物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 14. RECAPITULATION AND CONCLUSION. Page 1
Recapitulation of the difficulties on the theory of Natural Selection.Recapitulation of the general and special circumstances in its favour.Causes of the general belief in the immutability of species.How far the theory of natural selection may be extended.Effects of its adoption on the study of Natural history.Concluding remarks.
their constitutions or their reproductive systems should havebeen profoundly modified. Moreover, most of the varieties which havebeen experimentised.
As this whole volume is one long argument, it may be convenient to thereader to have the leading facts and inferences briefly recapitulated.
That many and grave objections may be advanced against the theory ofdescent with modification through natural selection, I do not deny. Ihave endeavoured to give to them their full force. Nothing at firstcan appear more difficult to believe than that the more complex organsand instincts should have been perfected, not by means superior to,though analogous with, human reason, but by the accumulation ofinnumerable slight variations, each good for the individual possessor.Nevertheless, this difficulty, though appearing to our imaginationinsuperably great, cannot be considered real if we admit the followingpropositions, namely,--that gradations in the perfection of any organor instinct, which we may consider, either do now exist or could haveexisted, each good of its kind,--that all organs and instincts are, inever so slight a degree, variable,--and, lastly, that there is astruggle for existence leading to the preservation of each profitabledeviation of structure or instinct. The truth of these propositionscannot, I think, be disputed.
It is, no doubt, extremely difficult even to conjecture by whatgradations many structures have been perfected, more especiallyamongst broken and failing groups of organic beings; but we see somany strange gradations in nature, as is proclaimed by the canon,"Natura non facit saltum," that we ought to be extremely cautious insaying that any organ or instinct, or any whole being, could not havearrived at its present state by many graduated steps. There are, itmust be admitted, cases of special difficulty on the theory of naturalselection; and one of the most curious of these is the existence oftwo or three defined castes of workers or sterile females in the samecommunity of ants; but I have attempted to show how this difficultycan be mastered.
With respect to the almost universal sterility of species when firstcrossed, which forms so remarkable a contrast with the almostuniversal fertility of varieties when crossed, I must refer the readerto the recapitulation of the facts given at the end of the eighthchapter, which seem to me conclusively to show that this sterility isno more a special endowment than is the incapacity of two trees to begrafted together, but that it is incidental on constitutionaldifferences in the reproductive systems of the intercrossed species.We see the truth of this conclusion in the vast difference in theresult, when the same two species are crossed reciprocally; that is,when one species is first used as the father and then as the mother.
The fertility of varieties when intercrossed and of their mongreloffspring cannot be considered as universal; nor is their very generalfertility surprising when we remember that it is not likely thateither their constitutions or their reproductive systems should havebeen profoundly modified. Moreover, most of the varieties which havebeen experimentised on have been produced under domestication; and asdomestication apparently tends to eliminate sterility, we ought not toexpect it also to produce sterility.
The sterility of hybrids is a very different case from that of firstcrosses, for their reproductive organs are more or less functionallyimpotent; whereas in first crosses the organs on both sides are in aperfect condition. As we continually see that organisms of all kindsare rendered in some degree sterile from their constitutions havingbeen disturbed by slightly different and new conditions of life, weneed not feel surprise at hybrids being in some degree sterile, fortheir constitutions can hardly fail to have been disturbed from beingcompounded of two distinct organisations. This parallelism issupported by another parallel, but directly opposite, class of facts;namely, that the vigour and fertility of all organic beings areincreased by slight changes in their conditions of life, and that theoffspring of slightly modified forms or varieties acquire from beingcrossed increased vigour and fertility. So that, on the one hand,considerable changes in the conditions of life and crosses betweengreatly modified forms, lessen fertility; and on the other hand,lesser changes in the conditions of life and crosses between lessmodified forms, increase fertility.
Turning to geographical distribution, the difficulties encountered onthe theory of descent with modification are grave enough. All theindividuals of the same species, and all the species of the samegenus, or even higher group, must have descended from common parents;and therefore, in however distant and isolated parts of the world theyare now found, they must in the course of successive generations havepassed from some one part to the others. We are often wholly unableeven to conjecture how this could have been effected. Yet, as we havereason to believe that some species have retained the same specificform for very long periods, enormously long as measured by years, toomuch stress ought not to be laid on the occasional wide diffusion ofthe same species; for during very long periods of time there willalways be a good chance for wide migration by many means. A broken orinterrupted range may often be accounted for by the extinction of thespecies in the intermediate regions. It cannot be denied that we areas yet very ignorant of the full extent of the various climatal andgeographical changes which have affected the earth during modernperiods; and such changes will obviously have greatly facilitatedmigration. As an example, I have attempted to show how potent has beenthe influence of the Glacial period on the distribution both of thesame and of representative species throughout the world. We are as yetprofoundly ignorant of the many occasional means of transport. Withrespect to distinct species of the same genus inhabiting very distantand isolated regions, as the process of modification has necessarilybeen slow, all the means of migration will have been possible during avery long period; and consequently the difficulty of the widediffusion of species of the same genus is in some degree lessened.
As on the theory of natural selection an interminable number ofintermediate forms must have existed, linking together all the speciesin each group by gradations as fine as our present varieties, it maybe asked, Why do we not see these linking forms all around us? Why arenot all organic beings blended together in an inextricable chaos? Withrespect to existing forms, we should remember that we have no right toexpect (excepting in rare cases) to discover DIRECTLY connecting linksbetween them, but only between each and some extinct and supplantedform. Even on a wide area, which has during a long period remainedcontinuous, and of which the climate and other conditions of lifechange insensibly in going from a district occupied by one speciesinto another district occupied by a closely allied species, we have nojust right to expect often to find intermediate varieties in theintermediate zone. For we have reason to believe that only a fewspecies are undergoing change at any one period; and all changes areslowly effected. I have also shown that the intermediate varietieswhich will at first probably exist in the intermediate zones, will beliable to be supplanted by the allied forms on either hand; and thelatter, from existing in greater numbers, will generally be modifiedand improved at a quicker rate than the intermediate varieties, whichexist in lesser numbers; so that the intermediate varieties will, inthe long run, be supplanted and exterminated.
On this doctrine of the extermination of an infinitude of connectinglinks, between the living and extinct inhabitants of the world, and ateach successive period between the extinct and still older species,why is not every geological formation charged with such links? Whydoes not every collection of fossil remains afford plain evidence ofthe gradation and mutation of the forms of life? We meet with no suchevidence, and this is the most obvious and forcible of the manyobjections which may be urged against my theory. Why, again, do wholegroups of allied species appear, though certainly they often falselyappear, to have come in suddenly on the several geological stages? Whydo we not find great piles of strata beneath the Silurian system,stored with the remains of the progenitors of the Silurian groups offossils? For certainly on my theory such strata must somewhere havebeen deposited at these ancient and utterly unknown epochs in theworld's history.
I can answer these questions and grave objections only on thesupposition that the geological record is far more imperfect than mostgeologists believe. It cannot be objected that there has not been timesufficient for any amount of organic change; for the lapse of time hasbeen so great as to be utterly inappreciable by the human intellect.The number of specimens in all our museums is absolutely as nothingcompared with the countless generations of countless species whichcertainly have existed. We should not be able to recognise a speciesas the parent of any one or more species if we were to examine themever so closely, unless we likewise possessed many of the intermediatelinks between their past or parent and present states; and these manylinks we could hardly ever expect to discover, owing to theimperfection of the geological record. Numerous existing doubtfulforms could be named which are probably varieties; but who willpretend that in future ages so many fossil links will be discovered,that naturalists will be able to decide, on the common view, whetheror not these doubtful forms are varieties? As long as most of thelinks between any two species are unknown, if any one link orintermediate variety be discovered, it will simply be classed asanother and distinct species. Only a small portion of the world hasbeen geologically explored. Only organic beings of certain classes canbe preserved in a fossil condition, at least in any great number.Widely ranging species vary most, and varieties are often at firstlocal,--both causes rendering the discovery of intermediate links lesslikely. Local varieties will not spread into other and distant regionsuntil they are considerably modified and improved; and when they dospread, if discovered in a geological formation, they will appear asif suddenly created there, and will be simply classed as new species.Most formations have been intermittent in their accumulation; andtheir duration, I am inclined to believe, has been shorter than theaverage duration of specific forms. Successive formations areseparated from each other by enormous blank intervals of time; forfossiliferous formations, thick enough to resist future degradation,can be accumulated only where much sediment is deposited on thesubsiding bed of the sea. During the alternate periods of elevationand of stationary level the record will be blank. During these latterperiods there will probably be more variability in the forms of life;during periods of subsidence, more extinction.
beautifully adapting each form to the most complexrelations of life. The theory of natural selection, even if we lookedno further than this, seems to me to be in itself probable. I havealready recapitulated, as fairly as I could, the opposed difficultiesand objections: .
With respect to the absence of fossiliferous formations beneath thelowest Silurian strata, I can only recur to the hypothesis given inthe ninth chapter. That the geological record is imperfect all willadmit; but that it is imperfect to the degree which I require, fewwill be inclined to admit. If we look to long enough intervals oftime, geology plainly declares that all species have changed; and theyhave changed in the manner which my theory requires, for they havechanged slowly and in a graduated manner. We clearly see this in thefossil remains from consecutive formations invariably being much moreclosely related to each other, than are the fossils from formationsdistant from each other in time.
Such is the sum of the several chief objections and difficulties whichmay justly be urged against my theory; and I have now brieflyrecapitulated the answers and explanations which can be given to them.I have felt these difficulties far too heavily during many years todoubt their weight. But it deserves especial notice that the moreimportant objections relate to questions on which we are confessedlyignorant; nor do we know how ignorant we are. We do not know all thepossible transitional gradations between the simplest and the mostperfect organs; it cannot be pretended that we know all the variedmeans of Distribution during the long lapse of years, or that we knowhow imperfect the Geological Record is. Grave as these severaldifficulties are, in my judgment they do not overthrow the theory ofdescent with modification.
Now let us turn to the other side of the argument. Under domesticationwe see much variability. This seems to be mainly due to thereproductive system being eminently susceptible to changes in theconditions of life; so that this system, when not rendered impotent,fails to reproduce offspring exactly like the parent-form. Variabilityis governed by many complex laws,--by correlation of growth, by useand disuse, and by the direct action of the physical conditions oflife. There is much difficulty in ascertaining how much modificationour domestic productions have undergone; but we may safely infer thatthe amount has been large, and that modifications can be inherited forlong periods. As long as the conditions of life remain the same, wehave reason to believe that a modification, which has already beeninherited for many generations, may continue to be inherited for analmost infinite number of generations. On the other hand we haveevidence that variability, when it has once come into play, does notwholly cease; for new varieties are still occasionally produced by ourmost anciently domesticated productions.
Man does not actually produce variability; he only unintentionallyexposes organic beings to new conditions of life, and then nature actson the organisation, and causes variability. But man can and doesselect the variations given to him by nature, and thus accumulate themin any desired manner. He thus adapts animals and plants for his ownbenefit or pleasure. He may do this methodically, or he may do itunconsciously by preserving the individuals most useful to him at thetime, without any thought of altering the breed. It is certain that hecan largely influence the character of a breed by selecting, in eachsuccessive generation, individual differences so slight as to be quiteinappreciable by an uneducated eye. This process of selection has beenthe great agency in the production of the most distinct and usefuldomestic breeds. That many of the breeds produced by man have to alarge extent the character of natural species, is shown by theinextricable doubts whether very many of them are varieties oraboriginal species.
There is no obvious reason why the principles which have acted soefficiently under domestication should not have acted under nature. Inthe preservation of favoured individuals and races, during theconstantly-recurrent Struggle for Existence, we see the most powerfuland ever-acting means of selection. The struggle for existenceinevitably follows from the high geometrical ratio of increase whichis common to all organic beings. This high rate of increase is provedby calculation, by the effects of a succession of peculiar seasons,and by the results of naturalisation, as explained in the thirdchapter. More individuals are born than can possibly survive. A grainin the balance will determine which individual shall live and whichshall die,--which variety or species shall increase in number, andwhich shall decrease, or finally become extinct. As the individuals ofthe same species come in all respects into the closest competitionwith each other, the struggle will generally be most severe betweenthem; it will be almost equally severe between the varieties of thesame species, and next in severity between the species of the samegenus. But the struggle will often be very severe between beings mostremote in the scale of nature. The slightest advantage in one being,at any age or during any season, over those with which it comes intocompetition, or better adaptation in however slight a degree to thesurrounding physical conditions, will turn the balance.
With animals having separated sexes there will in most cases be astruggle between the males for possession of the females. The mostvigorous individuals, or those which have most successfully struggledwith their conditions of life, will generally leave most progeny. Butsuccess will often depend on having special weapons or means ofdefence, or on the charms of the males; and the slightest advantagewill lead to victory.
As geology plainly proclaims that each land has undergone greatphysical changes, we might have expected that organic beings wouldhave varied under nature, in the same way as they generally havevaried under the changed conditions of domestication. And if there beany variability under nature, it would be an unaccountable fact ifnatural selection had not come into play. It has often been asserted,but the assertion is quite incapable of proof, that the amount ofvariation under nature is a strictly limited quantity. Man, thoughacting on external characters alone and often capriciously, canproduce within a short period a great result by adding up mereindividual differences in his domestic productions; and every oneadmits that there are at least individual differences in species undernature. But, besides such differences, all naturalists have admittedthe existence of varieties, which they think sufficiently distinct tobe worthy of record in systematic works. No one can draw any cleardistinction between individual differences and slight varieties; orbetween more plainly marked varieties and sub-species, and species.Let it be observed how naturalists differ in the rank which theyassign to the many representative forms in Europe and North America.
If then we have under nature variability and a powerful agent alwaysready to act and select, why should we doubt that variations in anyway useful to beings, under their excessively complex relations oflife, would be preserved, accumulated, and inherited? Why, if man canby patience select variations most useful to himself, should naturefail in selecting variations useful, under changing conditions oflife, to her living products? What limit can be put to this power,acting during long ages and rigidly scrutinising the wholeconstitution, structure, and habits of each creature,--favouring thegood and rejecting the bad? I can see no limit to this power, inslowly and beautifully adapting each form to the most complexrelations of life. The theory of natural selection, even if we lookedno further than this, seems to me to be in itself probable. I havealready recapitulated, as fairly as I could, the opposed difficultiesand objections: now let us turn to the special facts and arguments infavour of the theory.