物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 6. DIFFICULTIES ON THEORY. Page 2
If about a dozen genera of birds had become extinct or were unknown,who would have ventured to have surmised that birds might have existedwhich used their wings solely as flappers, like the logger-headed duck(Micropterus of Eyton); as fins in the water and front legs on theland, like the penguin; as sails, like the ostrich; and functionallyfor no purpose, like the Apteryx. Yet the structure of each of thesebirds is good for it, under the conditions of life to which it isexposed, for each has to live by a struggle; but it is not necessarilythe best possible under all possible conditions. It must not beinferred from these remarks that any of the grades of wing-structurehere alluded to, which perhaps may all have resulted from disuse,indicate the natural steps by which birds have acquired their perfectpower of flight; but they serve, at least, to show what diversifiedmeans of transition are possible.
Seeing that a few members of such water-breathing classes as theCrustacea and Mollusca are adapted to live on the land, and seeingthat we have flying birds and mammals, flying insects of the mostdiversified types, and formerly had flying reptiles, it is conceivablethat flying-fish, which now glide far through the air, slightly risingand turning by the aid of their fluttering fins, might have beenmodified into perfectly winged animals. If this had been effected, whowould have ever imagined that in an early transitional state they hadbeen inhabitants of the open ocean, and had used their incipientorgans of flight exclusively, as far as we know, to escape beingdevoured by other fish?
When we see any structure highly perfected for any particular habit,as the wings of a bird for flight, we should bear in mind that animalsdisplaying early transitional grades of the structure will seldomcontinue to exist to the present day, for they will have beensupplanted by the very process of perfection through naturalselection. Furthermore, we may conclude that transitional gradesbetween structures fitted for very different habits of life willrarely have been developed at an early period in great numbers andunder many subordinate forms. Thus, to return to our imaginaryillustration of the flying-fish, it does not seem probable that fishescapable of true flight would have been developed under manysubordinate forms, for taking prey of many kinds in many ways, on theland and in the water, until their organs of flight had come to a highstage of perfection, so as to have given them a decided advantage overother animals in the battle for life. Hence the chance of discoveringspecies with transitional grades of structure in a fossil conditionwill always be less, from their having existed in lesser numbers, thanin the case of species with fully developed structures.
I will now give two or three instances of diversified and of changedhabits in the individuals of the same species. When either caseoccurs, it would be easy for natural selection to fit the animal, bysome modification of its structure, for its changed habits, orexclusively for one of its several different habits. But it isdifficult to tell, and immaterial for us, whether habits generallychange first and structure afterwards; or whether slight modificationsof structure lead to changed habits; both probably often change almostsimultaneously. Of cases of changed habits it will suffice merely toallude to that of the many British insects which now feed on exoticplants, or exclusively on artificial substances. Of diversified habitsinnumerable instances could be given: I have often watched a tyrantflycatcher (Saurophagus sulphuratus) in South America, hovering overone spot and then proceeding to another, like a kestrel, and at othertimes standing stationary on the margin of water, and then dashinglike a kingfisher at a fish. In our own country the larger titmouse(Parus major) may be seen climbing branches, almost like a creeper; itoften, like a shrike, kills small birds by blows on the head; and Ihave many times seen and heard it hammering the seeds of the yew on abranch, and thus breaking them like a nuthatch. In North America theblack bear was seen by Hearne swimming for hours with widely openmouth, thus catching, like a whale, insects in the water. Even in soextreme a case as this, if the supply of insects were constant, and ifbetter adapted competitors did not already exist in the country, I cansee no difficulty in a race of bears being rendered, by naturalselection, more and more aquatic in their structure and habits, withlarger and larger mouths, till a creature was produced as monstrous asa whale.
As we sometimes see individuals of a species following habits widelydifferent from those both of their own species and of the otherspecies of the same genus, we might expect, on my theory, that suchindividuals would occasionally have given rise to new species, havinganomalous habits, and with their structure either slightly orconsiderably modified from that of their proper type. And suchinstances do occur in nature. Can a more striking instance ofadaptation be given than that of a woodpecker for climbing trees andfor seizing insects in the chinks of the bark? Yet in North Americathere are woodpeckers which feed largely on fruit, and others withelongated wings which chase insects on the wing; and on the plains ofLa Plata, where not a tree grows, there is a woodpecker, which inevery essential part of its organisation, even in its colouring, inthe harsh tone of its voice, and undulatory flight, told me plainly ofits close blood-relationship to our common species; yet it is awoodpecker which never climbs a tree!
Petrels are the most aerial and oceanic of birds, yet in the quietSounds of Tierra del Fuego, the Puffinuria berardi, in its generalhabits, in its astonishing power of diving, its manner of swimming,and of flying when unwillingly it takes flight, would be mistaken byany one for an auk or grebe; nevertheless, it is essentially a petrel,but with many parts of its organisation profoundly modified. On theother hand, the acutest observer by examining the dead body of thewater-ouzel would never have suspected its sub-aquatic habits; yetthis anomalous member of the strictly terrestrial thrush family whollysubsists by diving,--grasping the stones with its feet and using itswings under water.
He who believes that each being has been created as we now see it,must occasionally have felt surprise when he has met with an animalhaving habits and structure not at all in agreement. What can beplainer than that the webbed feet of ducks and geese are formed forswimming? yet there are upland geese with webbed feet which rarely ornever go near the water; and no one except Audubon has seen thefrigate-bird, which has all its four toes webbed, alight on thesurface of the sea. On the other hand, grebes and coots are eminentlyaquatic, although their toes are only bordered by membrane. What seemsplainer than that the long toes of grallatores are formed for walkingover swamps and floating plants, yet the water-hen is nearly asaquatic as the coot; and the landrail nearly as terrestrial as thequail or partridge. In such cases, and many others could be given,habits have changed without a corresponding change of structure. Thewebbed feet of the upland goose may be said to have become rudimentaryin function, though not in structure. In the frigate-bird, thedeeply-scooped membrane between the toes shows that structure hasbegun to change.
He who believes in separate and innumerable acts of creation will say,that in these cases it has pleased the Creator to cause a being of onetype to take the place of one of another type; but this seems to meonly restating the fact in dignified language. He who believes in thestruggle for existence and in the principle of natural selection, willacknowledge that every organic being is constantly endeavouring toincrease in numbers; and that if any one being vary ever so little,either in habits or structure, and thus gain an advantage over someother inhabitant of the country, it will seize on the place of thatinhabitant, however different it may be from its own place. Hence itwill cause him no surprise that there should be geese andfrigate-birds with webbed feet, either living on the dry land or mostrarely alighting on the water; that there should be long-toedcorncrakes living in meadows instead of in swamps; that there shouldbe woodpeckers where not a tree grows; that there should be divingthrushes, and petrels with the habits of auks.
ORGANS OF EXTREME PERFECTION AND COMPLICATION.
To suppose that the eye, with all its inimitable contrivances foradjusting the focus to different distances, for admitting differentamounts of light, and for the correction of spherical and chromaticaberration, could have been formed by natural selection, seems, Ifreely confess, absurd in the highest possible degree. Yet reasontells me, that if numerous gradations from a perfect and complex eyeto one very imperfect and simple, each grade being useful to itspossessor, can be shown to exist; if further, the eye does vary everso slightly, and the variations be inherited, which is certainly thecase; and if any variation or modification in the organ be ever usefulto an animal under changing conditions of life, then the difficulty ofbelieving that a perfect and complex eye could be formed by naturalselection, though insuperable by our imagination, can hardly beconsidered real. How a nerve comes to be sensitive to light, hardlyconcerns us more than how life itself first originated; but I mayremark that several facts make me suspect that any sensitive nerve maybe rendered sensitive to light, and likewise to those coarservibrations of the air which produce sound.
In looking for the gradations by which an organ in any species hasbeen perfected, we ought to look exclusively to its lineal ancestors;but this is scarcely ever possible, and we are forced in each case tolook to species of the same group, that is to the collateraldescendants from the same original parent-form, in order to see whatgradations are possible, and for the chance of some gradations havingbeen transmitted from the earlier stages of descent, in an unalteredor little altered condition. Amongst existing Vertebrata, we find buta small amount of gradation in the structure of the eye, and fromfossil species we can learn nothing on this head. In this great classwe should probably have to descend far beneath the lowest knownfossiliferous stratum to discover the earlier stages, by which the eyehas been perfected.
In the Articulata we can commence a series with an optic nerve merelycoated with pigment, and without any other mechanism; and from thislow stage, numerous gradations of structure, branching off in twofundamentally different lines, can be shown to exist, until we reach amoderately high stage of perfection. In certain crustaceans, forinstance, there is a double cornea, the inner one divided into facets,within each of which there is a lens-shaped swelling. In othercrustaceans the transparent cones which are coated by pigment, andwhich properly act only by excluding lateral pencils of light, areconvex at their upper ends and must act by convergence; and at theirlower ends there seems to be an imperfect vitreous substance. Withthese facts, here far too briefly and imperfectly given, which showthat there is much graduated diversity in the eyes of livingcrustaceans, and bearing in mind how small the number of livinganimals is in proportion to those which have become extinct, I can seeno very great difficulty (not more than in the case of many otherstructures) in believing that natural selection has converted thesimple apparatus of an optic nerve merely coated with pigment andinvested by transparent membrane, into an optical instrument asperfect as is possessed by any member of the great Articulate class.
He who will go thus far, if he find on finishing this treatise thatlarge bodies of facts, otherwise inexplicable, can be explained by thetheory of descent, ought not to hesitate to go further, and to admitthat a structure even as perfect as the eye of an eagle might beformed by natural selection, although in this case he does not knowany of the transitional grades. His reason ought to conquer hisimagination; though I have felt the difficulty far too keenly to besurprised at any degree of hesitation in extending the principle ofnatural selection to such startling lengths.
It is scarcely possible to avoid comparing the eye to a telescope. Weknow that this instrument has been perfected by the long-continuedefforts of the highest human intellects; and we naturally infer thatthe eye has been formed by a somewhat analogous process. But may notthis inference be presumptuous? Have we any right to assume that theCreator works by intellectual powers like those of man? If we mustcompare the eye to an optical instrument, we ought in imagination totake a thick layer of transparent tissue, with a nerve sensitive tolight beneath, and then suppose every part of this layer to becontinually changing slowly in density, so as to separate into layersof different densities and thicknesses, placed at different distancesfrom each other, and with the surfaces of each layer slowly changingin form. Further we must suppose that there is a power always intentlywatching each slight accidental alteration in the transparent layers;and carefully selecting each alteration which, under variedcircumstances, may in any way, or in any degree, tend to produce adistincter image. We must suppose each new state of the instrument tobe multiplied by the million; and each to be preserved till a betterbe produced, and then the old ones to be destroyed. In living bodies,variation will cause the slight alterations, generation will multiplythem almost infinitely, and natural selection will pick out withunerring skill each improvement. Let this process go on for millionson millions of years; and during each year on millions of individualsof many kinds; and may we not believe that a living optical instrumentmight thus be formed as superior to one of glass, as the works of theCreator are to those of man?
If it could be demonstrated that any complex organ existed, whichcould not possibly have been formed by numerous, successive, slightmodifications, my theory would absolutely break down. But I can findout no such case. No doubt many organs exist of which we do not knowthe transitional grades, more especially if we look to much-isolatedspecies, round which, according to my theory, there has been muchextinction. Or again, if we look to an organ common to all the membersof a large class, for in this latter case the organ must have beenfirst formed at an extremely remote period, since which all the manymembers of the class have been developed; and in order to discover theearly transitional grades through which the organ has passed, weshould have to look to very ancient ancestral forms, long since becomeextinct.
We should be extremely cautious in concluding that an organ could nothave been formed by transitional gradations of some kind. Numerouscases could be given amongst the lower animals of the same organperforming at the same time wholly distinct functions; thus thealimentary canal respires, digests, and excretes in the larva of thedragon-fly and in the fish Cobites. In the Hydra, the animal may beturned inside out, and the exterior surface will then digest and thestomach respire. In such cases natural selection might easilyspecialise, if any advantage were thus gained, a part or organ, whichhad performed two functions, for one function alone, and thus whollychange its nature by insensible steps. Two distinct organs sometimesperform simultaneously the same function in the same individual; togive one instance, there are fish with gills or branchiae that breathethe air dissolved in the water, at the same time that they breathefree air in their swimbladders, this latter organ having a ductuspneumaticus for its supply, and being divided by highly vascularpartitions. In these cases, one of the two organs might with ease bemodified and perfected so as to perform all the work by itself, beingaided during the process of modification by the other organ; and thenthis other organ might be modified for some other and quite distinctpurpose, or be quite obliterated.
The illustration of the swimbladder in fishes is a good one, becauseit shows us clearly the highly important fact that an organ originallyconstructed for one purpose, namely flotation, may be converted intoone for a wholly different purpose, namely respiration. Theswimbladder has, also, been worked in as an accessory to the auditoryorgans of certain fish, or, for I do not know which view is nowgenerally held, a part of the auditory apparatus has been worked in asa complement to the swimbladder. All physiologists admit that theswimbladder is homologous, or "ideally similar," in position andstructure with the lungs of the higher vertebrate animals: hence thereseems to me to be no great difficulty in believing that naturalselection has actually converted a swimbladder into a lung, or organused exclusively for respiration.
I can, indeed, hardly doubt that all vertebrate animals having truelungs have descended by ordinary generation from an ancient prototype,of which we know nothing, furnished with a floating apparatus orswimbladder. We can thus, as I infer from Professor Owen's interestingdescription of these parts, understand the strange fact that everyparticle of food and drink which we swallow has to pass over theorifice of the trachea, with some risk of falling into the lungs,notwithstanding the beautiful contrivance by which the glottis isclosed. In the higher Vertebrata the branchiae have whollydisappeared--the slits on the sides of the neck and the loop-likecourse of the arteries still marking in the embryo their formerposition. But it is conceivable that the now utterly lost branchiaemight have been gradually worked in by natural selection for somequite distinct purpose: in the same manner as, on the view entertainedby some naturalists that the branchiae and dorsal scales of Annelidsare homologous with the wings and wing-covers of insects, it isprobable that organs which at a very ancient period served forrespiration have been actually converted into organs of flight.
In considering transitions of organs, it is so important to bear inmind the probability of conversion from one function to another, thatI will give one more instance. Pedunculated cirripedes have two minutefolds of skin, called by me the ovigerous frena, which serve, throughthe means of a sticky secretion, to retain the eggs until they arehatched within the sack. These cirripedes have no branchiae, the wholesurface of the body and sack, including the small frena, serving forrespiration. The Balanidae or sessile cirripedes, on the other hand,have no ovigerous frena, the eggs lying loose at the bottom of thesack, in the well-enclosed shell; but they have large foldedbranchiae. Now I think no one will dispute that the ovigerous frena inthe one family are strictly homologous with the branchiae of the otherfamily; indeed, they graduate into each other. Therefore I do notdoubt that little folds of skin, which originally served as ovigerousfrena, but which, likewise, very slightly aided the act ofrespiration, have been gradually converted by natural selection intobranchiae, simply through an increase in their size and theobliteration of their adhesive glands. If all pedunculated cirripedeshad become extinct, and they have already suffered far more extinctionthan have sessile cirripedes, who would ever have imagined that thebranchiae in this latter family had originally existed as organs forpreventing the ova from being washed out of the sack?