物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 14. RECAPITULATION AND CONCLUSION. Page 2
On the view that species are only strongly marked and permanentvarieties, and that each species first existed as a variety, we cansee why it is that no line of demarcation can be drawn betweenspecies, commonly supposed to have been produced by special acts ofcreation, and varieties which are acknowledged to have been producedby secondary laws. On this same view we can understand how it is thatin each region where many species of a genus have been produced, andwhere they now flourish, these same species should present manyvarieties; for where the manufactory of species has been active, wemight expect, as a general rule, to find it still in action; and thisis the case if varieties be incipient species. Moreover, the speciesof the larger genera, which afford the greater number of varieties orincipient species, retain to a certain degree the character ofvarieties; for they differ from each other by a less amount ofdifference than do the species of smaller genera. The closely alliedspecies also of the larger genera apparently have restricted ranges,and they are clustered in little groups round other species--in whichrespects they resemble varieties. These are strange relations on theview of each species having been independently created, but areintelligible if all species first existed as varieties.
As each species tends by its geometrical ratio of reproduction toincrease inordinately in number; and as the modified descendants ofeach species will be enabled to increase by so much the more as theybecome more diversified in habits and structure, so as to be enabledto seize on many and widely different places in the economy of nature,there will be a constant tendency in natural selection to preserve themost divergent offspring of any one species. Hence during along-continued course of modification, the slight differences,characteristic of varieties of the same species, tend to be augmentedinto the greater differences characteristic of species of the samegenus. New and improved varieties will inevitably supplant andexterminate the older, less improved and intermediate varieties; andthus species are rendered to a large extent defined and distinctobjects. Dominant species belonging to the larger groups tend to givebirth to new and dominant forms; so that each large group tends tobecome still larger, and at the same time more divergent in character.But as all groups cannot thus succeed in increasing in size, for theworld would not hold them, the more dominant groups beat the lessdominant. This tendency in the large groups to go on increasing insize and diverging in character, together with the almost inevitablecontingency of much extinction, explains the arrangement of all theforms of life, in groups subordinate to groups, all within a few greatclasses, which we now see everywhere around us, and which hasprevailed throughout all time. This grand fact of the grouping of allorganic beings seems to me utterly inexplicable on the theory ofcreation.
As natural selection acts solely by accumulating slight, successive,favourable variations, it can produce no great or sudden modification;it can act only by very short and slow steps. Hence the canon of"Natura non facit saltum," which every fresh addition to our knowledgetends to make more strictly correct, is on this theory simplyintelligible. We can plainly see why nature is prodigal in variety,though niggard in innovation. But why this should be a law of natureif each species has been independently created, no man can explain.
Many other facts are, as it seems to me, explicable on this theory.How strange it is that a bird, under the form of woodpecker, shouldhave been created to prey on insects on the ground; that upland geese,which never or rarely swim, should have been created with webbed feet;that a thrush should have been created to dive and feed on sub-aquaticinsects; and that a petrel should have been created with habits andstructure fitting it for the life of an auk or grebe! and so on inendless other cases. But on the view of each species constantly tryingto increase in number, with natural selection always ready to adaptthe slowly varying descendants of each to any unoccupied orill-occupied place in nature, these facts cease to be strange, orperhaps might even have been anticipated.
As natural selection acts by competition, it adapts the inhabitants ofeach country only in relation to the degree of perfection of theirassociates; so that we need feel no surprise at the inhabitants of anyone country, although on the ordinary view supposed to have beenspecially created and adapted for that country, being beaten andsupplanted by the naturalised productions from another land. Nor oughtwe to marvel if all the contrivances in nature be not, as far as wecan judge, absolutely perfect; and if some of them be abhorrent to ourideas of fitness. We need not marvel at the sting of the bee causingthe bee's own death; at drones being produced in such vast numbers forone single act, and being then slaughtered by their sterile sisters;at the astonishing waste of pollen by our fir-trees; at theinstinctive hatred of the queen bee for her own fertile daughters; atichneumonidae feeding within the live bodies of caterpillars; and atother such cases. The wonder indeed is, on the theory of naturalselection, that more cases of the want of absolute perfection have notbeen observed.
The complex and little known laws governing variation are the same, asfar as we can see, with the laws which have governed the production ofso-called specific forms. In both cases physical conditions seem tohave produced but little direct effect; yet when varieties enter anyzone, they occasionally assume some of the characters of the speciesproper to that zone. In both varieties and species, use and disuseseem to have produced some effect; for it is difficult to resist thisconclusion when we look, for instance, at the logger-headed duck,which has wings incapable of flight, in nearly the same condition asin the domestic duck; or when we look at the burrowing tucutucu, whichis occasionally blind, and then at certain moles, which are habituallyblind and have their eyes covered with skin; or when we look at theblind animals inhabiting the dark caves of America and Europe. In bothvarieties and species correlation of growth seems to have played amost important part, so that when one part has been modified otherparts are necessarily modified. In both varieties and speciesreversions to long-lost characters occur. How inexplicable on thetheory of creation is the occasional appearance of stripes on theshoulder and legs of the several species of the horse-genus and intheir hybrids! How simply is this fact explained if we believe thatthese species have descended from a striped progenitor, in the samemanner as the several domestic breeds of pigeon have descended fromthe blue and barred rock-pigeon!
On the ordinary view of each species having been independentlycreated, why should the specific characters, or those by which thespecies of the same genus differ from each other, be more variablethan the generic characters in which they all agree? Why, forinstance, should the colour of a flower be more likely to vary in anyone species of a genus, if the other species, supposed to have beencreated independently, have differently coloured flowers, than if allthe species of the genus have the same coloured flowers? If speciesare only well-marked varieties, of which the characters have become ina high degree permanent, we can understand this fact; for they havealready varied since they branched off from a common progenitor incertain characters, by which they have come to be specificallydistinct from each other; and therefore these same characters would bemore likely still to be variable than the generic characters whichhave been inherited without change for an enormous period. It isinexplicable on the theory of creation why a part developed in a veryunusual manner in any one species of a genus, and therefore, as we maynaturally infer, of great importance to the species, should beeminently liable to variation; but, on my view, this part hasundergone, since the several species branched off from a commonprogenitor, an unusual amount of variability and modification, andtherefore we might expect this part generally to be still variable.But a part may be developed in the most unusual manner, like the wingof a bat, and yet not be more variable than any other structure, ifthe part be common to many subordinate forms, that is, if it has beeninherited for a very long period; for in this case it will have beenrendered constant by long-continued natural selection.
Glancing at instincts, marvellous as some are, they offer no greaterdifficulty than does corporeal structure on the theory of the naturalselection of successive, slight, but profitable modifications. We canthus understand why nature moves by graduated steps in endowingdifferent animals of the same class with their several instincts. Ihave attempted to show how much light the principle of gradationthrows on the admirable architectural powers of the hive-bee. Habit nodoubt sometimes comes into play in modifying instincts; but itcertainly is not indispensable, as we see, in the case of neuterinsects, which leave no progeny to inherit the effects oflong-continued habit. On the view of all the species of the same genushaving descended from a common parent, and having inherited much incommon, we can understand how it is that allied species, when placedunder considerably different conditions of life, yet should follownearly the same instincts; why the thrush of South America, forinstance, lines her nest with mud like our British species. On theview of instincts having been slowly acquired through naturalselection we need not marvel at some instincts being apparently notperfect and liable to mistakes, and at many instincts causing otheranimals to suffer.
If species be only well-marked and permanent varieties, we can at oncesee why their crossed offspring should follow the same complex laws intheir degrees and kinds of resemblance to their parents,--in beingabsorbed into each other by successive crosses, and in other suchpoints,--as do the crossed offspring of acknowledged varieties. On theother hand, these would be strange facts if species have beenindependently created, and varieties have been produced by secondarylaws.
If we admit that the geological record is imperfect in an extremedegree, then such facts as the record gives, support the theory ofdescent with modification. New species have come on the stage slowlyand at successive intervals; and the amount of change, after equalintervals of time, is widely different in different groups. Theextinction of species and of whole groups of species, which has playedso conspicuous a part in the history of the organic world, almostinevitably follows on the principle of natural selection; for oldforms will be supplanted by new and improved forms. Neither singlespecies nor groups of species reappear when the chain of ordinarygeneration has once been broken. The gradual diffusion of dominantforms, with the slow modification of their descendants, causes theforms of life, after long intervals of time, to appear as if they hadchanged simultaneously throughout the world. The fact of the fossilremains of each formation being in some degree intermediate incharacter between the fossils in the formations above and below, issimply explained by their intermediate position in the chain ofdescent. The grand fact that all extinct organic beings belong to thesame system with recent beings, falling either into the same or intointermediate groups, follows from the living and the extinct being theoffspring of common parents. As the groups which have descended froman ancient progenitor have generally diverged in character, theprogenitor with its early descendants will often be intermediate incharacter in comparison with its later descendants; and thus we cansee why the more ancient a fossil is, the oftener it stands in somedegree intermediate between existing and allied groups. Recent formsare generally looked at as being, in some vague sense, higher thanancient and extinct forms; and they are in so far higher as the laterand more improved forms have conquered the older and less improvedorganic beings in the struggle for life. Lastly, the law of the longendurance of allied forms on the same continent,--of marsupials inAustralia, of edentata in America, and other such cases,--isintelligible, for within a confined country, the recent and theextinct will naturally be allied by descent.
Looking to geographical distribution, if we admit that there has beenduring the long course of ages much migration from one part of theworld to another, owing to former climatal and geographical changesand to the many occasional and unknown means of dispersal, then we canunderstand, on the theory of descent with modification, most of thegreat leading facts in Distribution. We can see why there should be sostriking a parallelism in the distribution of organic beingsthroughout space, and in their geological succession throughout time;for in both cases the beings have been connected by the bond ofordinary generation, and the means of modification have been the same.We see the full meaning of the wonderful fact, which must have struckevery traveller, namely, that on the same continent, under the mostdiverse conditions, under heat and cold, on mountain and lowland, ondeserts and marshes, most of the inhabitants within each great classare plainly related; for they will generally be descendants of thesame progenitors and early colonists. On this same principle of formermigration, combined in most cases with modification, we canunderstand, by the aid of the Glacial period, the identity of some fewplants, and the close alliance of many others, on the most distantmountains, under the most different climates; and likewise the closealliance of some of the inhabitants of the sea in the northern andsouthern temperate zones, though separated by the whole intertropicalocean. Although two areas may present the same physical conditions oflife, we need feel no surprise at their inhabitants being widelydifferent, if they have been for a long period completely separatedfrom each other; for as the relation of organism to organism is themost important of all relations, and as the two areas will havereceived colonists from some third source or from each other, atvarious periods and in different proportions, the course ofmodification in the two areas will inevitably be different.
On this view of migration, with subsequent modification, we can seewhy oceanic islands should be inhabited by few species, but of these,that many should be peculiar. We can clearly see why those animalswhich cannot cross wide spaces of ocean, as frogs and terrestrialmammals, should not inhabit oceanic islands; and why, on the otherhand, new and peculiar species of bats, which can traverse the ocean,should so often be found on islands far distant from any continent.Such facts as the presence of peculiar species of bats, and theabsence of all other mammals, on oceanic islands, are utterlyinexplicable on the theory of independent acts of creation.
The existence of closely allied or representative species in any twoareas, implies, on the theory of descent with modification, that thesame parents formerly inhabited both areas; and we almost invariablyfind that wherever many closely allied species inhabit two areas, someidentical species common to both still exist. Wherever many closelyallied yet distinct species occur, many doubtful forms and varietiesof the same species likewise occur. It is a rule of high generalitythat the inhabitants of each area are related to the inhabitants ofthe nearest source whence immigrants might have been derived. We seethis in nearly all the plants and animals of the Galapagosarchipelago, of Juan Fernandez, and of the other American islandsbeing related in the most striking manner to the plants and animals ofthe neighbouring American mainland; and those of the Cape de Verdearchipelago and other African islands to the African mainland. It mustbe admitted that these facts receive no explanation on the theory ofcreation.
The fact, as we have seen, that all past and present organic beingsconstitute one grand natural system, with group subordinate to group,and with extinct groups often falling in between recent groups, isintelligible on the theory of natural selection with its contingenciesof extinction and divergence of character. On these same principles wesee how it is, that the mutual affinities of the species and generawithin each class are so complex and circuitous. We see why certaincharacters are far more serviceable than others forclassification;--why adaptive characters, though of paramountimportance to the being, are of hardly any importance inclassification; why characters derived from rudimentary parts, thoughof no service to the being, are often of high classificatory value;and why embryological characters are the most valuable of all. Thereal affinities of all organic beings are due to inheritance orcommunity of descent. The natural system is a genealogicalarrangement, in which we have to discover the lines of descent by themost permanent characters, however slight their vital importance maybe.
The framework of bones being the same in the hand of a man, wing of abat, fin of the porpoise, and leg of the horse,--the same number ofvertebrae forming the neck of the giraffe and of the elephant,--andinnumerable other such facts, at once explain themselves on the theoryof descent with slow and slight successive modifications. Thesimilarity of pattern in the wing and leg of a bat, though used forsuch different purpose,--in the jaws and legs of a crab,--in thepetals, stamens, and pistils of a flower, is likewise intelligible onthe view of the gradual modification of parts or organs, which werealike in the early progenitor of each class. On the principle ofsuccessive variations not always supervening at an early age, andbeing inherited at a corresponding not early period of life, we canclearly see why the embryos of mammals, birds, reptiles, and fishesshould be so closely alike, and should be so unlike the adult forms.We may cease marvelling at the embryo of an air-breathing mammal orbird having branchial slits and arteries running in loops, like thosein a fish which has to breathe the air dissolved in water, by the aidof well-developed branchiae.
Disuse, aided sometimes by natural selection, will often tend toreduce an organ, when it has become useless by changed habits or underchanged conditions of life; and we can clearly understand on this viewthe meaning of rudimentary organs. But disuse and selection willgenerally act on each creature, when it has come to maturity and hasto play its full part in the struggle for existence, and will thushave little power of acting on an organ during early life; hence theorgan will not be much reduced or rendered rudimentary at this earlyage. The calf, for instance, has inherited teeth, which never cutthrough the gums of the upper jaw, from an early progenitor havingwell-developed teeth; and we may believe, that the teeth in the matureanimal were reduced, during successive generations, by disuse or bythe tongue and palate having been fitted by natural selection tobrowse without their aid; whereas in the calf, the teeth have beenleft untouched by selection or disuse, and on the principle ofinheritance at corresponding ages have been inherited from a remoteperiod to the present day. On the view of each organic being and eachseparate organ having been specially created, how utterly inexplicableit is that parts, like the teeth in the embryonic calf or like theshrivelled wings under the soldered wing-covers of some beetles,should thus so frequently bear the plain stamp of inutility! Naturemay be said to have taken pains to reveal, by rudimentary organs andby homologous structures, her scheme of modification, which it seemsthat we wilfully will not understand.