物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 7. INSTINCT. Page 2
The occasional habit of birds laying their eggs in other birds' nests,either of the same or of a distinct species, is not very uncommon withthe Gallinaceae; and this perhaps explains the origin of a singularinstinct in the allied group of ostriches. For several hen ostriches,at least in the case of the American species, unite and lay first afew eggs in one nest and then in another; and these are hatched by themales. This instinct may probably be accounted for by the fact of thehens laying a large number of eggs; but, as in the case of the cuckoo,at intervals of two or three days. This instinct, however, of theAmerican ostrich has not as yet been perfected; for a surprisingnumber of eggs lie strewed over the plains, so that in one day'shunting I picked up no less than twenty lost and wasted eggs.
Many bees are parasitic, and always lay their eggs in the nests ofbees of other kinds. This case is more remarkable than that of thecuckoo; for these bees have not only their instincts but theirstructure modified in accordance with their parasitic habits; for theydo not possess the pollen-collecting apparatus which would benecessary if they had to store food for their own young. Some species,likewise, of Sphegidae (wasp-like insects) are parasitic on otherspecies; and M. Fabre has lately shown good reason for believing thatalthough the Tachytes nigra generally makes its own burrow and storesit with paralysed prey for its own larvae to feed on, yet that whenthis insect finds a burrow already made and stored by another sphex,it takes advantage of the prize, and becomes for the occasionparasitic. In this case, as with the supposed case of the cuckoo, Ican see no difficulty in natural selection making an occasional habitpermanent, if of advantage to the species, and if the insect whosenest and stored food are thus feloniously appropriated, be not thusexterminated.
SLAVE-MAKING INSTINCT.
This remarkable instinct was first discovered in the Formica(Polyerges) rufescens by Pierre Huber, a better observer even than hiscelebrated father. This ant is absolutely dependent on its slaves;without their aid, the species would certainly become extinct in asingle year. The males and fertile females do no work. The workers orsterile females, though most energetic and courageous in capturingslaves, do no other work. They are incapable of making their ownnests, or of feeding their own larvae. When the old nest is foundinconvenient, and they have to migrate, it is the slaves whichdetermine the migration, and actually carry their masters in theirjaws. So utterly helpless are the masters, that when Huber shut upthirty of them without a slave, but with plenty of the food which theylike best, and with their larvae and pupae to stimulate them to work,they did nothing; they could not even feed themselves, and manyperished of hunger. Huber then introduced a single slave (F. fusca),and she instantly set to work, fed and saved the survivors; made somecells and tended the larvae, and put all to rights. What can be moreextraordinary than these well-ascertained facts? If we had not knownof any other slave-making ant, it would have been hopeless to havespeculated how so wonderful an instinct could have been perfected.
Formica sanguinea was likewise first discovered by P. Huber to be aslave-making ant. This species is found in the southern parts ofEngland, and its habits have been attended to by Mr. F. Smith, of theBritish Museum, to whom I am much indebted for information on this andother subjects. Although fully trusting to the statements of Huber andMr. Smith, I tried to approach the subject in a sceptical frame ofmind, as any one may well be excused for doubting the truth of soextraordinary and odious an instinct as that of making slaves. Hence Iwill give the observations which I have myself made, in some littledetail. I opened fourteen nests of F. sanguinea, and found a fewslaves in all. Males and fertile females of the slave-species arefound only in their own proper communities, and have never beenobserved in the nests of F. sanguinea. The slaves are black and notabove half the size of their red masters, so that the contrast intheir appearance is very great. When the nest is slightly disturbed,the slaves occasionally come out, and like their masters are muchagitated and defend the nest: when the nest is much disturbed and thelarvae and pupae are exposed, the slaves work energetically with theirmasters in carrying them away to a place of safety. Hence, it isclear, that the slaves feel quite at home. During the months of Juneand July, on three successive years, I have watched for many hoursseveral nests in Surrey and Sussex, and never saw a slave either leaveor enter a nest. As, during these months, the slaves are very few innumber, I thought that they might behave differently when morenumerous; but Mr. Smith informs me that he has watched the nests atvarious hours during May, June and August, both in Surrey andHampshire, and has never seen the slaves, though present in largenumbers in August, either leave or enter the nest. Hence he considersthem as strictly household slaves. The masters, on the other hand, maybe constantly seen bringing in materials for the nest, and food of allkinds. During the present year, however, in the month of July, I cameacross a community with an unusually large stock of slaves, and Iobserved a few slaves mingled with their masters leaving the nest, andmarching along the same road to a tall Scotch-fir-tree, twenty-fiveyards distant, which they ascended together, probably in search ofaphides or cocci. According to Huber, who had ample opportunities forobservation, in Switzerland the slaves habitually work with theirmasters in making the nest, and they alone open and close the doors inthe morning and evening; and, as Huber expressly states, theirprincipal office is to search for aphides. This difference in theusual habits of the masters and slaves in the two countries, probablydepends merely on the slaves being captured in greater numbers inSwitzerland than in England.
One day I fortunately chanced to witness a migration from one nest toanother, and it was a most interesting spectacle to behold the masterscarefully carrying, as Huber has described, their slaves in theirjaws. Another day my attention was struck by about a score of theslave-makers haunting the same spot, and evidently not in search offood; they approached and were vigorously repulsed by an independentcommunity of the slave species (F. fusca); sometimes as many as threeof these ants clinging to the legs of the slave-making F. sanguinea.The latter ruthlessly killed their small opponents, and carried theirdead bodies as food to their nest, twenty-nine yards distant; but theywere prevented from getting any pupae to rear as slaves. I then dug upa small parcel of the pupae of F. fusca from another nest, and putthem down on a bare spot near the place of combat; they were eagerlyseized, and carried off by the tyrants, who perhaps fancied that,after all, they had been victorious in their late combat.
At the same time I laid on the same place a small parcel of the pupaeof another species, F. flava, with a few of these little yellow antsstill clinging to the fragments of the nest. This species issometimes, though rarely, made into slaves, as has been described byMr. Smith. Although so small a species, it is very courageous, and Ihave seen it ferociously attack other ants. In one instance I found tomy surprise an independent community of F. flava under a stone beneatha nest of the slave-making F. sanguinea; and when I had accidentallydisturbed both nests, the little ants attacked their big neighbourswith surprising courage. Now I was curious to ascertain whether F.sanguinea could distinguish the pupae of F. fusca, which theyhabitually make into slaves, from those of the little and furious F.flava, which they rarely capture, and it was evident that they did atonce distinguish them: for we have seen that they eagerly andinstantly seized the pupae of F. fusca, whereas they were muchterrified when they came across the pupae, or even the earth from thenest of F. flava, and quickly ran away; but in about a quarter of anhour, shortly after all the little yellow ants had crawled away, theytook heart and carried off the pupae.
One evening I visited another community of F. sanguinea, and found anumber of these ants entering their nest, carrying the dead bodies ofF. fusca (showing that it was not a migration) and numerous pupae. Itraced the returning file burthened with booty, for about forty yards,to a very thick clump of heath, whence I saw the last individual of F.sanguinea emerge, carrying a pupa; but I was not able to find thedesolated nest in the thick heath. The nest, however, must have beenclose at hand, for two or three individuals of F. fusca were rushingabout in the greatest agitation, and one was perched motionless withits own pupa in its mouth on the top of a spray of heath over itsravaged home.
Such are the facts, though they did not need confirmation by me, inregard to the wonderful instinct of making slaves. Let it be observedwhat a contrast the instinctive habits of F. sanguinea present withthose of the F. rufescens. The latter does not build its own nest,does not determine its own migrations, does not collect food foritself or its young, and cannot even feed itself: it is absolutelydependent on its numerous slaves. Formica sanguinea, on the otherhand, possesses much fewer slaves, and in the early part of the summerextremely few. The masters determine when and where a new nest shallbe formed, and when they migrate, the masters carry the slaves. Bothin Switzerland and England the slaves seem to have the exclusive careof the larvae, and the masters alone go on slave-making expeditions.In Switzerland the slaves and masters work together, making andbringing materials for the nest: both, but chiefly the slaves, tend,and milk as it may be called, their aphides; and thus both collectfood for the community. In England the masters alone usually leave thenest to collect building materials and food for themselves, theirslaves and larvae. So that the masters in this country receive muchless service from their slaves than they do in Switzerland.
By what steps the instinct of F. sanguinea originated I will notpretend to conjecture. But as ants, which are not slave-makers, will,as I have seen, carry off pupae of other species, if scattered neartheir nests, it is possible that pupae originally stored as food mightbecome developed; and the ants thus unintentionally reared would thenfollow their proper instincts, and do what work they could. If theirpresence proved useful to the species which had seized them--if itwere more advantageous to this species to capture workers than toprocreate them--the habit of collecting pupae originally for foodmight by natural selection be strengthened and rendered permanent forthe very different purpose of raising slaves. When the instinct wasonce acquired, if carried out to a much less extent even than in ourBritish F. sanguinea, which, as we have seen, is less aided by itsslaves than the same species in Switzerland, I can see no difficultyin natural selection increasing and modifying the instinct--alwayssupposing each modification to be of use to the species--until an antwas formed as abjectly dependent on its slaves as is the Formicarufescens.
CELL-MAKING INSTINCT OF THE HIVE-BEE.
I will not here enter on minute details on this subject, but willmerely give an outline of the conclusions at which I have arrived. Hemust be a dull man who can examine the exquisite structure of a comb,so beautifully adapted to its end, without enthusiastic admiration. Wehear from mathematicians that bees have practically solved a reconditeproblem, and have made their cells of the proper shape to hold thegreatest possible amount of honey, with the least possible consumptionof precious wax in their construction. It has been remarked that askilful workman, with fitting tools and measures, would find it verydifficult to make cells of wax of the true form, though this isperfectly effected by a crowd of bees working in a dark hive. Grantwhatever instincts you please, and it seems at first quiteinconceivable how they can make all the necessary angles and planes,or even perceive when they are correctly made. But the difficulty isnot nearly so great as it at first appears: all this beautiful workcan be shown, I think, to follow from a few very simple instincts.
I was led to investigate this subject by Mr. Waterhouse, who has shownthat the form of the cell stands in close relation to the presence ofadjoining cells; and the following view may, perhaps, be consideredonly as a modification of his theory. Let us look to the greatprinciple of gradation, and see whether Nature does not reveal to usher method of work. At one end of a short series we have humble-bees,which use their old cocoons to hold honey, sometimes adding to themshort tubes of wax, and likewise making separate and very irregularrounded cells of wax. At the other end of the series we have the cellsof the hive-bee, placed in a double layer: each cell, as is wellknown, is an hexagonal prism, with the basal edges of its six sidesbevelled so as to join on to a pyramid, formed of three rhombs. Theserhombs have certain angles, and the three which form the pyramidalbase of a single cell on one side of the comb, enter into thecomposition of the bases of three adjoining cells on the oppositeside. In the series between the extreme perfection of the cells of thehive-bee and the simplicity of those of the humble-bee, we have thecells of the Mexican Melipona domestica, carefully described andfigured by Pierre Huber. The Melipona itself is intermediate instructure between the hive and humble bee, but more nearly related tothe latter: it forms a nearly regular waxen comb of cylindrical cells,in which the young are hatched, and, in addition, some large cells ofwax for holding honey. These latter cells are nearly spherical and ofnearly equal sizes, and are aggregated into an irregular mass. But theimportant point to notice, is that these cells are always made at thatdegree of nearness to each other, that they would have intersected orbroken into each other, if the spheres had been completed; but this isnever permitted, the bees building perfectly flat walls of wax betweenthe spheres which thus tend to intersect. Hence each cell consists ofan outer spherical portion and of two, three, or more perfectly flatsurfaces, according as the cell adjoins two, three or more othercells. When one cell comes into contact with three other cells, which,from the spheres being nearly of the same size, is very frequently andnecessarily the case, the three flat surfaces are united into apyramid; and this pyramid, as Huber has remarked, is manifestly agross imitation of the three-sided pyramidal basis of the cell of thehive-bee. As in the cells of the hive-bee, so here, the three planesurfaces in any one cell necessarily enter into the construction ofthree adjoining cells. It is obvious that the Melipona saves wax bythis manner of building; for the flat walls between the adjoiningcells are not double, but are of the same thickness as the outerspherical portions, and yet each flat portion forms a part of twocells.
Reflecting on this case, it occurred to me that if the Melipona hadmade its spheres at some given distance from each other, and had madethem of equal sizes and had arranged them symmetrically in a doublelayer, the resulting structure would probably have been as perfect asthe comb of the hive-bee. Accordingly I wrote to Professor Miller, ofCambridge, and this geometer has kindly read over the followingstatement, drawn up from his information, and tells me that it isstrictly correct:--
Hence we may safely conclude that if we could slightly modify theinstincts already possessed by the Melipona, and in themselves notvery wonderful, this bee would make a structure as wonderfully perfectas that of the hive-bee. We must suppose the Melipona to make hercells truly spherical, and of equal sizes; and this would not be verysurprising, seeing that she already does so to a certain extent, andseeing what perfectly cylindrical burrows in wood many insects canmake, apparently by turning round on a fixed point. We must supposethe Melipona to arrange her cells in level layers, as she already doesher cylindrical cells; and we must further suppose, and this is thegreatest difficulty, that she can somehow judge accurately at whatdistance to stand from her fellow-labourers when several are makingtheir spheres; but she is already so far enabled to judge of distance,that she always describes her spheres so as to intersect largely; andthen she unites the points of intersection by perfectly flat surfaces.We have further to suppose, but this is no difficulty, that afterhexagonal prisms have been formed by the intersection of adjoiningspheres in the same layer, she can prolong the hexagon to any lengthrequisite to hold the stock of honey; in the same way as the rudehumble-bee adds cylinders of wax to the circular mouths of her oldcocoons. By such modifications of instincts in themselves not verywonderful,--hardly more wonderful than those which guide a bird tomake its nest,--I believe that the hive-bee has acquired, throughnatural selection, her inimitable architectural powers.
But this theory can be tested by experiment. Following the example ofMr. Tegetmeier, I separated two combs, and put between them a long,thick, square strip of wax: the bees instantly began to excavateminute circular pits in it; and as they deepened these little pits,they made them wider and wider until they were converted into shallowbasins, appearing to the eye perfectly true or parts of a sphere, andof about the diameter of a cell. It was most interesting to me toobserve that wherever several bees had begun to excavate these basinsnear together, they had begun their work at such a distance from eachother, that by the time the basins had acquired the above stated width(i.e. about the width of an ordinary cell), and were in depth aboutone sixth of the diameter of the sphere of which they formed a part,the rims of the basins intersected or broke into each other. As soonas this occurred, the bees ceased to excavate, and began to build upflat walls of wax on the lines of intersection between the basins, sothat each hexagonal prism was built upon the festooned edge of asmooth basin, instead of on the straight edges of a three-sidedpyramid as in the case of ordinary cells.