect than if we combine both into one general system. Withrespect to the Vertebrata, whole pages could be filled with strikingillustrations from our great palaeontologist, Owen, showing howextinct animals fall in between existing groups. Cuvier ranked theRuminants and Pachyderms, as the two most distinct orders of mammals;but Owen has discovered so many fossil links, that he has had to alterthe whole classification of these two orders; and has placed certainpachyderms in the same sub-order with ruminants: for example, hedissolves by fine gradations the apparently wide difference betweenthe pig and the camel. In regard to the Invertebrata, Barrande, and ahigher authority could not be named, asserts that he is every daytaught that palaeozoic animals, though belonging to the same orders,families, or genera with those living at the present day, were not atthis early epoch limited in such distinct groups as they now are.
Some writers have objected to any extinct species or group of speciesbeing considered as intermediate between living species or groups. Ifby this term it is meant that an extinct form is directly intermediatein all its characters between two living forms, the objection isprobably valid. But I apprehend that in a perfectly naturalclassification many fossil species would have to stand between livingspecies, and some extinct genera between living genera, even betweengenera belonging to distinct families. The most common case,especially with respect to very distinct groups, such as fish andreptiles, seems to be, that supposing them to be distinguished at thepresent day from each other by a dozen characters, the ancient membersof the same two groups would be distinguished by a somewhat lessernumber of characters, so that the two groups, though formerly quitedistinct, at that period made some small approach to each other.
speciesbeing considered as intermediate between living species or groups. Ifby this term it is meant that an extinct form is directly intermediatein.
It is a common belief that the more ancient a form is, by so much themore it tends to connect by some of its characters groups now widelyseparated from each other. This remark no doubt must be restricted tothose groups which have undergone much change in the course ofgeological ages; and it would be difficult to prove the truth of theproposition, for every now and then even a living animal, as theLepidosiren, is discovered having affinities directed towards verydistinct groups. Yet if we compare the older Reptiles and Batrachians,the older Fish, the older Cephalopods, and the eocene Mammals, withthe more recent members of the same classes, we must admit that thereis some truth in the remark.
Let us see how far these several facts and inferences accord with thetheory of descent with modification. As the subject is somewhatcomplex, I must request the reader to turn to the diagram in thefourth chapter. We may suppose that the numbered letters representgenera, and the dotted lines diverging from them the species in eachgenus. The diagram is much too simple, too few genera and too fewspecies being given, but this is unimportant for us. The horizontallines may represent successive geological formations, and all theforms beneath the uppermost line may be considered as extinct. Thethree existing genera, a14, q14, p14, will form a small family; b14and f14 a closely allied family or sub-family; and o14, e14, m14, athird family. These three families, together with the many extinctgenera on the several lines of descent diverging from the parent-formA, will form an order; for all will have inherited something in commonfrom their ancient and common progenitor. On the principle of thecontinued tendency to divergence of character, which was formerlyillustrated by this diagram, the more recent any form is, the more itwill generally differ from its ancient progenitor. Hence we canunderstand the rule that the most ancient fossils differ most fromexisting forms. We must not, however, assume that divergence ofcharacter is a necessary contingency; it depends solely on thedescendants from a species being thus enabled to seize on many anddifferent places in the economy of nature. Therefore it is quitepossible, as we have seen in the case of some Silurian forms, that aspecies might go on being slightly modified in relation to itsslightly altered conditions of life, and yet retain throughout a vastperiod the same general characteristics. This is represented in thediagram by the letter F14.
All the many forms, extinct and recent, descended from A, make, asbefore remarked, one order; and this order, from the continued effectsof extinction and divergence of character, has become divided intoseveral sub-families and families, some of which are supposed to haveperished at different periods, and some to have endured to the presentday.
By looking at the diagram we can see that if many of the extinctforms, supposed to be embedded in the successive formations, werediscovered at several points low down in the series, the threeexisting families on the uppermost line would be rendered lessdistinct from each other. If, for instance, the genera a1, a5, a10,f8, m3, m6, m9 were disinterred, these three families would be soclosely linked together that they probably would have to be unitedinto one great family, in nearly the same manner as has occurred withruminants and pachyderms. Yet he who objected to call the extinctgenera, which thus linked the living genera of three familiestogether, intermediate in character, would be justified, as they areintermediate, not directly, but only by a long and circuitous coursethrough many widely different forms. If many extinct forms were to bediscovered above one of the middle horizontal lines or geologicalformations--for instance, above Number VI.--but none from beneath thisline, then only the two families on the left hand (namely, a14, etc.,and b14, etc.) would have to be united into one family; and the twoother families (namely, a14 to f14 now including five genera, and o14to m14) would yet remain distinct. These two families, however, wouldbe less distinct from each other than they were before the discoveryof the fossils. If, for instance, we suppose the existing genera ofthe two families to differ from each other by a dozen characters, inthis case the genera, at the early period marked VI., would differ bya lesser number of characters; for at this early stage of descent theyhave not diverged in character from the common progenitor of theorder, nearly so much as they subsequently diverged. Thus it comesthat ancient and extinct genera are often in some slight degreeintermediate in character between their modified descendants, orbetween their collateral relations.