物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 7. INSTINCT. Page 3
I then put into the hive, instead of a thick, square piece of wax, athin and narrow, knife-edged ridge, coloured with vermilion. The beesinstantly began on both sides to excavate little basins near to eachother, in the same way as before; but the ridge of wax was so thin,that the bottoms of the basins, if they had been excavated to the samedepth as in the former experiment, would have broken into each otherfrom the opposite sides. The bees, however, did not suffer this tohappen, and they stopped their excavations in due time; so that thebasins, as soon as they had been a little deepened, came to have flatbottoms; and these flat bottoms, formed by thin little plates of thevermilion wax having been left ungnawed, were situated, as far as theeye could judge, exactly along the planes of imaginary intersectionbetween the basins on the opposite sides of the ridge of wax. Inparts, only little bits, in other parts, large portions of a rhombicplate had been left between the opposed basins, but the work, from theunnatural state of things, had not been neatly performed. The beesmust have worked at very nearly the same rate on the opposite sides ofthe ridge of vermilion wax, as they circularly gnawed away anddeepened the basins on both sides, in order to have succeeded in thusleaving flat plates between the basins, by stopping work along theintermediate planes or planes of intersection.
Considering how flexible thin wax is, I do not see that there is anydifficulty in the bees, whilst at work on the two sides of a strip ofwax, perceiving when they have gnawed the wax away to the properthinness, and then stopping their work. In ordinary combs it hasappeared to me that the bees do not always succeed in working atexactly the same rate from the opposite sides; for I have noticedhalf-completed rhombs at the base of a just-commenced cell, which wereslightly concave on one side, where I suppose that the bees hadexcavated too quickly, and convex on the opposed side, where the beeshad worked less quickly. In one well-marked instance, I put the combback into the hive, and allowed the bees to go on working for a shorttime, and again examined the cell, and I found that the rhombic platehad been completed, and had become PERFECTLY FLAT: it was absolutelyimpossible, from the extreme thinness of the little rhombic plate,that they could have effected this by gnawing away the convex side;and I suspect that the bees in such cases stand in the opposed cellsand push and bend the ductile and warm wax (which as I have tried iseasily done) into its proper intermediate plane, and thus flatten it.
From the experiment of the ridge of vermilion wax, we can clearly seethat if the bees were to build for themselves a thin wall of wax, theycould make their cells of the proper shape, by standing at the properdistance from each other, by excavating at the same rate, and byendeavouring to make equal spherical hollows, but never allowing thespheres to break into each other. Now bees, as may be clearly seen byexamining the edge of a growing comb, do make a rough, circumferentialwall or rim all round the comb; and they gnaw into this from theopposite sides, always working circularly as they deepen each cell.They do not make the whole three-sided pyramidal base of any one cellat the same time, but only the one rhombic plate which stands on theextreme growing margin, or the two plates, as the case may be; andthey never complete the upper edges of the rhombic plates, until thehexagonal walls are commenced. Some of these statements differ fromthose made by the justly celebrated elder Huber, but I am convinced oftheir accuracy; and if I had space, I could show that they areconformable with my theory.
Huber's statement that the very first cell is excavated out of alittle parallel-sided wall of wax, is not, as far as I have seen,strictly correct; the first commencement having always been a littlehood of wax; but I will not here enter on these details. We see howimportant a part excavation plays in the construction of the cells;but it would be a great error to suppose that the bees cannot build upa rough wall of wax in the proper position--that is, along the planeof intersection between two adjoining spheres. I have severalspecimens showing clearly that they can do this. Even in the rudecircumferential rim or wall of wax round a growing comb, flexures maysometimes be observed, corresponding in position to the planes of therhombic basal plates of future cells. But the rough wall of wax has inevery case to be finished off, by being largely gnawed away on bothsides. The manner in which the bees build is curious; they always makethe first rough wall from ten to twenty times thicker than theexcessively thin finished wall of the cell, which will ultimately beleft. We shall understand how they work, by supposing masons first topile up a broad ridge of cement, and then to begin cutting it awayequally on both sides near the ground, till a smooth, very thin wallis left in the middle; the masons always piling up the cut-awaycement, and adding fresh cement, on the summit of the ridge. We shallthus have a thin wall steadily growing upward; but always crowned by agigantic coping. From all the cells, both those just commenced andthose completed, being thus crowned by a strong coping of wax, thebees can cluster and crawl over the comb without injuring the delicatehexagonal walls, which are only about one four-hundredth of an inch inthickness; the plates of the pyramidal basis being about twice asthick. By this singular manner of building, strength is continuallygiven to the comb, with the utmost ultimate economy of wax.
It seems at first to add to the difficulty of understanding how thecells are made, that a multitude of bees all work together; one beeafter working a short time at one cell going to another, so that, asHuber has stated, a score of individuals work even at the commencementof the first cell. I was able practically to show this fact, bycovering the edges of the hexagonal walls of a single cell, or theextreme margin of the circumferential rim of a growing comb, with anextremely thin layer of melted vermilion wax; and I invariably foundthat the colour was most delicately diffused by the bees--asdelicately as a painter could have done with his brush--by atoms ofthe coloured wax having been taken from the spot on which it had beenplaced, and worked into the growing edges of the cells all round. Thework of construction seems to be a sort of balance struck between manybees, all instinctively standing at the same relative distance fromeach other, all trying to sweep equal spheres, and then building up,or leaving ungnawed, the planes of intersection between these spheres.It was really curious to note in cases of difficulty, as when twopieces of comb met at an angle, how often the bees would entirely pulldown and rebuild in different ways the same cell, sometimes recurringto a shape which they had at first rejected.
When bees have a place on which they can stand in their properpositions for working,--for instance, on a slip of wood, placeddirectly under the middle of a comb growing downwards so that the combhas to be built over one face of the slip--in this case the bees canlay the foundations of one wall of a new hexagon, in its strictlyproper place, projecting beyond the other completed cells. It sufficesthat the bees should be enabled to stand at their proper relativedistances from each other and from the walls of the last completedcells, and then, by striking imaginary spheres, they can build up awall intermediate between two adjoining spheres; but, as far as I haveseen, they never gnaw away and finish off the angles of a cell till alarge part both of that cell and of the adjoining cells has beenbuilt. This capacity in bees of laying down under certaincircumstances a rough wall in its proper place between twojust-commenced cells, is important, as it bears on a fact, which seemsat first quite subversive of the foregoing theory; namely, that thecells on the extreme margin of wasp-combs are sometimes strictlyhexagonal; but I have not space here to enter on this subject. Nordoes there seem to me any great difficulty in a single insect (as inthe case of a queen-wasp) making hexagonal cells, if she workalternately on the inside and outside of two or three cells commencedat the same time, always standing at the proper relative distance fromthe parts of the cells just begun, sweeping spheres or cylinders, andbuilding up intermediate planes. It is even conceivable that an insectmight, by fixing on a point at which to commence a cell, and thenmoving outside, first to one point, and then to five other points, atthe proper relative distances from the central point and from eachother, strike the planes of intersection, and so make an isolatedhexagon: but I am not aware that any such case has been observed; norwould any good be derived from a single hexagon being built, as in itsconstruction more materials would be required than for a cylinder.
different breeds of cattle in relation to an artificiallyimperfect state of the male sex; for oxen of certain breeds havelonger horns than in other breeds, in comparison with the horns of thebulls or.
As natural selection acts only by the accumulation of slightmodifications of structure or instinct, each profitable to theindividual under its conditions of life, it may reasonably be asked,how a long and graduated succession of modified architecturalinstincts, all tending towards the present perfect plan ofconstruction, could have profited the progenitors of the hive-bee? Ithink the answer is not difficult: it is known that bees are oftenhard pressed to get sufficient nectar; and I am informed by Mr.Tegetmeier that it has been experimentally found that no less thanfrom twelve to fifteen pounds of dry sugar are consumed by a hive ofbees for the secretion of each pound of wax; so that a prodigiousquantity of fluid nectar must be collected and consumed by the bees ina hive for the secretion of the wax necessary for the construction oftheir combs. Moreover, many bees have to remain idle for many daysduring the process of secretion. A large store of honey isindispensable to support a large stock of bees during the winter; andthe security of the hive is known mainly to depend on a large numberof bees being supported. Hence the saving of wax by largely savinghoney must be a most important element of success in any family ofbees. Of course the success of any species of bee may be dependent onthe number of its parasites or other enemies, or on quite distinctcauses, and so be altogether independent of the quantity of honeywhich the bees could collect. But let us suppose that this lattercircumstance determined, as it probably often does determine, thenumbers of a humble-bee which could exist in a country; and let usfurther suppose that the community lived throughout the winter, andconsequently required a store of honey: there can in this case be nodoubt that it would be an advantage to our humble-bee, if a slightmodification of her instinct led her to make her waxen cells neartogether, so as to intersect a little; for a wall in common even totwo adjoining cells, would save some little wax. Hence it wouldcontinually be more and more advantageous to our humble-bee, if shewere to make her cells more and more regular, nearer together, andaggregated into a mass, like the cells of the Melipona; for in thiscase a large part of the bounding surface of each cell would serve tobound other cells, and much wax would be saved. Again, from the samecause, it would be advantageous to the Melipona, if she were to makeher cells closer together, and more regular in every way than atpresent; for then, as we have seen, the spherical surfaces wouldwholly disappear, and would all be replaced by plane surfaces; and theMelipona would make a comb as perfect as that of the hive-bee. Beyondthis stage of perfection in architecture, natural selection could notlead; for the comb of the hive-bee, as far as we can see, isabsolutely perfect in economising wax.
Thus, as I believe, the most wonderful of all known instincts, that ofthe hive-bee, can be explained by natural selection having takenadvantage of numerous, successive, slight modifications of simplerinstincts; natural selection having by slow degrees, more and moreperfectly, led the bees to sweep equal spheres at a given distancefrom each other in a double layer, and to build up and excavate thewax along the planes of intersection. The bees, of course, no moreknowing that they swept their spheres at one particular distance fromeach other, than they know what are the several angles of thehexagonal prisms and of the basal rhombic plates. The motive power ofthe process of natural selection having been economy of wax; thatindividual swarm which wasted least honey in the secretion of wax,having succeeded best, and having transmitted by inheritance its newlyacquired economical instinct to new swarms, which in their turn willhave had the best chance of succeeding in the struggle for existence.
No doubt many instincts of very difficult explanation could be opposedto the theory of natural selection,--cases, in which we cannot see howan instinct could possibly have originated; cases, in which nointermediate gradations are known to exist; cases of instinct ofapparently such trifling importance, that they could hardly have beenacted on by natural selection; cases of instincts almost identicallythe same in animals so remote in the scale of nature, that we cannotaccount for their similarity by inheritance from a common parent, andmust therefore believe that they have been acquired by independentacts of natural selection. I will not here enter on these severalcases, but will confine myself to one special difficulty, which atfirst appeared to me insuperable, and actually fatal to my wholetheory. I allude to the neuters or sterile females ininsect-communities: for these neuters often differ widely in instinctand in structure from both the males and fertile females, and yet,from being sterile, they cannot propagate their kind.
The subject well deserves to be discussed at great length, but I willhere take only a single case, that of working or sterile ants. How theworkers have been rendered sterile is a difficulty; but not muchgreater than that of any other striking modification of structure; forit can be shown that some insects and other articulate animals in astate of nature occasionally become sterile; and if such insects hadbeen social, and it had been profitable to the community that a numbershould have been annually born capable of work, but incapable ofprocreation, I can see no very great difficulty in this being effectedby natural selection. But I must pass over this preliminarydifficulty. The great difficulty lies in the working ants differingwidely from both the males and the fertile females in structure, as inthe shape of the thorax and in being destitute of wings and sometimesof eyes, and in instinct. As far as instinct alone is concerned, theprodigious difference in this respect between the workers and theperfect females, would have been far better exemplified by thehive-bee. If a working ant or other neuter insect had been an animalin the ordinary state, I should have unhesitatingly assumed that allits characters had been slowly acquired through natural selection;namely, by an individual having been born with some slight profitablemodification of structure, this being inherited by its offspring,which again varied and were again selected, and so onwards. But withthe working ant we have an insect differing greatly from its parents,yet absolutely sterile; so that it could never have transmittedsuccessively acquired modifications of structure or instinct to itsprogeny. It may well be asked how is it possible to reconcile thiscase with the theory of natural selection?
First, let it be remembered that we have innumerable instances, bothin our domestic productions and in those in a state of nature, of allsorts of differences of structure which have become correlated tocertain ages, and to either sex. We have differences correlated notonly to one sex, but to that short period alone when the reproductivesystem is active, as in the nuptial plumage of many birds, and in thehooked jaws of the male salmon. We have even slight differences in thehorns of different breeds of cattle in relation to an artificiallyimperfect state of the male sex; for oxen of certain breeds havelonger horns than in other breeds, in comparison with the horns of thebulls or cows of these same breeds. Hence I can see no real difficultyin any character having become correlated with the sterile conditionof certain members of insect-communities: the difficulty lies inunderstanding how such correlated modifications of structure couldhave been slowly accumulated by natural selection.
This difficulty, though appearing insuperable, is lessened, or, as Ibelieve, disappears, when it is remembered that selection may beapplied to the family, as well as to the individual, and may thus gainthe desired end. Thus, a well-flavoured vegetable is cooked, and theindividual is destroyed; but the horticulturist sows seeds of the samestock, and confidently expects to get nearly the same variety;breeders of cattle wish the flesh and fat to be well marbled together;the animal has been slaughtered, but the breeder goes with confidenceto the same family. I have such faith in the powers of selection, thatI do not doubt that a breed of cattle, always yielding oxen withextraordinarily long horns, could be slowly formed by carefullywatching which individual bulls and cows, when matched, produced oxenwith the longest horns; and yet no one ox could ever have propagatedits kind. Thus I believe it has been with social insects: a slightmodification of structure, or instinct, correlated with the sterilecondition of certain members of the community, has been advantageousto the community: consequently the fertile males and females of thesame community flourished, and transmitted to their fertile offspringa tendency to produce sterile members having the same modification.And I believe that this process has been repeated, until thatprodigious amount of difference between the fertile and sterilefemales of the same species has been produced, which we see in manysocial insects.
But we have not as yet touched on the climax of the difficulty;namely, the fact that the neuters of several ants differ, not onlyfrom the fertile females and males, but from each other, sometimes toan almost incredible degree, and are thus divided into two or eventhree castes. The castes, moreover, do not generally graduate intoeach other, but are perfectly well defined; being as distinct fromeach other, as are any two species of the same genus, or rather as anytwo genera of the same family. Thus in Eciton, there are working andsoldier neuters, with jaws and instincts extraordinarily different: inCryptocerus, the workers of one caste alone carry a wonderful sort ofshield on their heads, the use of which is quite unknown: in theMexican Myrmecocystus, the workers of one caste never leave the nest;they are fed by the workers of another caste, and they have anenormously developed abdomen which secretes a sort of honey, supplyingthe place of that excreted by the aphides, or the domestic cattle asthey may be called, which our European ants guard or imprison.