物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 7. INSTINCT. Page 1
Instincts comparable with habits, but different in their origin.Instincts graduated.Aphides and ants.Instincts variable.Domestic instincts, their origin.Natural instincts of the cuckoo, ostrich, and parasitic bees.Slave-making ants.Hive-bee, its cell-making instinct.Difficulties on the theory of the Natural Selection of instincts.Neuter or sterile insects.Summary.
The subject of instinct might have been worked into the previouschapters; but I have thought that it would be more convenient to treatthe subject separately, especially as so wonderful an instinct as thatof the hive-bee making its cells will probably have occurred to manyreaders, as a difficulty sufficient to overthrow my whole theory. Imust premise, that I have nothing to do with the origin of the primarymental powers, any more than I have with that of life itself. We areconcerned only with the diversities of instinct and of the othermental qualities of animals within the same class.
I will not attempt any definition of instinct. It would be easy toshow that several distinct mental actions are commonly embraced bythis term; but every one understands what is meant, when it is saidthat instinct impels the cuckoo to migrate and to lay her eggs inother birds' nests. An action, which we ourselves should requireexperience to enable us to perform, when performed by an animal, moreespecially by a very young one, without any experience, and whenperformed by many individuals in the same way, without their knowingfor what purpose it is performed, is usually said to be instinctive.But I could show that none of these characters of instinct areuniversal. A little dose, as Pierre Huber expresses it, of judgment orreason, often comes into play, even in animals very low in the scaleof nature.
Frederick Cuvier and several of the older metaphysicians have comparedinstinct with habit. This comparison gives, I think, a remarkablyaccurate notion of the frame of mind under which an instinctive actionis performed, but not of its origin. How unconsciously many habitualactions are performed, indeed not rarely in direct opposition to ourconscious will! yet they may be modified by the will or reason. Habitseasily become associated with other habits, and with certain periodsof time and states of the body. When once acquired, they often remainconstant throughout life. Several other points of resemblance betweeninstincts and habits could be pointed out. As in repeating awell-known song, so in instincts, one action follows another by a sortof rhythm; if a person be interrupted in a song, or in repeatinganything by rote, he is generally forced to go back to recover thehabitual train of thought: so P. Huber found it was with acaterpillar, which makes a very complicated hammock; for if he took acaterpillar which had completed its hammock up to, say, the sixthstage of construction, and put it into a hammock completed up only tothe third stage, the caterpillar simply re-performed the fourth,fifth, and sixth stages of construction. If, however, a caterpillarwere taken out of a hammock made up, for instance, to the third stage,and were put into one finished up to the sixth stage, so that much ofits work was already done for it, far from feeling the benefit ofthis, it was much embarrassed, and, in order to complete its hammock,seemed forced to start from the third stage, where it had left off,and thus tried to complete the already finished work. If we supposeany habitual action to become inherited--and I think it can be shownthat this does sometimes happen--then the resemblance between whatoriginally was a habit and an instinct becomes so close as not to bedistinguished. If Mozart, instead of playing the pianoforte at threeyears old with wonderfully little practice, had played a tune with nopractice at all, he might truly be said to have done so instinctively.But it would be the most serious error to suppose that the greaternumber of instincts have been acquired by habit in one generation, andthen transmitted by inheritance to succeeding generations. It can beclearly shown that the most wonderful instincts with which we areacquainted, namely, those of the hive-bee and of many ants, could notpossibly have been thus acquired.
It will be universally admitted that instincts are as important ascorporeal structure for the welfare of each species, under its presentconditions of life. Under changed conditions of life, it is at leastpossible that slight modifications of instinct might be profitable toa species; and if it can be shown that instincts do vary ever solittle, then I can see no difficulty in natural selection preservingand continually accumulating variations of instinct to any extent thatmay be profitable. It is thus, as I believe, that all the most complexand wonderful instincts have originated. As modifications of corporealstructure arise from, and are increased by, use or habit, and arediminished or lost by disuse, so I do not doubt it has been withinstincts. But I believe that the effects of habit are of quitesubordinate importance to the effects of the natural selection of whatmay be called accidental variations of instincts;--that is ofvariations produced by the same unknown causes which produce slightdeviations of bodily structure.
No complex instinct can possibly be produced through naturalselection, except by the slow and gradual accumulation of numerous,slight, yet profitable, variations. Hence, as in the case of corporealstructures, we ought to find in nature, not the actual transitionalgradations by which each complex instinct has been acquired--for thesecould be found only in the lineal ancestors of each species--but weought to find in the collateral lines of descent some evidence of suchgradations; or we ought at least to be able to show that gradations ofsome kind are possible; and this we certainly can do. I have beensurprised to find, making allowance for the instincts of animalshaving been but little observed except in Europe and North America,and for no instinct being known amongst extinct species, how verygenerally gradations, leading to the most complex instincts, can bediscovered. The canon of "Natura non facit saltum" applies with almostequal force to instincts as to bodily organs. Changes of instinct maysometimes be facilitated by the same species having differentinstincts at different periods of life, or at different seasons of theyear, or when placed under different circumstances, etc.; in whichcase either one or the other instinct might be preserved by naturalselection. And such instances of diversity of instinct in the samespecies can be shown to occur in nature.
Again as in the case of corporeal structure, and conformably with mytheory, the instinct of each species is good for itself, but hasnever, as far as we can judge, been produced for the exclusive good ofothers. One of the strongest instances of an animal apparentlyperforming an action for the sole good of another, with which I amacquainted, is that of aphides voluntarily yielding their sweetexcretion to ants: that they do so voluntarily, the following factsshow. I removed all the ants from a group of about a dozen aphides ona dock-plant, and prevented their attendance during several hours.After this interval, I felt sure that the aphides would want toexcrete. I watched them for some time through a lens, but not oneexcreted; I then tickled and stroked them with a hair in the samemanner, as well as I could, as the ants do with their antennae; butnot one excreted. Afterwards I allowed an ant to visit them, and itimmediately seemed, by its eager way of running about, to be wellaware what a rich flock it had discovered; it then began to play withits antennae on the abdomen first of one aphis and then of another;and each aphis, as soon as it felt the antennae, immediately lifted upits abdomen and excreted a limpid drop of sweet juice, which waseagerly devoured by the ant. Even the quite young aphides behaved inthis manner, showing that the action was instinctive, and not theresult of experience. But as the excretion is extremely viscid, it isprobably a convenience to the aphides to have it removed; andtherefore probably the aphides do not instinctively excrete for thesole good of the ants. Although I do not believe that any animal inthe world performs an action for the exclusive good of another of adistinct species, yet each species tries to take advantage of theinstincts of others, as each takes advantage of the weaker bodilystructure of others. So again, in some few cases, certain instinctscannot be considered as absolutely perfect; but as details on this andother such points are not indispensable, they may be here passed over.
As some degree of variation in instincts under a state of nature, andthe inheritance of such variations, are indispensable for the actionof natural selection, as many instances as possible ought to have beenhere given; but want of space prevents me. I can only assert, thatinstincts certainly do vary--for instance, the migratory instinct,both in extent and direction, and in its total loss. So it is with thenests of birds, which vary partly in dependence on the situationschosen, and on the nature and temperature of the country inhabited,but often from causes wholly unknown to us: Audubon has given severalremarkable cases of differences in nests of the same species in thenorthern and southern United States. Fear of any particular enemy iscertainly an instinctive quality, as may be seen in nestling birds,though it is strengthened by experience, and by the sight of fear ofthe same enemy in other animals. But fear of man is slowly acquired,as I have elsewhere shown, by various animals inhabiting desertislands; and we may see an instance of this, even in England, in thegreater wildness of all our large birds than of our small birds; forthe large birds have been most persecuted by man. We may safelyattribute the greater wildness of our large birds to this cause; forin uninhabited islands large birds are not more fearful than small;and the magpie, so wary in England, is tame in Norway, as is thehooded crow in Egypt.
That the general disposition of individuals of the same species, bornin a state of nature, is extremely diversified, can be shown by amultitude of facts. Several cases also, could be given, of occasionaland strange habits in certain species, which might, if advantageous tothe species, give rise, through natural selection, to quite newinstincts. But I am well aware that these general statements, withoutfacts given in detail, can produce but a feeble effect on the reader'smind. I can only repeat my assurance, that I do not speak without goodevidence.
The possibility, or even probability, of inherited variations ofinstinct in a state of nature will be strengthened by brieflyconsidering a few cases under domestication. We shall thus also beenabled to see the respective parts which habit and the selection ofso-called accidental variations have played in modifying the mentalqualities of our domestic animals. A number of curious and authenticinstances could be given of the inheritance of all shades ofdisposition and tastes, and likewise of the oddest tricks, associatedwith certain frames of mind or periods of time. But let us look to thefamiliar case of the several breeds of dogs: it cannot be doubted thatyoung pointers (I have myself seen a striking instance) will sometimespoint and even back other dogs the very first time that they are takenout; retrieving is certainly in some degree inherited by retrievers;and a tendency to run round, instead of at, a flock of sheep, byshepherd-dogs. I cannot see that these actions, performed withoutexperience by the young, and in nearly the same manner by eachindividual, performed with eager delight by each breed, and withoutthe end being known,--for the young pointer can no more know that hepoints to aid his master, than the white butterfly knows why she laysher eggs on the leaf of the cabbage,--I cannot see that these actionsdiffer essentially from true instincts. If we were to see one kind ofwolf, when young and without any training, as soon as it scented itsprey, stand motionless like a statue, and then slowly crawl forwardwith a peculiar gait; and another kind of wolf rushing round, insteadof at, a herd of deer, and driving them to a distant point, we shouldassuredly call these actions instinctive. Domestic instincts, as theymay be called, are certainly far less fixed or invariable than naturalinstincts; but they have been acted on by far less rigorous selection,and have been transmitted for an incomparably shorter period, underless fixed conditions of life.
How strongly these domestic instincts, habits, and dispositions areinherited, and how curiously they become mingled, is well shown whendifferent breeds of dogs are crossed. Thus it is known that a crosswith a bull-dog has affected for many generations the courage andobstinacy of greyhounds; and a cross with a greyhound has given to awhole family of shepherd-dogs a tendency to hunt hares. These domesticinstincts, when thus tested by crossing, resemble natural instincts,which in a like manner become curiously blended together, and for along period exhibit traces of the instincts of either parent: forexample, Le Roy describes a dog, whose great-grandfather was a wolf,and this dog showed a trace of its wild parentage only in one way, bynot coming in a straight line to his master when called.
Domestic instincts are sometimes spoken of as actions which havebecome inherited solely from long-continued and compulsory habit, butthis, I think, is not true. No one would ever have thought ofteaching, or probably could have taught, the tumbler-pigeon totumble,--an action which, as I have witnessed, is performed by youngbirds, that have never seen a pigeon tumble. We may believe that someone pigeon showed a slight tendency to this strange habit, and thatthe long-continued selection of the best individuals in successivegenerations made tumblers what they now are; and near Glasgow thereare house-tumblers, as I hear from Mr. Brent, which cannot flyeighteen inches high without going head over heels. It may be doubtedwhether any one would have thought of training a dog to point, had notsome one dog naturally shown a tendency in this line; and this isknown occasionally to happen, as I once saw in a pure terrier. Whenthe first tendency was once displayed, methodical selection and theinherited effects of compulsory training in each successive generationwould soon complete the work; and unconscious selection is still atwork, as each man tries to procure, without intending to improve thebreed, dogs which will stand and hunt best. On the other hand, habitalone in some cases has sufficed; no animal is more difficult to tamethan the young of the wild rabbit; scarcely any animal is tamer thanthe young of the tame rabbit; but I do not suppose that domesticrabbits have ever been selected for tameness; and I presume that wemust attribute the whole of the inherited change from extreme wildnessto extreme tameness, simply to habit and long-continued closeconfinement.
Natural instincts are lost under domestication: a remarkable instanceof this is seen in those breeds of fowls which very rarely or neverbecome "broody," that is, never wish to sit on their eggs. Familiarityalone prevents our seeing how universally and largely the minds of ourdomestic animals have been modified by domestication. It is scarcelypossible to doubt that the love of man has become instinctive in thedog. All wolves, foxes, jackals, and species of the cat genus, whenkept tame, are most eager to attack poultry, sheep, and pigs; and thistendency has been found incurable in dogs which have been brought homeas puppies from countries, such as Tierra del Fuego and Australia,where the savages do not keep these domestic animals. How rarely, onthe other hand, do our civilised dogs, even when quite young, requireto be taught not to attack poultry, sheep, and pigs! No doubt theyoccasionally do make an attack, and are then beaten; and if not cured,they are destroyed; so that habit, with some degree of selection, hasprobably concurred in civilising by inheritance our dogs. On the otherhand, young chickens have lost, wholly by habit, that fear of the dogand cat which no doubt was originally instinctive in them, in the sameway as it is so plainly instinctive in young pheasants, though rearedunder a hen. It is not that chickens have lost all fear, but fear onlyof dogs and cats, for if the hen gives the danger-chuckle, they willrun (more especially young turkeys) from under her, and concealthemselves in the surrounding grass or thickets; and this is evidentlydone for the instinctive purpose of allowing, as we see in wildground-birds, their mother to fly away. But this instinct retained byour chickens has become useless under domestication, for themother-hen has almost lost by disuse the power of flight.
Hence, we may conclude, that domestic instincts have been acquired andnatural instincts have been lost partly by habit, and partly by manselecting and accumulating during successive generations, peculiarmental habits and actions, which at first appeared from what we mustin our ignorance call an accident. In some cases compulsory habitalone has sufficed to produce such inherited mental changes; in othercases compulsory habit has done nothing, and all has been the resultof selection, pursued both methodically and unconsciously; but in mostcases, probably, habit and selection have acted together.
We shall, perhaps, best understand how instincts in a state of naturehave become modified by selection, by considering a few cases. I willselect only three, out of the several which I shall have to discuss inmy future work,--namely, the instinct which leads the cuckoo to layher eggs in other birds' nests; the slave-making instinct of certainants; and the comb-making power of the hive-bee: these two latterinstincts have generally, and most justly, been ranked by naturalistsas the most wonderful of all known instincts.
It is now commonly admitted that the more immediate and final cause ofthe cuckoo's instinct is, that she lays her eggs, not daily, but atintervals of two or three days; so that, if she were to make her ownnest and sit on her own eggs, those first laid would have to be leftfor some time unincubated, or there would be eggs and young birds ofdifferent ages in the same nest. If this were the case, the process oflaying and hatching might be inconveniently long, more especially asshe has to migrate at a very early period; and the first hatched youngwould probably have to be fed by the male alone. But the Americancuckoo is in this predicament; for she makes her own nest and has eggsand young successively hatched, all at the same time. It has beenasserted that the American cuckoo occasionally lays her eggs in otherbirds' nests; but I hear on the high authority of Dr. Brewer, thatthis is a mistake. Nevertheless, I could give several instances ofvarious birds which have been known occasionally to lay their eggs inother birds' nests. Now let us suppose that the ancient progenitor ofour European cuckoo had the habits of the American cuckoo; but thatoccasionally she laid an egg in another bird's nest. If the old birdprofited by this occasional habit, or if the young were made morevigorous by advantage having been taken of the mistaken maternalinstinct of another bird, than by their own mother's care, encumberedas she can hardly fail to be by having eggs and young of differentages at the same time; then the old birds or the fostered young wouldgain an advantage. And analogy would lead me to believe, that theyoung thus reared would be apt to follow by inheritance the occasionaland aberrant habit of their mother, and in their turn would be apt tolay their eggs in other birds' nests, and thus be successful inrearing their young. By a continued process of this nature, I believethat the strange instinct of our cuckoo could be, and has been,generated. I may add that, according to Dr. Gray and to some otherobservers, the European cuckoo has not utterly lost all maternal loveand care for her own offspring.