物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 13. MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY: EMBRYOLOGY: RUDIMENTARY ORGANS. Page 2
It may be worth while to illustrate this view of classification, bytaking the case of languages. If we possessed a perfect pedigree ofmankind, a genealogical arrangement of the races of man would affordthe best classification of the various languages now spoken throughoutthe world; and if all extinct languages, and all intermediate andslowly changing dialects, had to be included, such an arrangementwould, I think, be the only possible one. Yet it might be that somevery ancient language had altered little, and had given rise to fewnew languages, whilst others (owing to the spreading and subsequentisolation and states of civilisation of the several races, descendedfrom a common race) had altered much, and had given rise to many newlanguages and dialects. The various degrees of difference in thelanguages from the same stock, would have to be expressed by groupssubordinate to groups; but the proper or even only possiblearrangement would still be genealogical; and this would be strictlynatural, as it would connect together all languages, extinct andmodern, by the closest affinities, and would give the filiation andorigin of each tongue.
In confirmation of this view, let us glance at the classification ofvarieties, which are believed or known to have descended from onespecies. These are grouped under species, with sub-varieties undervarieties; and with our domestic productions, several other grades ofdifference are requisite, as we have seen with pigeons. The origin ofthe existence of groups subordinate to groups, is the same withvarieties as with species, namely, closeness of descent with variousdegrees of modification. Nearly the same rules are followed inclassifying varieties, as with species. Authors have insisted on thenecessity of classing varieties on a natural instead of an artificialsystem; we are cautioned, for instance, not to class two varieties ofthe pine-apple together, merely because their fruit, though the mostimportant part, happens to be nearly identical; no one puts theswedish and common turnips together, though the esculent and thickenedstems are so similar. Whatever part is found to be most constant, isused in classing varieties: thus the great agriculturist Marshall saysthe horns are very useful for this purpose with cattle, because theyare less variable than the shape or colour of the body, etc.; whereaswith sheep the horns are much less serviceable, because less constant.In classing varieties, I apprehend if we had a real pedigree, agenealogical classification would be universally preferred; and it hasbeen attempted by some authors. For we might feel sure, whether therehad been more or less modification, the principle of inheritance wouldkeep the forms together which were allied in the greatest number ofpoints. In tumbler pigeons, though some sub-varieties differ from theothers in the important character of having a longer beak, yet all arekept together from having the common habit of tumbling; but theshort-faced breed has nearly or quite lost this habit; nevertheless,without any reasoning or thinking on the subject, these tumblers arekept in the same group, because allied in blood and alike in someother respects. If it could be proved that the Hottentot had descendedfrom the Negro, I think he would be classed under the Negro group,however much he might differ in colour and other important charactersfrom negroes.
With species in a state of nature, every naturalist has in factbrought descent into his classification; for he includes in his lowestgrade, or that of a species, the two sexes; and how enormously thesesometimes differ in the most important characters, is known to everynaturalist: scarcely a single fact can be predicated in common of themales and hermaphrodites of certain cirripedes, when adult, and yet noone dreams of separating them. The naturalist includes as one speciesthe several larval stages of the same individual, however much theymay differ from each other and from the adult; as he likewise includesthe so-called alternate generations of Steenstrup, which can only in atechnical sense be considered as the same individual. He includesmonsters; he includes varieties, not solely because they closelyresemble the parent-form, but because they are descended from it. Hewho believes that the cowslip is descended from the primrose, orconversely, ranks them together as a single species, and gives asingle definition. As soon as three Orchidean forms (Monochanthus,Myanthus, and Catasetum), which had previously been ranked as threedistinct genera, were known to be sometimes produced on the samespike, they were immediately included as a single species. But it maybe asked, what ought we to do, if it could be proved that one speciesof kangaroo had been produced, by a long course of modification, froma bear? Ought we to rank this one species with bears, and what shouldwe do with the other species? The supposition is of coursepreposterous; and I might answer by the argumentum ad hominem, and askwhat should be done if a perfect kangaroo were seen to come out of thewomb of a bear? According to all analogy, it would be ranked withbears; but then assuredly all the other species of the kangaroo familywould have to be classed under the bear genus. The whole case ispreposterous; for where there has been close descent in common, therewill certainly be close resemblance or affinity.
As descent has universally been used in classing together theindividuals of the same species, though the males and females andlarvae are sometimes extremely different; and as it has been used inclassing varieties which have undergone a certain, and sometimes aconsiderable amount of modification, may not this same element ofdescent have been unconsciously used in grouping species under genera,and genera under higher groups, though in these cases the modificationhas been greater in degree, and has taken a longer time to complete? Ibelieve it has thus been unconsciously used; and only thus can Iunderstand the several rules and guides which have been followed byour best systematists. We have no written pedigrees; we have to makeout community of descent by resemblances of any kind. Therefore wechoose those characters which, as far as we can judge, are the leastlikely to have been modified in relation to the conditions of life towhich each species has been recently exposed. Rudimentary structureson this view are as good as, or even sometimes better than, otherparts of the organisation. We care not how trifling a character maybe--let it be the mere inflection of the angle of the jaw, the mannerin which an insect's wing is folded, whether the skin be covered byhair or feathers--if it prevail throughout many and different species,especially those having very different habits of life, it assumes highvalue; for we can account for its presence in so many forms with suchdifferent habits, only by its inheritance from a common parent. We mayerr in this respect in regard to single points of structure, but whenseveral characters, let them be ever so trifling, occur togetherthroughout a large group of beings having different habits, we mayfeel almost sure, on the theory of descent, that these characters havebeen inherited from a common ancestor. And we know that suchcorrelated or aggregated characters have especial value inclassification.
We can understand why a species or a group of species may depart, inseveral of its most important characteristics, from its allies, andyet be safely classed with them. This may be safely done, and is oftendone, as long as a sufficient number of characters, let them be everso unimportant, betrays the hidden bond of community of descent. Lettwo forms have not a single character in common, yet if these extremeforms are connected together by a chain of intermediate groups, we mayat once infer their community of descent, and we put them all into thesame class. As we find organs of high physiological importance--thosewhich serve to preserve life under the most diverse conditions ofexistence--are generally the most constant, we attach especial valueto them; but if these same organs, in another group or section of agroup, are found to differ much, we at once value them less in ourclassification. We shall hereafter, I think, clearly see whyembryological characters are of such high classificatory importance.Geographical distribution may sometimes be brought usefully into playin classing large and widely-distributed genera, because all thespecies of the same genus, inhabiting any distinct and isolatedregion, have in all probability descended from the same parents.
We can understand, on these views, the very important distinctionbetween real affinities and analogical or adaptive resemblances.Lamarck first called attention to this distinction, and he has beenably followed by Macleay and others. The resemblance, in the shape ofthe body and in the fin-like anterior limbs, between the dugong, whichis a pachydermatous animal, and the whale, and between both thesemammals and fishes, is analogical. Amongst insects there areinnumerable instances: thus Linnaeus, misled by external appearances,actually classed an homopterous insect as a moth. We see something ofthe same kind even in our domestic varieties, as in the thickenedstems of the common and swedish turnip. The resemblance of thegreyhound and racehorse is hardly more fanciful than the analogieswhich have been drawn by some authors between very distinct animals.On my view of characters being of real importance for classification,only in so far as they reveal descent, we can clearly understand whyanalogical or adaptive character, although of the utmost importance tothe welfare of the being, are almost valueless to the systematist. Foranimals, belonging to two most distinct lines of descent, may readilybecome adapted to similar conditions, and thus assume a close externalresemblance; but such resemblances will not reveal--will rather tendto conceal their blood-relationship to their proper lines of descent.We can also understand the apparent paradox, that the very samecharacters are analogical when one class or order is compared withanother, but give true affinities when the members of the same classor order are compared one with another: thus the shape of the body andfin-like limbs are only analogical when whales are compared withfishes, being adaptations in both classes for swimming through thewater; but the shape of the body and fin-like limbs serve ascharacters exhibiting true affinity between the several members of thewhale family; for these cetaceans agree in so many characters, greatand small, that we cannot doubt that they have inherited their generalshape of body and structure of limbs from a common ancestor. So it iswith fishes.
As members of distinct classes have often been adapted by successiveslight modifications to live under nearly similar circumstances,--toinhabit for instance the three elements of land, air, and water,--wecan perhaps understand how it is that a numerical parallelism hassometimes been observed between the sub-groups in distinct classes. Anaturalist, struck by a parallelism of this nature in any one class,by arbitrarily raising or sinking the value of the groups in otherclasses (and all our experience shows that this valuation has hithertobeen arbitrary), could easily extend the parallelism over a widerange; and thus the septenary, quinary, quaternary, and ternaryclassifications have probably arisen.
As the modified descendants of dominant species, belonging to thelarger genera, tend to inherit the advantages, which made the groupsto which they belong large and their parents dominant, they are almostsure to spread widely, and to seize on more and more places in theeconomy of nature. The larger and more dominant groups thus tend to goon increasing in size; and they consequently supplant many smaller andfeebler groups. Thus we can account for the fact that all organisms,recent and extinct, are included under a few great orders, under stillfewer classes, and all in one great natural system. As showing how fewthe higher groups are in number, and how widely spread they arethroughout the world, the fact is striking, that the discovery ofAustralia has not added a single insect belonging to a new order; andthat in the vegetable kingdom, as I learn from Dr. Hooker, it hasadded only two or three orders of small size.
In the chapter on geological succession I attempted to show, on theprinciple of each group having generally diverged much in characterduring the long-continued process of modification, how it is that themore ancient forms of life often present characters in some slightdegree intermediate between existing groups. A few old andintermediate parent-forms having occasionally transmitted to thepresent day descendants but little modified, will give to us ourso-called osculant or aberrant groups. The more aberrant any form is,the greater must be the number of connecting forms which on my theoryhave been exterminated and utterly lost. And we have some evidence ofaberrant forms having suffered severely from extinction, for they aregenerally represented by extremely few species; and such species as dooccur are generally very distinct from each other, which again impliesextinction. The genera Ornithorhynchus and Lepidosiren, for example,would not have been less aberrant had each been represented by a dozenspecies instead of by a single one; but such richness in species, as Ifind after some investigation, does not commonly fall to the lot ofaberrant genera. We can, I think, account for this fact only bylooking at aberrant forms as failing groups conquered by moresuccessful competitors, with a few members preserved by some unusualcoincidence of favourable circumstances.
Mr. Waterhouse has remarked that, when a member belonging to one groupof animals exhibits an affinity to a quite distinct group, thisaffinity in most cases is general and not special: thus, according toMr. Waterhouse, of all Rodents, the bizcacha is most nearly related toMarsupials; but in the points in which it approaches this order, itsrelations are general, and not to any one marsupial species more thanto another. As the points of affinity of the bizcacha to Marsupialsare believed to be real and not merely adaptive, they are due on mytheory to inheritance in common. Therefore we must suppose either thatall Rodents, including the bizcacha, branched off from some veryancient Marsupial, which will have had a character in some degreeintermediate with respect to all existing Marsupials; or that bothRodents and Marsupials branched off from a common progenitor, and thatboth groups have since undergone much modification in divergentdirections. On either view we may suppose that the bizcacha hasretained, by inheritance, more of the character of its ancientprogenitor than have other Rodents; and therefore it will not bespecially related to any one existing Marsupial, but indirectly to allor nearly all Marsupials, from having partially retained the characterof their common progenitor, or of an early member of the group. On theother hand, of all Marsupials, as Mr. Waterhouse has remarked, thephascolomys resembles most nearly, not any one species, but thegeneral order of Rodents. In this case, however, it may be stronglysuspected that the resemblance is only analogical, owing to thephascolomys having become adapted to habits like those of a Rodent.The elder De Candolle has made nearly similar observations on thegeneral nature of the affinities of distinct orders of plants.
On the principle of the multiplication and gradual divergence incharacter of the species descended from a common parent, together withtheir retention by inheritance of some characters in common, we canunderstand the excessively complex and radiating affinities by whichall the members of the same family or higher group are connectedtogether. For the common parent of a whole family of species, nowbroken up by extinction into distinct groups and sub-groups, will havetransmitted some of its characters, modified in various ways anddegrees, to all; and the several species will consequently be relatedto each other by circuitous lines of affinity of various lengths (asmay be seen in the diagram so often referred to), mounting up throughmany predecessors. As it is difficult to show the blood-relationshipbetween the numerous kindred of any ancient and noble family, even bythe aid of a genealogical tree, and almost impossible to do thiswithout this aid, we can understand the extraordinary difficulty whichnaturalists have experienced in describing, without the aid of adiagram, the various affinities which they perceive between the manyliving and extinct members of the same great natural class.
Extinction, as we have seen in the fourth chapter, has played animportant part in defining and widening the intervals between theseveral groups in each class. We may thus account even for thedistinctness of whole classes from each other--for instance, of birdsfrom all other vertebrate animals--by the belief that many ancientforms of life have been utterly lost, through which the earlyprogenitors of birds were formerly connected with the earlyprogenitors of the other vertebrate classes. There has been lessentire extinction of the forms of life which once connected fisheswith batrachians. There has been still less in some other classes, asin that of the Crustacea, for here the most wonderfully diverse formsare still tied together by a long, but broken, chain of affinities.Extinction has only separated groups: it has by no means made them;for if every form which has ever lived on this earth were suddenly toreappear, though it would be quite impossible to give definitions bywhich each group could be distinguished from other groups, as allwould blend together by steps as fine as those between the finestexisting varieties, nevertheless a natural classification, or at leasta natural arrangement, would be possible. We shall see this by turningto the diagram: the letters, A to L, may represent eleven Siluriangenera, some of which have produced large groups of modifieddescendants. Every intermediate link between these eleven genera andtheir primordial parent, and every intermediate link in each branchand sub-branch of their descendants, may be supposed to be stillalive; and the links to be as fine as those between the finestvarieties. In this case it would be quite impossible to give anydefinition by which the several members of the several groups could bedistinguished from their more immediate parents; or these parents fromtheir ancient and unknown progenitor. Yet the natural arrangement inthe diagram would still hold good; and, on the principle ofinheritance, all the forms descended from A, or from I, would havesomething in common. In a tree we can specify this or that branch,though at the actual fork the two unite and blend together. We couldnot, as I have said, define the several groups; but we could pick outtypes, or forms, representing most of the characters of each group,whether large or small, and thus give a general idea of the value ofthe differences between them. This is what we should be driven to, ifwe were ever to succeed in collecting all the forms in any class whichhave lived throughout all time and space. We shall certainly neversucceed in making so perfect a collection: nevertheless, in certainclasses, we are tending in this direction; and Milne Edwards haslately insisted, in an able paper, on the high importance of lookingto types, whether or not we can separate and define the groups towhich such types belong.