物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 14. RECAPITULATION AND CONCLUSION. Page 3
I have now recapitulated the chief facts and considerations which havethoroughly convinced me that species have changed, and are stillslowly changing by the preservation and accumulation of successiveslight favourable variations. Why, it may be asked, have all the mosteminent living naturalists and geologists rejected this view of themutability of species? It cannot be asserted that organic beings in astate of nature are subject to no variation; it cannot be proved thatthe amount of variation in the course of long ages is a limitedquantity; no clear distinction has been, or can be, drawn betweenspecies and well-marked varieties. It cannot be maintained thatspecies when intercrossed are invariably sterile, and varietiesinvariably fertile; or that sterility is a special endowment and signof creation. The belief that species were immutable productions wasalmost unavoidable as long as the history of the world was thought tobe of short duration; and now that we have acquired some idea of thelapse of time, we are too apt to assume, without proof, that thegeological record is so perfect that it would have afforded us plainevidence of the mutation of species, if they had undergone mutation.
But the chief cause of our natural unwillingness to admit that onespecies has given birth to other and distinct species, is that we arealways slow in admitting any great change of which we do not see theintermediate steps. The difficulty is the same as that felt by so manygeologists, when Lyell first insisted that long lines of inland cliffshad been formed, and great valleys excavated, by the slow action ofthe coast-waves. The mind cannot possibly grasp the full meaning ofthe term of a hundred million years; it cannot add up and perceive thefull effects of many slight variations, accumulated during an almostinfinite number of generations.
Although I am fully convinced of the truth of the views given in thisvolume under the form of an abstract, I by no means expect to convinceexperienced naturalists whose minds are stocked with a multitude offacts all viewed, during a long course of years, from a point of viewdirectly opposite to mine. It is so easy to hide our ignorance undersuch expressions as the "plan of creation," "unity of design," etc.,and to think that we give an explanation when we only restate a fact.Any one whose disposition leads him to attach more weight tounexplained difficulties than to the explanation of a certain numberof facts will certainly reject my theory. A few naturalists, endowedwith much flexibility of mind, and who have already begun to doubt onthe immutability of species, may be influenced by this volume; but Ilook with confidence to the future, to young and rising naturalists,who will be able to view both sides of the question with impartiality.Whoever is led to believe that species are mutable will do goodservice by conscientiously expressing his conviction; for only thuscan the load of prejudice by which this subject is overwhelmed beremoved.
Several eminent naturalists have of late published their belief that amultitude of reputed species in each genus are not real species; butthat other species are real, that is, have been independently created.This seems to me a strange conclusion to arrive at. They admit that amultitude of forms, which till lately they themselves thought werespecial creations, and which are still thus looked at by the majorityof naturalists, and which consequently have every externalcharacteristic feature of true species,--they admit that these havebeen produced by variation, but they refuse to extend the same view toother and very slightly different forms. Nevertheless they do notpretend that they can define, or even conjecture, which are thecreated forms of life, and which are those produced by secondary laws.They admit variation as a vera causa in one case, they arbitrarilyreject it in another, without assigning any distinction in the twocases. The day will come when this will be given as a curiousillustration of the blindness of preconceived opinion. These authorsseem no more startled at a miraculous act of creation than at anordinary birth. But do they really believe that at innumerable periodsin the earth's history certain elemental atoms have been commandedsuddenly to flash into living tissues? Do they believe that at eachsupposed act of creation one individual or many were produced? Wereall the infinitely numerous kinds of animals and plants created aseggs or seed, or as full grown? and in the case of mammals, were theycreated bearing the false marks of nourishment from the mother's womb?Although naturalists very properly demand a full explanation of everydifficulty from those who believe in the mutability of species, ontheir own side they ignore the whole subject of the first appearanceof species in what they consider reverent silence.
It may be asked how far I extend the doctrine of the modification ofspecies. The question is difficult to answer, because the moredistinct the forms are which we may consider, by so much the argumentsfall away in force. But some arguments of the greatest weight extendvery far. All the members of whole classes can be connected togetherby chains of affinities, and all can be classified on the sameprinciple, in groups subordinate to groups. Fossil remains sometimestend to fill up very wide intervals between existing orders. Organs ina rudimentary condition plainly show that an early progenitor had theorgan in a fully developed state; and this in some instancesnecessarily implies an enormous amount of modification in thedescendants. Throughout whole classes various structures are formed onthe same pattern, and at an embryonic age the species closely resembleeach other. Therefore I cannot doubt that the theory of descent withmodification embraces all the members of the same class. I believethat animals have descended from at most only four or fiveprogenitors, and plants from an equal or lesser number.
Analogy would lead me one step further, namely, to the belief that allanimals and plants have descended from some one prototype. But analogymay be a deceitful guide. Nevertheless all living things have much incommon, in their chemical composition, their germinal vesicles, theircellular structure, and their laws of growth and reproduction. We seethis even in so trifling a circumstance as that the same poison oftensimilarly affects plants and animals; or that the poison secreted bythe gall-fly produces monstrous growths on the wild rose or oak-tree.Therefore I should infer from analogy that probably all the organicbeings which have ever lived on this earth have descended from someone primordial form, into which life was first breathed. When theviews entertained in this volume on the origin of species, or whenanalogous views are generally admitted, we can dimly foresee thatthere will be a considerable revolution in natural history.Systematists will be able to pursue their labours as at present; butthey will not be incessantly haunted by the shadowy doubt whether thisor that form be in essence a species. This I feel sure, and I speakafter experience, will be no slight relief. The endless disputeswhether or not some fifty species of British brambles are true specieswill cease. Systematists will have only to decide (not that this willbe easy) whether any form be sufficiently constant and distinct fromother forms, to be capable of definition; and if definable, whetherthe differences be sufficiently important to deserve a specific name.This latter point will become a far more essential consideration thanit is at present; for differences, however slight, between any twoforms, if not blended by intermediate gradations, are looked at bymost naturalists as sufficient to raise both forms to the rank ofspecies. Hereafter we shall be compelled to acknowledge that the onlydistinction between species and well-marked varieties is, that thelatter are known, or believed, to be connected at the present day byintermediate gradations, whereas species were formerly thus connected.Hence, without quite rejecting the consideration of the presentexistence of intermediate gradations between any two forms, we shallbe led to weigh more carefully and to value higher the actual amountof difference between them. It is quite possible that forms nowgenerally acknowledged to be merely varieties may hereafter be thoughtworthy of specific names, as with the primrose and cowslip; and inthis case scientific and common language will come into accordance. Inshort, we shall have to treat species in the same manner as thosenaturalists treat genera, who admit that genera are merely artificialcombinations made for convenience. This may not be a cheeringprospect; but we shall at least be freed from the vain search for theundiscovered and undiscoverable essence of the term species.
The other and more general departments of natural history will risegreatly in interest. The terms used by naturalists of affinity,relationship, community of type, paternity, morphology, adaptivecharacters, rudimentary and aborted organs, etc., will cease to bemetaphorical, and will have a plain signification. When we no longerlook at an organic being as a savage looks at a ship, as at somethingwholly beyond his comprehension; when we regard every production ofnature as one which has had a history; when we contemplate everycomplex structure and instinct as the summing up of many contrivances,each useful to the possessor, nearly in the same way as when we lookat any great mechanical invention as the summing up of the labour, theexperience, the reason, and even the blunders of numerous workmen;when we thus view each organic being, how far more interesting, Ispeak from experience, will the study of natural history become!
A grand and almost untrodden field of inquiry will be opened, on thecauses and laws of variation, on correlation of growth, on the effectsof use and disuse, on the direct action of external conditions, and soforth. The study of domestic productions will rise immensely in value.A new variety raised by man will be a far more important andinteresting subject for study than one more species added to theinfinitude of already recorded species. Our classifications will cometo be, as far as they can be so made, genealogies; and will then trulygive what may be called the plan of creation. The rules forclassifying will no doubt become simpler when we have a definiteobject in view. We possess no pedigrees or armorial bearings; and wehave to discover and trace the many diverging lines of descent in ournatural genealogies, by characters of any kind which have long beeninherited. Rudimentary organs will speak infallibly with respect tothe nature of long-lost structures. Species and groups of species,which are called aberrant, and which may fancifully be called livingfossils, will aid us in forming a picture of the ancient forms oflife. Embryology will reveal to us the structure, in some degreeobscured, of the prototypes of each great class.
When we can feel assured that all the individuals of the same species,and all the closely allied species of most genera, have within a notvery remote period descended from one parent, and have migrated fromsome one birthplace; and when we better know the many means ofmigration, then, by the light which geology now throws, and willcontinue to throw, on former changes of climate and of the level ofthe land, we shall surely be enabled to trace in an admirable mannerthe former migrations of the inhabitants of the whole world. Even atpresent, by comparing the differences of the inhabitants of the sea onthe opposite sides of a continent, and the nature of the variousinhabitants of that continent in relation to their apparent means ofimmigration, some light can be thrown on ancient geography.
The noble science of Geology loses glory from the extreme imperfectionof the record. The crust of the earth with its embedded remains mustnot be looked at as a well-filled museum, but as a poor collectionmade at hazard and at rare intervals. The accumulation of each greatfossiliferous formation will be recognised as having depended on anunusual concurrence of circumstances, and the blank intervals betweenthe successive stages as having been of vast duration. But we shall beable to gauge with some security the duration of these intervals by acomparison of the preceding and succeeding organic forms. We must becautious in attempting to correlate as strictly contemporaneous twoformations, which include few identical species, by the generalsuccession of their forms of life. As species are produced andexterminated by slowly acting and still existing causes, and not bymiraculous acts of creation and by catastrophes; and as the mostimportant of all causes of organic change is one which is almostindependent of altered and perhaps suddenly altered physicalconditions, namely, the mutual relation of organism to organism,--theimprovement of one being entailing the improvement or theextermination of others; it follows, that the amount of organic changein the fossils of consecutive formations probably serves as a fairmeasure of the lapse of actual time. A number of species, however,keeping in a body might remain for a long period unchanged, whilstwithin this same period, several of these species, by migrating intonew countries and coming into competition with foreign associates,might become modified; so that we must not overrate the accuracy oforganic change as a measure of time. During early periods of theearth's history, when the forms of life were probably fewer andsimpler, the rate of change was probably slower; and at the first dawnof life, when very few forms of the simplest structure existed, therate of change may have been slow in an extreme degree. The wholehistory of the world, as at present known, although of a length quiteincomprehensible by us, will hereafter be recognised as a merefragment of time, compared with the ages which have elapsed since thefirst creature, the progenitor of innumerable extinct and livingdescendants, was created.
In the distant future I see open fields for far more importantresearches. Psychology will be based on a new foundation, that of thenecessary acquirement of each mental power and capacity by gradation.Light will be thrown on the origin of man and his history.
Authors of the highest eminence seem to be fully satisfied with theview that each species has been independently created. To my mind itaccords better with what we know of the laws impressed on matter bythe Creator, that the production and extinction of the past andpresent inhabitants of the world should have been due to secondarycauses, like those determining the birth and death of the individual.When I view all beings not as special creations, but as the linealdescendants of some few beings which lived long before the first bedof the Silurian system was deposited, they seem to me to becomeennobled. Judging from the past, we may safely infer that not oneliving species will transmit its unaltered likeness to a distantfuturity. And of the species now living very few will transmit progenyof any kind to a far distant futurity; for the manner in which allorganic beings are grouped, shows that the greater number of speciesof each genus, and all the species of many genera, have left nodescendants, but have become utterly extinct. We can so far take aprophetic glance into futurity as to foretel that it will be thecommon and widely-spread species, belonging to the larger and dominantgroups, which will ultimately prevail and procreate new and dominantspecies. As all the living forms of life are the lineal descendants ofthose which lived long before the Silurian epoch, we may feel certainthat the ordinary succession by generation has never once been broken,and that no cataclysm has desolated the whole world. Hence we may lookwith some confidence to a secure future of equally inappreciablelength. And as natural selection works solely by and for the good ofeach being, all corporeal and mental endowments will tend to progresstowards perfection.
It is interesting to contemplate an entangled bank, clothed with manyplants of many kinds, with birds singing on the bushes, with variousinsects flitting about, and with worms crawling through the dampearth, and to reflect that these elaborately constructed forms, sodifferent from each other, and dependent on each other in so complex amanner, have all been produced by laws acting around us. These laws,taken in the largest sense, being Growth with Reproduction;Inheritance which is almost implied by reproduction; Variability fromthe indirect and direct action of the external conditions of life, andfrom use and disuse; a Ratio of Increase so high as to lead to aStruggle for Life, and as a consequence to Natural Selection,entailing Divergence of Character and the Extinction of less-improvedforms. Thus, from the war of nature, from famine and death, the mostexalted object which we are capable of conceiving, namely, theproduction of the higher animals, directly follows. There is grandeurin this view of life, with its several powers, having been originallybreathed into a few forms or into one; and that, whilst this planethas gone cycling on according to the fixed law of gravity, from sosimple a beginning endless forms most beautiful and most wonderfulhave been, and are being, evolved.