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 3
Finally, we have seen that natural selection, which results from thestruggle for existence, and which almost inevitably induces extinctionand divergence of character in the many descendants from one dominantparent-species, explains that great and universal feature in theaffinities of all organic beings, namely, their subordination in groupunder group. We use the element of descent in classing the individualsof both sexes and of all ages, although having few characters incommon, under one species; we use descent in classing acknowledgedvarieties, however different they may be from their parent; and Ibelieve this element of descent is the hidden bond of connexion whichnaturalists have sought under the term of the Natural System. On thisidea of the natural system being, in so far as it has been perfected,genealogical in its arrangement, with the grades of difference betweenthe descendants from a common parent, expressed by the terms genera,families, orders, etc., we can understand the rules which we arecompelled to follow in our classification. We can understand why wevalue certain resemblances far more than others; why we are permittedto use rudimentary and useless organs, or others of triflingphysiological importance; why, in comparing one group with a distinctgroup, we summarily reject analogical or adaptive characters, and yetuse these same characters within the limits of the same group. We canclearly see how it is that all living and extinct forms can be groupedtogether in one great system; and how the several members of eachclass are connected together by the most complex and radiating linesof affinities. We shall never, probably, disentangle the inextricableweb of affinities between the members of any one class; but when wehave a distinct object in view, and do not look to some unknown planof creation, we may hope to make sure but slow progress.
MORPHOLOGY.
We have seen that the members of the same class, independently oftheir habits of life, resemble each other in the general plan of theirorganisation. This resemblance is often expressed by the term "unityof type;" or by saying that the several parts and organs in thedifferent species of the class are homologous. The whole subject isincluded under the general name of Morphology. This is the mostinteresting department of natural history, and may be said to be itsvery soul. What can be more curious than that the hand of a man,formed for grasping, that of a mole for digging, the leg of the horse,the paddle of the porpoise, and the wing of the bat, should all beconstructed on the same pattern, and should include the same bones, inthe same relative positions? Geoffroy St. Hilaire has insistedstrongly on the high importance of relative connexion in homologousorgans: the parts may change to almost any extent in form and size,and yet they always remain connected together in the same order. Wenever find, for instance, the bones of the arm and forearm, or of thethigh and leg, transposed. Hence the same names can be given to thehomologous bones in widely different animals. We see the same greatlaw in the construction of the mouths of insects: what can be moredifferent than the immensely long spiral proboscis of a sphinx-moth,the curious folded one of a bee or bug, and the great jaws of abeetle?--yet all these organs, serving for such different purposes,are formed by infinitely numerous modifications of an upper lip,mandibles, and two pairs of maxillae. Analogous laws govern theconstruction of the mouths and limbs of crustaceans. So it is with theflowers of plants.
Nothing can be more hopeless than to attempt to explain thissimilarity of pattern in members of the same class, by utility or bythe doctrine of final causes. The hopelessness of the attempt has beenexpressly admitted by Owen in his most interesting work on the 'Natureof Limbs.' On the ordinary view of the independent creation of eachbeing, we can only say that so it is;--that it has so pleased theCreator to construct each animal and plant.
The explanation is manifest on the theory of the natural selection ofsuccessive slight modifications,--each modification being profitablein some way to the modified form, but often affecting by correlationof growth other parts of the organisation. In changes of this nature,there will be little or no tendency to modify the original pattern, orto transpose parts. The bones of a limb might be shortened and widenedto any extent, and become gradually enveloped in thick membrane, so asto serve as a fin; or a webbed foot might have all its bones, orcertain bones, lengthened to any extent, and the membrane connectingthem increased to any extent, so as to serve as a wing: yet in allthis great amount of modification there will be no tendency to alterthe framework of bones or the relative connexion of the several parts.If we suppose that the ancient progenitor, the archetype as it may becalled, of all mammals, had its limbs constructed on the existinggeneral pattern, for whatever purpose they served, we can at onceperceive the plain signification of the homologous construction of thelimbs throughout the whole class. So with the mouths of insects, wehave only to suppose that their common progenitor had an upper lip,mandibles, and two pair of maxillae, these parts being perhaps verysimple in form; and then natural selection will account for theinfinite diversity in structure and function of the mouths of insects.Nevertheless, it is conceivable that the general pattern of an organmight become so much obscured as to be finally lost, by the atrophyand ultimately by the complete abortion of certain parts, by thesoldering together of other parts, and by the doubling ormultiplication of others,--variations which we know to be within thelimits of possibility. In the paddles of the extinct giganticsea-lizards, and in the mouths of certain suctorial crustaceans, thegeneral pattern seems to have been thus to a certain extent obscured.
There is another and equally curious branch of the present subject;namely, the comparison not of the same part in different members of aclass, but of the different parts or organs in the same individual.Most physiologists believe that the bones of the skull are homologouswith--that is correspond in number and in relative connexion with--theelemental parts of a certain number of vertebrae. The anterior andposterior limbs in each member of the vertebrate and articulateclasses are plainly homologous. We see the same law in comparing thewonderfully complex jaws and legs in crustaceans. It is familiar toalmost every one, that in a flower the relative position of thesepals, petals, stamens, and pistils, as well as their intimatestructure, are intelligible on the view that they consist ofmetamorphosed leaves, arranged in a spire. In monstrous plants, weoften get direct evidence of the possibility of one organ beingtransformed into another; and we can actually see in embryoniccrustaceans and in many other animals, and in flowers, that organs,which when mature become extremely different, are at an early stage ofgrowth exactly alike.
How inexplicable are these facts on the ordinary view of creation! Whyshould the brain be enclosed in a box composed of such numerous andsuch extraordinarily shaped pieces of bone? As Owen has remarked, thebenefit derived from the yielding of the separate pieces in the act ofparturition of mammals, will by no means explain the same constructionin the skulls of birds. Why should similar bones have been created inthe formation of the wing and leg of a bat, used as they are for suchtotally different purposes? Why should one crustacean, which has anextremely complex mouth formed of many parts, consequently always havefewer legs; or conversely, those with many legs have simpler mouths?Why should the sepals, petals, stamens, and pistils in any individualflower, though fitted for such widely different purposes, be allconstructed on the same pattern?
On the theory of natural selection, we can satisfactorily answer thesequestions. In the vertebrata, we see a series of internal vertebraebearing certain processes and appendages; in the articulata, we seethe body divided into a series of segments, bearing externalappendages; and in flowering plants, we see a series of successivespiral whorls of leaves. An indefinite repetition of the same part ororgan is the common characteristic (as Owen has observed) of all lowor little-modified forms; therefore we may readily believe that theunknown progenitor of the vertebrata possessed many vertebrae; theunknown progenitor of the articulata, many segments; and the unknownprogenitor of flowering plants, many spiral whorls of leaves. We haveformerly seen that parts many times repeated are eminently liable tovary in number and structure; consequently it is quite probable thatnatural selection, during a long-continued course of modification,should have seized on a certain number of the primordially similarelements, many times repeated, and have adapted them to the mostdiverse purposes. And as the whole amount of modification will havebeen effected by slight successive steps, we need not wonder atdiscovering in such parts or organs, a certain degree of fundamentalresemblance, retained by the strong principle of inheritance.
In the great class of molluscs, though we can homologise the parts ofone species with those of another and distinct species, we canindicate but few serial homologies; that is, we are seldom enabled tosay that one part or organ is homologous with another in the sameindividual. And we can understand this fact; for in molluscs, even inthe lowest members of the class, we do not find nearly so muchindefinite repetition of any one part, as we find in the other greatclasses of the animal and vegetable kingdoms.
Naturalists frequently speak of the skull as formed of metamorphosedvertebrae: the jaws of crabs as metamorphosed legs; the stamens andpistils of flowers as metamorphosed leaves; but it would in thesecases probably be more correct, as Professor Huxley has remarked, tospeak of both skull and vertebrae, both jaws and legs, etc.,--ashaving been metamorphosed, not one from the other, but from somecommon element. Naturalists, however, use such language only in ametaphorical sense: they are far from meaning that during a longcourse of descent, primordial organs of any kind--vertebrae in the onecase and legs in the other--have actually been modified into skulls orjaws. Yet so strong is the appearance of a modification of this naturehaving occurred, that naturalists can hardly avoid employing languagehaving this plain signification. On my view these terms may be usedliterally; and the wonderful fact of the jaws, for instance, of a crabretaining numerous characters, which they would probably have retainedthrough inheritance, if they had really been metamorphosed during along course of descent from true legs, or from some simple appendage,is explained.
EMBRYOLOGY.
It has already been casually remarked that certain organs in theindividual, which when mature become widely different and serve fordifferent purposes, are in the embryo exactly alike. The embryos,also, of distinct animals within the same class are often strikinglysimilar: a better proof of this cannot be given, than a circumstancementioned by Agassiz, namely, that having forgotten to ticket theembryo of some vertebrate animal, he cannot now tell whether it bethat of a mammal, bird, or reptile. The vermiform larvae of moths,flies, beetles, etc., resemble each other much more closely than dothe mature insects; but in the case of larvae, the embryos are active,and have been adapted for special lines of life. A trace of the law ofembryonic resemblance, sometimes lasts till a rather late age: thusbirds of the same genus, and of closely allied genera, often resembleeach other in their first and second plumage; as we see in the spottedfeathers in the thrush group. In the cat tribe, most of the speciesare striped or spotted in lines; and stripes can be plainlydistinguished in the whelp of the lion. We occasionally though rarelysee something of this kind in plants: thus the embryonic leaves of theulex or furze, and the first leaves of the phyllodineous acaceas, arepinnate or divided like the ordinary leaves of the leguminosae.
The case, however, is different when an animal during any part of itsembryonic career is active, and has to provide for itself. The periodof activity may come on earlier or later in life; but whenever itcomes on, the adaptation of the larva to its conditions of life isjust as perfect and as beautiful as in the adult animal. From suchspecial adaptations, the similarity of the larvae or active embryos ofallied animals is sometimes much obscured; and cases could be given ofthe larvae of two species, or of two groups of species, differingquite as much, or even more, from each other than do their adultparents. In most cases, however, the larvae, though active, still obeymore or less closely the law of common embryonic resemblance.Cirripedes afford a good instance of this: even the illustrious Cuvierdid not perceive that a barnacle was, as it certainly is, acrustacean; but a glance at the larva shows this to be the case in anunmistakeable manner. So again the two main divisions of cirripedes,the pedunculated and sessile, which differ widely in externalappearance, have larvae in all their several stages barelydistinguishable.
The embryo in the course of development generally rises inorganisation: I use this expression, though I am aware that it ishardly possible to define clearly what is meant by the organisationbeing higher or lower. But no one probably will dispute that thebutterfly is higher than the caterpillar. In some cases, however, themature animal is generally considered as lower in the scale than thelarva, as with certain parasitic crustaceans. To refer once again tocirripedes: the larvae in the first stage have three pairs of legs, avery simple single eye, and a probosciformed mouth, with which theyfeed largely, for they increase much in size. In the second stage,answering to the chrysalis stage of butterflies, they have six pairsof beautifully constructed natatory legs, a pair of magnificentcompound eyes, and extremely complex antennae; but they have a closedand imperfect mouth, and cannot feed: their function at this stage is,to search by their well-developed organs of sense, and to reach bytheir active powers of swimming, a proper place on which to becomeattached and to undergo their final metamorphosis. When this iscompleted they are fixed for life: their legs are now converted intoprehensile organs; they again obtain a well-constructed mouth; butthey have no antennae, and their two eyes are now reconverted into aminute, single, and very simple eye-spot. In this last and completestate, cirripedes may be considered as either more highly or morelowly organised than they were in the larval condition. But in somegenera the larvae become developed either into hermaphrodites havingthe ordinary structure, or into what I have called complemental males:and in the latter, the development has assuredly been retrograde; forthe male is a mere sack, which lives for a short time, and isdestitute of mouth, stomach, or other organ of importance, exceptingfor reproduction.
We are so much accustomed to see differences in structure between theembryo and the adult, and likewise a close similarity in the embryosof widely different animals within the same class, that we might beled to look at these facts as necessarily contingent in some manner ongrowth. But there is no obvious reason why, for instance, the wing ofa bat, or the fin of a porpoise, should not have been sketched outwith all the parts in proper proportion, as soon as any structurebecame visible in the embryo. And in some whole groups of animals andin certain members of other groups, the embryo does not at any perioddiffer widely from the adult: thus Owen has remarked in regard tocuttle-fish, "there is no metamorphosis; the cephalopodic character ismanifested long before the parts of the embryo are completed;" andagain in spiders, "there is nothing worthy to be called ametamorphosis." The larvae of insects, whether adapted to the mostdiverse and active habits, or quite inactive, being fed by theirparents or placed in the midst of proper nutriment, yet nearly allpass through a similar worm-like stage of development; but in some fewcases, as in that of Aphis, if we look to the admirable drawings byProfessor Huxley of the development of this insect, we see no trace ofthe vermiform stage.
How, then, can we explain these several facts in embryology,--namelythe very general, but not universal difference in structure betweenthe embryo and the adult;--of parts in the same individual embryo,which ultimately become very unlike and serve for diverse purposes,being at this early period of growth alike;--of embryos of differentspecies within the same class, generally, but not universally,resembling each other;--of the structure of the embryo not beingclosely related to its conditions of existence, except when the embryobecomes at any period of life active and has to provide foritself;--of the embryo apparently having sometimes a higherorganisation than the mature animal, into which it is developed. Ibelieve that all these facts can be explained, as follows, on the viewof descent with modification.
It is commonly assumed, perhaps from monstrosities often affecting theembryo at a very early period, that slight variations necessarilyappear at an equally early period. But we have little evidence on thishead--indeed the evidence rather points the other way; for it isnotorious that breeders of cattle, horses, and various fancy animals,cannot positively tell, until some time after the animal has beenborn, what its merits or form will ultimately turn out. We see thisplainly in our own children; we cannot always tell whether the childwill be tall or short, or what its precise features will be. Thequestion is not, at what period of life any variation has been caused,but at what period it is fully displayed. The cause may have acted,and I believe generally has acted, even before the embryo is formed;and the variation may be due to the male and female sexual elementshaving been affected by the conditions to which either parent, ortheir ancestors, have been exposed. Nevertheless an effect thus causedat a very early period, even before the formation of the embryo, mayappear late in life; as when an hereditary disease, which appears inold age alone, has been communicated to the offspring from thereproductive element of one parent. Or again, as when the horns ofcross-bred cattle have been affected by the shape of the horns ofeither parent. For the welfare of a very young animal, as long as itremains in its mother's womb, or in the egg, or as long as it isnourished and protected by its parent, it must be quite unimportantwhether most of its characters are fully acquired a little earlier orlater in life. It would not signify, for instance, to a bird whichobtained its food best by having a long beak, whether or not itassumed a beak of this particular length, as long as it was fed by itsparents. Hence, I conclude, that it is quite possible, that each ofthe many successive modifications, by which each species has acquiredits present structure, may have supervened at a not very early periodof life; and some direct evidence from our domestic animals supportsthis view. But in other cases it is quite possible that eachsuccessive modification, or most of them, may have appeared at anextremely early period.