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 4
I have stated in the first chapter, that there is some evidence torender it probable, that at whatever age any variation first appearsin the parent, it tends to reappear at a corresponding age in theoffspring. Certain variations can only appear at corresponding ages,for instance, peculiarities in the caterpillar, cocoon, or imagostates of the silk-moth; or, again, in the horns of almost full-growncattle. But further than this, variations which, for all that we cansee, might have appeared earlier or later in life, tend to appear at acorresponding age in the offspring and parent. I am far from meaningthat this is invariably the case; and I could give a good many casesof variations (taking the word in the largest sense) which havesupervened at an earlier age in the child than in the parent.
These two principles, if their truth be admitted, will, I believe,explain all the above specified leading facts in embryology. But firstlet us look at a few analogous cases in domestic varieties. Someauthors who have written on Dogs, maintain that the greyhound andbulldog, though appearing so different, are really varieties mostclosely allied, and have probably descended from the same wild stock;hence I was curious to see how far their puppies differed from eachother: I was told by breeders that they differed just as much as theirparents, and this, judging by the eye, seemed almost to be the case;but on actually measuring the old dogs and their six-days old puppies,I found that the puppies had not nearly acquired their full amount ofproportional difference. So, again, I was told that the foals of cartand race-horses differed as much as the full-grown animals; and thissurprised me greatly, as I think it probable that the differencebetween these two breeds has been wholly caused by selection underdomestication; but having had careful measurements made of the dam andof a three-days old colt of a race and heavy cart-horse, I find thatthe colts have by no means acquired their full amount of proportionaldifference.
As the evidence appears to me conclusive, that the several domesticbreeds of Pigeon have descended from one wild species, I comparedyoung pigeons of various breeds, within twelve hours after beinghatched; I carefully measured the proportions (but will not here givedetails) of the beak, width of mouth, length of nostril and of eyelid,size of feet and length of leg, in the wild stock, in pouters,fantails, runts, barbs, dragons, carriers, and tumblers. Now some ofthese birds, when mature, differ so extraordinarily in length and formof beak, that they would, I cannot doubt, be ranked in distinctgenera, had they been natural productions. But when the nestling birdsof these several breeds were placed in a row, though most of themcould be distinguished from each other, yet their proportionaldifferences in the above specified several points were incomparablyless than in the full-grown birds. Some characteristic points ofdifference--for instance, that of the width of mouth--could hardly bedetected in the young. But there was one remarkable exception to thisrule, for the young of the short-faced tumbler differed from the youngof the wild rock-pigeon and of the other breeds, in all itsproportions, almost exactly as much as in the adult state.
The two principles above given seem to me to explain these facts inregard to the later embryonic stages of our domestic varieties.Fanciers select their horses, dogs, and pigeons, for breeding, whenthey are nearly grown up: they are indifferent whether the desiredqualities and structures have been acquired earlier or later in life,if the full-grown animal possesses them. And the cases just given,more especially that of pigeons, seem to show that the characteristicdifferences which give value to each breed, and which have beenaccumulated by man's selection, have not generally first appeared atan early period of life, and have been inherited by the offspring at acorresponding not early period. But the case of the short-facedtumbler, which when twelve hours old had acquired its properproportions, proves that this is not the universal rule; for here thecharacteristic differences must either have appeared at an earlierperiod than usual, or, if not so, the differences must have beeninherited, not at the corresponding, but at an earlier age.
Now let us apply these facts and the above two principles--whichlatter, though not proved true, can be shown to be in some degreeprobable--to species in a state of nature. Let us take a genus ofbirds, descended on my theory from some one parent-species, and ofwhich the several new species have become modified through naturalselection in accordance with their diverse habits. Then, from the manyslight successive steps of variation having supervened at a ratherlate age, and having been inherited at a corresponding age, the youngof the new species of our supposed genus will manifestly tend toresemble each other much more closely than do the adults, just as wehave seen in the case of pigeons. We may extend this view to wholefamilies or even classes. The fore-limbs, for instance, which servedas legs in the parent-species, may become, by a long course ofmodification, adapted in one descendant to act as hands, in another aspaddles, in another as wings; and on the above two principles--namelyof each successive modification supervening at a rather late age, andbeing inherited at a corresponding late age--the fore-limbs in theembryos of the several descendants of the parent-species will stillresemble each other closely, for they will not have been modified. Butin each individual new species, the embryonic fore-limbs will differgreatly from the fore-limbs in the mature animal; the limbs in thelatter having undergone much modification at a rather late period oflife, and having thus been converted into hands, or paddles, or wings.Whatever influence long-continued exercise or use on the one hand, anddisuse on the other, may have in modifying an organ, such influencewill mainly affect the mature animal, which has come to its fullpowers of activity and has to gain its own living; and the effectsthus produced will be inherited at a corresponding mature age. Whereasthe young will remain unmodified, or be modified in a lesser degree,by the effects of use and disuse.
In certain cases the successive steps of variation might supervene,from causes of which we are wholly ignorant, at a very early period oflife, or each step might be inherited at an earlier period than thatat which it first appeared. In either case (as with the short-facedtumbler) the young or embryo would closely resemble the matureparent-form. We have seen that this is the rule of development incertain whole groups of animals, as with cuttle-fish and spiders, andwith a few members of the great class of insects, as with Aphis. Withrespect to the final cause of the young in these cases not undergoingany metamorphosis, or closely resembling their parents from theirearliest age, we can see that this would result from the two followingcontingencies; firstly, from the young, during a course ofmodification carried on for many generations, having to provide fortheir own wants at a very early stage of development, and secondly,from their following exactly the same habits of life with theirparents; for in this case, it would be indispensable for the existenceof the species, that the child should be modified at a very early agein the same manner with its parents, in accordance with their similarhabits. Some further explanation, however, of the embryo notundergoing any metamorphosis is perhaps requisite. If, on the otherhand, it profited the young to follow habits of life in any degreedifferent from those of their parent, and consequently to beconstructed in a slightly different manner, then, on the principle ofinheritance at corresponding ages, the active young or larvae mighteasily be rendered by natural selection different to any conceivableextent from their parents. Such differences might, also, becomecorrelated with successive stages of development; so that the larvae,in the first stage, might differ greatly from the larvae in the secondstage, as we have seen to be the case with cirripedes. The adult mightbecome fitted for sites or habits, in which organs of locomotion or ofthe senses, etc., would be useless; and in this case the finalmetamorphosis would be said to be retrograde.
As all the organic beings, extinct and recent, which have ever livedon this earth have to be classed together, and as all have beenconnected by the finest gradations, the best, or indeed, if ourcollections were nearly perfect, the only possible arrangement, wouldbe genealogical. Descent being on my view the hidden bond of connexionwhich naturalists have been seeking under the term of the naturalsystem. On this view we can understand how it is that, in the eyes ofmost naturalists, the structure of the embryo is even more importantfor classification than that of the adult. For the embryo is theanimal in its less modified state; and in so far it reveals thestructure of its progenitor. In two groups of animal, however muchthey may at present differ from each other in structure and habits, ifthey pass through the same or similar embryonic stages, we may feelassured that they have both descended from the same or nearly similarparents, and are therefore in that degree closely related. Thus,community in embryonic structure reveals community of descent. It willreveal this community of descent, however much the structure of theadult may have been modified and obscured; we have seen, for instance,that cirripedes can at once be recognised by their larvae as belongingto the great class of crustaceans. As the embryonic state of eachspecies and group of species partially shows us the structure of theirless modified ancient progenitors, we can clearly see why ancient andextinct forms of life should resemble the embryos of theirdescendants,--our existing species. Agassiz believes this to be a lawof nature; but I am bound to confess that I only hope to see the lawhereafter proved true. It can be proved true in those cases alone inwhich the ancient state, now supposed to be represented in manyembryos, has not been obliterated, either by the successive variationsin a long course of modification having supervened at a very earlyage, or by the variations having been inherited at an earlier periodthan that at which they first appeared. It should also be borne inmind, that the supposed law of resemblance of ancient forms of life tothe embryonic stages of recent forms, may be true, but yet, owing tothe geological record not extending far enough back in time, mayremain for a long period, or for ever, incapable of demonstration.
Thus, as it seems to me, the leading facts in embryology, which aresecond in importance to none in natural history, are explained on theprinciple of slight modifications not appearing, in the manydescendants from some one ancient progenitor, at a very early periodin the life of each, though perhaps caused at the earliest, and beinginherited at a corresponding not early period. Embryology risesgreatly in interest, when we thus look at the embryo as a picture,more or less obscured, of the common parent-form of each great classof animals.
RUDIMENTARY, ATROPHIED, OR ABORTED ORGANS.
Organs or parts in this strange condition, bearing the stamp ofinutility, are extremely common throughout nature. For instance,rudimentary mammae are very general in the males of mammals: I presumethat the "bastard-wing" in birds may be safely considered as a digitin a rudimentary state: in very many snakes one lobe of the lungs isrudimentary; in other snakes there are rudiments of the pelvis andhind limbs. Some of the cases of rudimentary organs are extremelycurious; for instance, the presence of teeth in foetal whales, whichwhen grown up have not a tooth in their heads; and the presence ofteeth, which never cut through the gums, in the upper jaws of ourunborn calves. It has even been stated on good authority thatrudiments of teeth can be detected in the beaks of certain embryonicbirds. Nothing can be plainer than that wings are formed for flight,yet in how many insects do we see wings so reduced in size as to beutterly incapable of flight, and not rarely lying under wing-cases,firmly soldered together!
The meaning of rudimentary organs is often quite unmistakeable: forinstance there are beetles of the same genus (and even of the samespecies) resembling each other most closely in all respects, one ofwhich will have full-sized wings, and another mere rudiments ofmembrane; and here it is impossible to doubt, that the rudimentsrepresent wings. Rudimentary organs sometimes retain theirpotentiality, and are merely not developed: this seems to be the casewith the mammae of male mammals, for many instances are on record ofthese organs having become well developed in full-grown males, andhaving secreted milk. So again there are normally four developed andtwo rudimentary teats in the udders of the genus Bos, but in ourdomestic cows the two sometimes become developed and give milk. Inindividual plants of the same species the petals sometimes occur asmere rudiments, and sometimes in a well-developed state. In plantswith separated sexes, the male flowers often have a rudiment of apistil; and Kolreuter found that by crossing such male plants with anhermaphrodite species, the rudiment of the pistil in the hybridoffspring was much increased in size; and this shows that the rudimentand the perfect pistil are essentially alike in nature.
An organ serving for two purposes, may become rudimentary or utterlyaborted for one, even the more important purpose; and remain perfectlyefficient for the other. Thus in plants, the office of the pistil isto allow the pollen-tubes to reach the ovules protected in the ovariumat its base. The pistil consists of a stigma supported on the style;but in some Compositae, the male florets, which of course cannot befecundated, have a pistil, which is in a rudimentary state, for it isnot crowned with a stigma; but the style remains well developed, andis clothed with hairs as in other compositae, for the purpose ofbrushing the pollen out of the surrounding anthers. Again, an organmay become rudimentary for its proper purpose, and be used for adistinct object: in certain fish the swim-bladder seems to berudimentary for its proper function of giving buoyancy, but has becomeconverted into a nascent breathing organ or lung. Other similarinstances could be given.
Rudimentary organs in the individuals of the same species are veryliable to vary in degree of development and in other respects.Moreover, in closely allied species, the degree to which the sameorgan has been rendered rudimentary occasionally differs much. Thislatter fact is well exemplified in the state of the wings of thefemale moths in certain groups. Rudimentary organs may be utterlyaborted; and this implies, that we find in an animal or plant no traceof an organ, which analogy would lead us to expect to find, and whichis occasionally found in monstrous individuals of the species. Thus inthe snapdragon (antirrhinum) we generally do not find a rudiment of afifth stamen; but this may sometimes be seen. In tracing thehomologies of the same part in different members of a class, nothingis more common, or more necessary, than the use and discovery ofrudiments. This is well shown in the drawings given by Owen of thebones of the leg of the horse, ox, and rhinoceros.
It is an important fact that rudimentary organs, such as teeth in theupper jaws of whales and ruminants, can often be detected in theembryo, but afterwards wholly disappear. It is also, I believe, auniversal rule, that a rudimentary part or organ is of greater sizerelatively to the adjoining parts in the embryo, than in the adult; sothat the organ at this early age is less rudimentary, or even cannotbe said to be in any degree rudimentary. Hence, also, a rudimentaryorgan in the adult, is often said to have retained its embryoniccondition.
I have now given the leading facts with respect to rudimentary organs.In reflecting on them, every one must be struck with astonishment: forthe same reasoning power which tells us plainly that most parts andorgans are exquisitely adapted for certain purposes, tells us withequal plainness that these rudimentary or atrophied organs, areimperfect and useless. In works on natural history rudimentary organsare generally said to have been created "for the sake of symmetry," orin order "to complete the scheme of nature;" but this seems to me noexplanation, merely a restatement of the fact. Would it be thoughtsufficient to say that because planets revolve in elliptic coursesround the sun, satellites follow the same course round the planets,for the sake of symmetry, and to complete the scheme of nature? Aneminent physiologist accounts for the presence of rudimentary organs,by supposing that they serve to excrete matter in excess, or injuriousto the system; but can we suppose that the minute papilla, which oftenrepresents the pistil in male flowers, and which is formed merely ofcellular tissue, can thus act? Can we suppose that the formation ofrudimentary teeth which are subsequently absorbed, can be of anyservice to the rapidly growing embryonic calf by the excretion ofprecious phosphate of lime? When a man's fingers have been amputated,imperfect nails sometimes appear on the stumps: I could as soonbelieve that these vestiges of nails have appeared, not from unknownlaws of growth, but in order to excrete horny matter, as that therudimentary nails on the fin of the manatee were formed for thispurpose.
On my view of descent with modification, the origin of rudimentaryorgans is simple. We have plenty of cases of rudimentary organs in ourdomestic productions,--as the stump of a tail in tailless breeds,--thevestige of an ear in earless breeds,--the reappearance of minutedangling horns in hornless breeds of cattle, more especially,according to Youatt, in young animals,--and the state of the wholeflower in the cauliflower. We often see rudiments of various parts inmonsters. But I doubt whether any of these cases throw light on theorigin of rudimentary organs in a state of nature, further than byshowing that rudiments can be produced; for I doubt whether speciesunder nature ever undergo abrupt changes. I believe that disuse hasbeen the main agency; that it has led in successive generations to thegradual reduction of various organs, until they have becomerudimentary,--as in the case of the eyes of animals inhabiting darkcaverns, and of the wings of birds inhabiting oceanic islands, whichhave seldom been forced to take flight, and have ultimately lost thepower of flying. Again, an organ useful under certain conditions,might become injurious under others, as with the wings of beetlesliving on small and exposed islands; and in this case naturalselection would continue slowly to reduce the organ, until it wasrendered harmless and rudimentary.
Any change in function, which can be effected by insensibly smallsteps, is within the power of natural selection; so that an organrendered, during changed habits of life, useless or injurious for onepurpose, might easily be modified and used for another purpose. Or anorgan might be retained for one alone of its former functions. Anorgan, when rendered useless, may well be variable, for its variationscannot be checked by natural selection. At whatever period of lifedisuse or selection reduces an organ, and this will generally be whenthe being has come to maturity and to its full powers of action, theprinciple of inheritance at corresponding ages will reproduce theorgan in its reduced state at the same age, and consequently willseldom affect or reduce it in the embryo. Thus we can understand thegreater relative size of rudimentary organs in the embryo, and theirlesser relative size in the adult. But if each step of the process ofreduction were to be inherited, not at the corresponding age, but atan extremely early period of life (as we have good reason to believeto be possible) the rudimentary part would tend to be wholly lost, andwe should have a case of complete abortion. The principle, also, ofeconomy, explained in a former chapter, by which the materials formingany part or structure, if not useful to the possessor, will be savedas far as is possible, will probably often come into play; and thiswill tend to cause the entire obliteration of a rudimentary organ.
As the presence of rudimentary organs is thus due to the tendency inevery part of the organisation, which has long existed, to beinherited--we can understand, on the genealogical view ofclassification, how it is that systematists have found rudimentaryparts as useful as, or even sometimes more useful than, parts of highphysiological importance. Rudimentary organs may be compared with theletters in a word, still retained in the spelling, but become uselessin the pronunciation, but which serve as a clue in seeking for itsderivation. On the view of descent with modification, we may concludethat the existence of organs in a rudimentary, imperfect, and uselesscondition, or quite aborted, far from presenting a strange difficulty,as they assuredly do on the ordinary doctrine of creation, might evenhave been anticipated, and can be accounted for by the laws ofinheritance.
SUMMARY.
In this chapter I have attempted to show, that the subordination ofgroup to group in all organisms throughout all time; that the natureof the relationship, by which all living and extinct beings are unitedby complex, radiating, and circuitous lines of affinities into onegrand system; the rules followed and the difficulties encountered bynaturalists in their classifications; the value set upon characters,if constant and prevalent, whether of high vital importance, or of themost trifling importance, or, as in rudimentary organs, of noimportance; the wide opposition in value between analogical oradaptive characters, and characters of true affinity; and other suchrules;--all naturally follow on the view of the common parentage ofthose forms which are considered by naturalists as allied, togetherwith their modification through natural selection, with itscontingencies of extinction and divergence of character. Inconsidering this view of classification, it should be borne in mindthat the element of descent has been universally used in rankingtogether the sexes, ages, and acknowledged varieties of the samespecies, however different they may be in structure. If we extend theuse of this element of descent,--the only certainly known cause ofsimilarity in organic beings,--we shall understand what is meant bythe natural system: it is genealogical in its attempted arrangement,with the grades of acquired difference marked by the terms varieties,species, genera, families, orders, and classes.
On this same view of descent with modification, all the great facts inMorphology become intelligible,--whether we look to the same patterndisplayed in the homologous organs, to whatever purpose applied, ofthe different species of a class; or to the homologous partsconstructed on the same pattern in each individual animal and plant.
On the principle of successive slight variations, not necessarily orgenerally supervening at a very early period of life, and beinginherited at a corresponding period, we can understand the greatleading facts in Embryology; namely, the resemblance in an individualembryo of the homologous parts, which when matured will become widelydifferent from each other in structure and function; and theresemblance in different species of a class of the homologous parts ororgans, though fitted in the adult members for purposes as differentas possible. Larvae are active embryos, which have become speciallymodified in relation to their habits of life, through the principle ofmodifications being inherited at corresponding ages. On this sameprinciple--and bearing in mind, that when organs are reduced in size,either from disuse or selection, it will generally be at that periodof life when the being has to provide for its own wants, and bearingin mind how strong is the principle of inheritance--the occurrence ofrudimentary organs and their final abortion, present to us noinexplicable difficulties; on the contrary, their presence might havebeen even anticipated. The importance of embryological characters andof rudimentary organs in classification is intelligible, on the viewthat an arrangement is only so far natural as it is genealogical.
Finally, the several classes of facts which have been considered inthis chapter, seem to me to proclaim so plainly, that the innumerablespecies, genera, and families of organic beings, with which this worldis peopled, have all descended, each within its own class or group,from common parents, and have all been modified in the course ofdescent, that I should without hesitation adopt this view, even if itwere unsupported by other facts or arguments.