Hello,Welcome!Log In to My AccountorCreate Account

Instincts comparable with habits, but different in their origin --Instincts graduated -- Aphides and ants -- Instincts variable -- Domesticinstincts, their origin -- Natural instincts of the cuckoo, molothrus,ostrich, and parasitic bees -- Slave-making ants -- Hive-bee, itscell-making instinct -- Changes of instinct and structure not necessarilysimultaneous -- Difficulties of the theory of the Natural Selection ofinstincts -- Neuter or sterile insects -- Summary.

Many instincts are so wonderful that their development will probably appearto the reader a difficulty sufficient to overthrow my whole theory. I mayhere premise, that I have nothing to do with the origin of the mentalpowers, any more than I have with that of life itself. We are concernedonly with the diversities of instinct and of the other mental faculties inanimals of the same class.

I will not attempt any definition of instinct. It would be easy to showthat several distinct mental actions are commonly embraced by this term;but every one understands what is meant, when it is said that instinctimpels the cuckoo to migrate and to lay her eggs in other birds' nests. Anaction, which we ourselves require experience to enable us to perform, whenperformed by an animal, more especially by a very young one, withoutexperience, and when performed by many individuals in the same way, withouttheir knowing for what purpose it is performed, is usually said to beinstinctive. But I could show that none of these characters are universal. A little dose of judgment or reason, as Pierre Huber expresses it, oftencomes into play, even with animals low in the scale of nature.

Frederick Cuvier and several of the older metaphysicians have comparedinstinct with habit. This comparison gives, I think, an accurate notion ofthe frame of mind under which an instinctive action is performed, but notnecessarily of its origin. How unconsciously many habitual actions areperformed, indeed not rarely in direct opposition to our conscious will!yet they may be modified by the will or reason. Habits easily becomeassociated with other habits, with certain periods of time and states ofthe body. When once acquired, they often remain constant throughout life. Several other points of resemblance between instincts and habits could bepointed out. As in repeating a well-known song, so in instincts, oneaction follows another by a sort of rhythm; if a person be interrupted in asong, or in repeating anything by rote, he is generally forced to go backto recover the habitual 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 sixth stage ofconstruction, and put it into a hammock completed up only to the thirdstage, the caterpillar simply re-performed the fourth, fifth, and sixthstages of construction. If, however, a caterpillar were taken out of ahammock made up, for instance, to the third stage, and were put into onefinished up to the sixth stage, so that much of its work was already donefor it, far from deriving any benefit from this, it was much embarrassed,and, in order to complete its hammock, seemed forced to start from thethird stage, where it had left off, and thus tried to complete the alreadyfinished work.

If we suppose any habitual action to become inherited--and 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 three yearsold with wonderfully little practice, had played a tune with no practice atall, be might truly be said to have done so instinctively. But it would bea serious error to suppose that the greater number of instincts have beenacquired by habit in one generation, and then transmitted by inheritance tosucceeding generations. It can be clearly shown that the most wonderfulinstincts with which we are acquainted, namely, those of the hive-bee andof many ants, could not possibly have been acquired by habit.

It will be universally admitted that instincts are as important ascorporeal structures 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 to aspecies; and if it can be shown that instincts do vary ever so little, thenI can see no difficulty in natural selection preserving and continuallyaccumulating variations of instinct to any extent that was profitable. Itis thus, as I believe, that all the most complex and wonderful instinctshave originated. As modifications of corporeal structure arise from, andare increased by, use or habit, and are diminished or lost by disuse, so Ido not doubt it has been with instincts. But I believe that the effects ofhabit are in many cases of subordinate importance to the effects of thenatural selection of what may be called spontaneous variations ofinstincts;--that is of variations produced by the same unknown causes whichproduce slight deviations of bodily structure.

No complex instinct can possibly be produced through natural selection,except by the slow and gradual accumulation of numerous, slight, yetprofitable, variations. Hence, as in the case of corporeal structures, weought to find in nature, not the actual transitional gradations by whicheach complex instinct has been acquired--for these could be found only inthe lineal ancestors of each species--but we ought to find in thecollateral lines of descent some evidence of such gradations; or we oughtat least to be able to show that gradations of some kind are possible; andthis we certainly can do. I have been surprised to find, making allowancefor the instincts of animals having been but little observed, except inEurope and North America, and for no instinct being known among extinctspecies, how very generally gradations, leading to the most complexinstincts, can be discovered. Changes of instinct may sometimes befacilitated by the same species having different instincts at differentperiods of life, or at different seasons of the year, or when placed underdifferent circumstances, etc.; in which case either the one or the otherinstinct might be preserved by natural selection. And such instances ofdiversity of instinct in the same species can be shown to occur in nature.

Again, as in the case of corporeal structure, and conformably to my theory,the instinct of each species is good for itself, but has never, as far aswe can judge, been produced for the exclusive good of others. One of thestrongest instances of an animal apparently performing an action for thesole good of another, with which I am acquainted, is that of aphidesvoluntarily yielding, as was first observed by Huber, their sweet excretionto ants: that they do so voluntarily, the following facts show. I removedall the ants from a group of about a dozen aphides on a dock-plant, andprevented their attendance during several hours. After this interval, Ifelt sure that the aphides would want to excrete. I watched them for sometime through a lens, but not one excreted; I then tickled and stroked themwith a hair in the same manner, as well as I could, as the ants do withtheir antennae; but not one excreted. Afterwards, I allowed an ant tovisit them, and it immediately seemed, by its eager way of running about tobe well aware what a rich flock it had discovered; it then began to playwith its antennae on the abdomen first of one aphis and then of another;and each, as soon as it felt the antennae, immediately lifted up itsabdomen and excreted a limpid drop of sweet juice, which was eagerlydevoured by the ant. Even the quite young aphides behaved in this manner,showing that the action was instinctive, and not the result of experience. It is certain, from the observations of Huber, that the aphides show nodislike to the ants: if the latter be not present they are at lastcompelled to eject their excretion. But as the excretion is extremelyviscid, it is no doubt a convenience to the aphides to have it removed;therefore probably they do not excrete solely for the good of the ants. Although there is no evidence that any animal performs an action for theexclusive good of another species, yet each tries to take advantage of theinstincts of others, as each takes advantage of the weaker bodily structureof other species. So again certain instincts cannot be considered asabsolutely perfect; but as details on this and other such points are notindispensable, they may be here passed over.

As some degree of variation in instincts under a state of nature, and theinheritance of such variations, are indispensable for the action of naturalselection, as many instances as possible ought to be given; but want ofspace prevents me. I can only assert that instincts certainly do vary--forinstance, the migratory instinct, both in extent and direction, and in itstotal loss. So it is with the nests of birds, which vary partly independence on the situations chosen, and on the nature and temperature ofthe country inhabited, but often from causes wholly unknown to us. Audubonhas given several remarkable cases of differences in the nests of the samespecies in the northern and southern United States. Why, it has beenasked, if instinct be variable, has it not granted to the bee "the abilityto use some other material when wax was deficient?" But what other naturalmaterial could bees use? They will work, as I have seen, with wax hardenedwith vermilion or softened with lard. Andrew Knight observed that hisbees, instead of laboriously collecting propolis, used a cement of wax andturpentine, with which he had covered decorticated trees. It has latelybeen shown that bees, instead of searching for pollen, will gladly use avery different substance, namely, oatmeal. Fear of any particular enemy iscertainly an instinctive quality, as may be seen in nestling birds, thoughit is strengthened by experience, and by the sight of fear of the sameenemy in other animals. The fear of man is slowly acquired, as I haveelsewhere shown, by the various animals which inhabit desert islands; andwe see an instance of this, even in England, in the greater wildness of allour large birds in comparison with our small birds; for the large birdshave been most persecuted by man. We may safely attribute the greaterwildness of our large birds to this cause; for in uninhabited islands largebirds are not more fearful than small; and the magpie, so wary in England,is tame in Norway, as is the hooded crow in Egypt.

That the mental qualities of animals of the same kind, born in a state ofnature, vary much, could be shown by many facts. Several cases could alsobe adduced of occasional and strange habits in wild animals, which, ifadvantageous to the species, might have given rise, through naturalselection, to new instincts. But I am well aware that these generalstatements, without the facts in detail, can produce but a feeble effect onthe reader's mind. I can only repeat my assurance, that I do not speakwithout good evidence.

INHERITED CHANGES OF HABIT OR INSTINCT IN DOMESTICATED ANIMALS.

The possibility, or even probability, of inherited variations of instinctin a state of nature will be strengthened by briefly considering a fewcases under domestication. We shall thus be enabled to see the part whichhabit and the selection of so-called spontaneous variations have played inmodifying the mental qualities of our domestic animals. It is notorioushow much domestic animals vary in their mental qualities. With cats, forinstance, one naturally takes to catching rats, and another mice, and thesetendencies are known to be inherited. One cat, according to Mr. St. John,always brought home game birds, another hares or rabbits, and anotherhunted on marshy ground and almost nightly caught woodcocks or snipes. Anumber of curious and authentic instances could be given of various shadesof disposition and taste, and likewise of the oddest tricks, associatedwith certain frames of mind or periods of time. But let us look to thefamiliar case of the breeds of dogs: it cannot be doubted that youngpointers (I have myself seen striking instances) will sometimes point andeven back other dogs the very first time that they are taken out;retrieving is certainly in some degree inherited by retrievers; and atendency to run round, instead of at, a flock of sheep, by shepherd-dogs. I cannot see that these actions, performed without experience by the young,and in nearly the same manner by each individual, performed with eagerdelight by each breed, and without the end being known--for the youngpointer can no more know that he points to aid his master, than the whitebutterfly knows why she lays her eggs on the leaf of the cabbage--I cannotsee that these actions differ essentially from true instincts. If we wereto behold one kind of wolf, when young and without any training, as soon asit scented its prey, stand motionless like a statue, and then slowly crawlforward with a peculiar gait; and another kind of wolf rushing round,instead of at, a herd of deer, and driving them to a distant point, weshould assuredly call these actions instinctive. Domestic instincts, asthey may be called, are certainly far less fixed than natural instincts;but they have been acted on by far less rigorous selection, and have beentransmitted for an incomparably shorter period, under less fixed conditionsof 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 cross with abull-dog has affected for many generations the courage and obstinacy ofgreyhounds; and a cross with a greyhound has given to a whole family ofshepherd-dogs a tendency to hunt hares. These domestic instincts, whenthus tested by crossing, resemble natural instincts, which in a like mannerbecome curiously blended together, and for a long period exhibit traces ofthe instincts of either parent: for example, Le Roy describes a dog, whosegreat-grandfather was a wolf, and this dog showed a trace of its wildparentage only in one way, by not coming in a straight line to his master,when called.

Domestic instincts are sometimes spoken of as actions which have becomeinherited solely from long-continued and compulsory habit, but this is nottrue. No one would ever have thought of teaching, or probably could havetaught, the tumbler-pigeon to tumble--an action which, as I have witnessed,is performed by young birds, that have never seen a pigeon tumble. We maybelieve that some one pigeon showed a slight tendency to this strangehabit, and that the long-continued selection of the best individuals insuccessive generations made tumblers what they now are; and near Glasgowthere are 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 not someone dog naturally shown a tendency in this line; and this is knownoccasionally to happen, as I once saw, in a pure terrier: the act ofpointing is probably, as many have thought, only the exaggerated pause ofan animal preparing to spring on its prey. When the first tendency topoint was once displayed, methodical selection and the inherited effects ofcompulsory training in each successive generation would soon complete thework; and unconscious selection is still in progress, as each man tries toprocure, without intending to improve the breed, dogs which stand and huntbest. On the other hand, habit alone in some cases has sufficed; hardlyany animal is more difficult to tame than the young of the wild rabbit;scarcely any animal is tamer than the young of the tame rabbit; but I canhardly suppose that domestic rabbits have often been selected for tamenessalone; so that we must attribute at least the greater part of the inheritedchange from extreme wildness to extreme tameness, to habit andlong-continued close confinement.

Natural instincts are lost under domestication: a remarkable instance ofthis is seen in those breeds of fowls which very rarely or never become"broody," that is, never wish to sit on their eggs. Familiarity aloneprevents our seeing how largely and how permanently the minds of ourdomestic animals have been modified. It is scarcely possible to doubt thatthe love of man has become instinctive in the dog. All wolves, foxes,jackals and species of the cat genus, when kept tame, are most eager toattack poultry, sheep and pigs; and this tendency has been found incurablein dogs which have been brought home as puppies from countries such asTierra del Fuego and Australia, where the savages do not keep thesedomestic animals. How rarely, on the other hand, do our civilised dogs,even when quite young, require to be taught not to attack poultry, sheep,and pigs! No doubt they occasionally do make an attack, and are thenbeaten; and if not cured, they are destroyed; so that habit and some degreeof selection have probably concurred in civilising by inheritance our dogs. On the other hand, young chickens have lost wholly by habit, that fear ofthe dog and cat which no doubt was originally instinctive in them, for I aminformed by Captain Hutton that the young chickens of the parent stock, theGallus bankiva, when reared in India under a hen, are at first excessivelywild. So it is with young pheasants reared in England under a hen. It isnot that chickens have lost all fear, but fear only of dogs and cats, forif the hen gives the danger chuckle they will run (more especially youngturkeys) from under her and conceal themselves in the surrounding grass orthickets; and this is evidently done for the instinctive purpose ofallowing, as we see in wild ground-birds, their mother to fly away. Butthis instinct retained by our chickens has become useless underdomestication, for the mother-hen has almost lost by disuse the power offlight.

Hence, we may conclude that under domestication instincts have beenacquired and natural instincts have been lost, partly by habit and partlyby man selecting and accumulating, during successive generations, peculiarmental habits and actions, which at first appeared from what we must in ourignorance call an accident. In some cases compulsory habit alone hassufficed to produce inherited mental changes; in other cases compulsoryhabit has done nothing, and all has been the result of selection, pursuedboth methodically and unconsciously; but in most cases habit and selectionhave probably concurred.

SPECIAL INSTINCTS.

We shall, perhaps, best understand how instincts in a state of nature havebecome modified by selection by considering a few cases. I will selectonly three, namely, the instinct which leads the cuckoo to lay her eggs inother birds' nests; the slave-making instinct of certain ants; and thecell-making power of the hive-bee: these two latter instincts havegenerally and justly been ranked by naturalists as the most wonderful ofall known instincts.

INSTINCTS OF THE CUCKOO.

It is supposed by some naturalists that the more immediate cause of theinstinct of the cuckoo is that she lays her eggs, not daily, but atintervals of two or three days; so that, if she were to make her own nestand sit on her own eggs, those first laid would have to be left for sometime unincubated or there would be eggs and young birds of different agesin the same nest. If this were the case the process of laying and hatchingmight be inconveniently long, more especially as she migrates at a veryearly period; and the first hatched young would probably have to be fed bythe male alone. But the American cuckoo is in this predicament, for shemakes her own nest and has eggs and young successively hatched, all at thesame time. It has been both asserted and denied that the American cuckoooccasionally lays her eggs in other birds' nests; but I have lately heardfrom Dr. Merrill, of Iowa, that he once found in Illinois a young cuckoo,together with a young jay in the nest of a blue jay (Garrulus cristatus);and as both were nearly full feathered, there could be no mistake in theiridentification. I could also give several instances of various birds whichhave been known occasionally to lay their eggs in other birds' nests. Nowlet us suppose that the ancient progenitor of our European cuckoo had thehabits of the American cuckoo, and that she occasionally laid an egg inanother bird's nest. If the old bird profited by this occasional habitthrough being enabled to emigrate earlier or through any other cause; or ifthe young were made more vigorous by advantage being taken of the mistakeninstinct of another species than when reared by their own mother,encumbered as she could hardly fail to be by having eggs and young ofdifferent ages at the same time, then the old birds or the fostered youngwould gain an advantage. And analogy would lead us to believe that theyoung thus reared would be apt to follow by inheritance the occasional andaberrant habit of their mother, and in their turn would be apt to lay theireggs in other birds' nests, and thus be more successful in rearing theiryoung. By a continued process of this nature, I believe that the strangeinstinct of our cuckoo has been generated. It has, also recently beenascertained on sufficient evidence, by Adolf Muller, that the cuckoooccasionally lays her eggs on the bare ground, sits on them and feeds heryoung. This rare event is probably a case of reversion to the long-lost,aboriginal instinct of nidification.

It has been objected that I have not noticed other related instincts andadaptations of structure in the cuckoo, which are spoken of as necessarilyco-ordinated. But in all cases, speculation on an instinct known to usonly in a single species, is useless, for we have hitherto had no facts toguide us. Until recently the instincts of the European and of the non-parasitic American cuckoo alone were known; now, owing to Mr. Ramsay'sobservations, we have learned something about three Australian species,which lay their eggs in other birds' nests. The chief points to bereferred to are three: first, that the common cuckoo, with rareexceptions, lays only one egg in a nest, so that the large and voraciousyoung bird receives ample food. Secondly, that the eggs are remarkablysmall, not exceeding those of the skylark--a bird about one-fourth as largeas the cuckoo. That the small size of the egg is a real case of adaptationwe may infer from the fact of the mon-parasitic American cuckoo layingfull-sized eggs. Thirdly, that the young cuckoo, soon after birth, has theinstinct, the strength and a properly shaped back for ejecting its foster-brothers, which then perish from cold and hunger. This has been boldlycalled a beneficent arrangement, in order that the young cuckoo may getsufficient food, and that its foster-brothers may perish before they hadacquired much feeling!

Turning now to the Australian species: though these birds generally layonly one egg in a nest, it is not rare to find two and even three eggs inthe same nest. In the bronze cuckoo the eggs vary greatly in size, fromeight to ten lines in length. Now, if it had been of an advantage to thisspecies to have laid eggs even smaller than those now laid, so as to havedeceived certain foster-parents, or, as is more probable, to have beenhatched within a shorter period (for it is asserted that there is arelation between the size of eggs and the period of their incubation), thenthere is no difficulty in believing that a race or species might have beenformed which would have laid smaller and smaller eggs; for these would havebeen more safely hatched and reared. Mr. Ramsay remarks that two of theAustralian cuckoos, when they lay their eggs in an open nest, manifest adecided preference for nests containing eggs similar in colour to theirown. The European species apparently manifests some tendency towards asimilar instinct, but not rarely departs from it, as is shown by her layingher dull and pale-coloured eggs in the nest of the hedge-warbler withbright greenish-blue eggs. Had our cuckoo invariably displayed the aboveinstinct, it would assuredly have been added to those which it is assumedmust all have been acquired together. The eggs of the Australian bronzecuckoo vary, according to Mr. Ramsay, to an extraordinary degree in colour;so that in this respect, as well as in size, natural selection might havesecured and fixed any advantageous variation.

In the case of the European cuckoo, the offspring of the foster-parents arecommonly ejected from the nest within three days after the cuckoo ishatched; and as the latter at this age is in a most helpless condition, Mr.Gould was formerly inclined to believe that the act of ejection wasperformed by the foster-parents themselves. But he has now received atrustworthy account of a young cuckoo which was actually seen, while stillblind and not able even to hold up its own head, in the act of ejecting itsfoster-brothers. One of these was replaced in the nest by the observer,and was again thrown out. With respect to the means by which this strangeand odious instinct was acquired, if it were of great importance for theyoung cuckoo, as is probably the case, to receive as much food as possiblesoon after birth, I can see no special difficulty in its having graduallyacquired, during successive generations, the blind desire, the strength,and structure necessary for the work of ejection; for those cuckoos whichhad such habits and structure best developed would be the most securelyreared. The first step towards the acquisition of the proper instinctmight have been mere unintentional restlessness on the part of the youngbird, when somewhat advanced in age and strength; the habit having beenafterwards improved, and transmitted to an earlier age. I can see no moredifficulty in this than in the unhatched young of other birds acquiring theinstinct to break through their own shells; or than in young snakesacquiring in their upper jaws, as Owen has remarked, a transitory sharptooth for cutting through the tough egg-shell. For if each part is liableto individual variations at all ages, and the variations tend to beinherited at a corresponding or earlier age--propositions which cannot bedisputed--then the instincts and structure of the young could be slowlymodified as surely as those of the adult; and both cases must stand or falltogether with the whole theory of natural selection.

Some species of Molothrus, a widely distinct genus of American birds,allied to our starlings, have parasitic habits like those of the cuckoo;and the species present an interesting gradation in the perfection of theirinstincts. The sexes of Molothrus badius are stated by an excellentobserver, Mr. Hudson, sometimes to live promiscuously together in flocks,and sometimes to pair. They either build a nest of their own or seize onone belonging to some other bird, occasionally throwing out the nestlingsof the stranger. They either lay their eggs in the nest thus appropriated,or oddly enough build one for themselves on the top of it. They usuallysit on their own eggs and rear their own young; but Mr. Hudson says it isprobable that they are occasionally parasitic, for he has seen the young ofthis species following old birds of a distinct kind and clamouring to befed by them. The parasitic habits of another species of Molothrus, the M.bonariensis, are much more highly developed than those of the last, but arestill far from perfect. This bird, as far as it is known, invariably laysits eggs in the nests of strangers; but it is remarkable that severaltogether sometimes commence to build an irregular untidy nest of their own,placed in singular ill-adapted situations, as on the leaves of a largethistle. They never, however, as far as Mr. Hudson has ascertained,complete a nest for themselves. They often lay so many eggs--from fifteento twenty--in the same foster-nest, that few or none can possibly behatched. They have, moreover, the extraordinary habit of pecking holes inthe eggs, whether of their own species or of their foster parents, whichthey find in the appropriated nests. They drop also many eggs on the bareground, which are thus wasted. A third species, the M. pecoris of NorthAmerica, has acquired instincts as perfect as those of the cuckoo, for itnever lays more than one egg in a foster-nest, so that the young bird issecurely reared. Mr. Hudson is a strong disbeliever in evolution, but heappears to have been so much struck by the imperfect instincts of theMolothrus bonariensis that he quotes my words, and asks, "Must we considerthese habits, not as especially endowed or created instincts, but as smallconsequences of one general law, namely, transition?"

Various birds, as has already been remarked, occasionally lay their eggs inthe nests of other birds. This habit is not very uncommon with theGallinaceae, and throws some light on the singular instinct of the ostrich.In this family several hen birds unite and lay first a few eggs in one nestand then in another; and these are hatched by the males. This instinct mayprobably be accounted for by the fact of the hens laying a large number ofeggs, but, as with the cuckoo, at intervals of two or three days. Theinstinct, however, of the American ostrich, as in the case of the Molothrusbonariensis, has not as yet been perfected; for a surprising number of eggslie strewed over the plains, so that in one day's hunting I picked up noless than twenty lost and wasted eggs.

Many bees are parasitic, and regularly lay their eggs in the nests of otherkinds of bees. This case is more remarkable than that of the cuckoo; forthese bees have not only had their instincts but their structure modifiedin accordance with their parasitic habits; for they do not possess thepollen-collecting apparatus which would have been indispensable if they hadstored up food for their own young. Some species of Sphegidae (wasp-likeinsects) are likewise parasitic; and M. Fabre has lately shown good reasonfor believing that, although the Tachytes nigra generally makes its ownburrow and stores it with paralysed prey for its own larvae, yet that, whenthis insect finds a burrow already made and stored by another sphex, ittakes advantage of the prize, and becomes for the occasion parasitic. Inthis case, as with that of the Molothrus or cuckoo, I can see no difficultyin natural selection making an occasional habit permanent, if of advantageto the species, and if the insect whose nest and stored food arefeloniously appropriated, be not thus exterminated.

SLAVE-MAKING INSTINCT.

This remarkable instinct was first discovered in the Formica (Polyerges)rufescens by Pierre Huber, a better observer even than his celebratedfather. This ant is absolutely dependent on its slaves; without their aid,the species would certainly become extinct in a single year. The males andfertile females do no work of any kind, and the workers or sterile females,though most energetic and courageous in capturing slaves, do no other work. They are incapable of making their own nests, or of feeding their ownlarvae. When the old nest is found inconvenient, and they have to migrate,it is the slaves which determine the migration, and actually carry theirmasters in their jaws. So utterly helpless are the masters, that whenHuber shut up thirty of them without a slave, but with plenty of the foodwhich they like best, and with their larvae and pupae to stimulate them towork, they did nothing; they could not even feed themselves, and manyperished of hunger. Huber then introduced a single slave (F. fusca), andshe instantly set to work, fed and saved the survivors; made some cells andtended the larvae, and put all to rights. What can be more extraordinarythan these well-ascertained facts? If we had not known of any otherslave-making ant, it would have been hopeless to speculate how so wonderfulan instinct could have been perfected.

Another species, Formica sanguinea, was likewise first discovered by P.Huber to be a slave-making ant. This species is found in the southernparts of England, and its habits have been attended to by Mr. F. Smith, ofthe British Museum, to whom I am much indebted for information on this andother subjects. Although fully trusting to the statements of Huber and Mr.Smith, I tried to approach the subject in a sceptical frame of mind, as anyone may well be excused for doubting the existence of so extraordinary aninstinct as that of making slaves. Hence, I will give the observationswhich I made in some little detail. I opened fourteen nests of F.sanguinea, and found a few slaves in all. Males and fertile females of theslave-species (F. fusca) are found only in their own proper communities,and have never been observed in the nests of F. sanguinea. The slaves areblack and not above half the size of their red masters, so that thecontrast in their appearance is great. When the nest is slightlydisturbed, the slaves occasionally come out, and like their masters aremuch agitated and defend the nest: when the nest is much disturbed, andthe larvae and pupae are exposed, the slaves work energetically togetherwith their masters in carrying them away to a place of safety. Hence, itis clear that the slaves feel quite at home. During the months of June andJuly, on three successive years, I watched for many hours several nests inSurrey and Sussex, and never saw a slave either leave or enter a nest. As,during these months, the slaves are very few in number, I thought that theymight behave differently when more numerous; but Mr. Smith informs me thathe has watched the nests at various hours during May, June and August, bothin Surrey and Hampshire, and has never seen the slaves, though present inlarge numbers in August, either leave or enter the nest. Hence, heconsiders them as strictly household slaves. The masters, on the otherhand, may be constantly seen bringing in materials for the nest, and foodof all kinds. During the year 1860, however, in the month of July, I cameacross a community with an unusually large stock of slaves, and I observeda few slaves mingled with their masters leaving the nest, and marchingalong the same road to a tall Scotch-fir tree, twenty-five yards distant,which they ascended together, probably in search of aphides or cocci. According to Huber, who had ample opportunities for observation, the slavesin Switzerland habitually work with their masters in making the nest, andthey alone open and close the doors in the morning and evening; and, asHuber expressly states, their principal office is to search for aphides. This difference in the usual habits of the masters and slaves in the twocountries, probably depends merely on the slaves being captured in greaternumbers in Switzerland than in England.

One day I fortunately witnessed a migration of F. sanguinea from one nestto another, and it was a most interesting spectacle to behold the masterscarefully carrying their slaves in their jaws instead of being carried bythem, as in the case of F. rufescens. Another day my attention was struckby about a score of the slave-makers haunting the same spot, and evidentlynot in search of food; they approached and were vigorously repulsed by anindependent community of the slave species (F. fusca); sometimes as many asthree of these ants clinging to the legs of the slave-making F. sanguinea. The latter ruthlessly killed their small opponents and carried their deadbodies as food to their nest, twenty-nine yards distant; but they wereprevented from getting any pupae to rear as slaves. I then dug up a smallparcel of the pupae of F. fusca from another nest, and put them down on abare spot near the place of combat; they were eagerly seized and carriedoff by the tyrants, who perhaps fancied that, after all, they had beenvictorious in their late combat.

At the same time I laid on the same place a small parcel of the pupae ofanother species, F. flava, with a few of these little yellow ants stillclinging to the fragments of their nest. This species is sometimes, thoughrarely, made into slaves, as has been described by Mr. Smith. Although sosmall a species, it is very courageous, and I have seen it ferociouslyattack other ants. In one instance I found to my surprise an independentcommunity of F. flava under a stone beneath a nest of the slave-making F.sanguinea; and when I had accidentally disturbed both nests, the littleants attacked their big neighbours with surprising courage. Now I wascurious to ascertain whether F. sanguinea could distinguish the pupae of F.fusca, which they habitually make into slaves, from those of the little andfurious F. flava, which they rarely capture, and it was evident that theydid at once distinguish them; for we have seen that they eagerly andinstantly seized the pupae of F. fusca, whereas they were much terrifiedwhen they came across the pupae, or even the earth from the nest, of F.flava, and quickly ran away; but in about a quarter of an hour, shortlyafter all the little yellow ants had crawled away, they took heart andcarried off the pupae.

One evening I visited another community of F. sanguinea, and found a numberof these ants returning home and entering their nests, carrying the deadbodies of F. fusca (showing that it was not a migration) and numerouspupae. I traced a long file of ants burdened with booty, for about fortyyards back, to a very thick clump of heath, whence I saw the lastindividual of F. sanguinea emerge, carrying a pupa; but I was not able tofind the desolated nest in the thick heath. The nest, however, must havebeen close at hand, for two or three individuals of F. fusca were rushingabout in the greatest agitation, and one was perched motionless with itsown pupa in its mouth on the top of a spray of heath, an image of despairover its ravaged home.

Such are the facts, though they did not need confirmation by me, in regardto the wonderful instinct of making slaves. Let it be observed what acontrast the instinctive habits of F. sanguinea present with those of thecontinental F. rufescens. The latter does not build its own nest, does notdetermine its own migrations, does not collect food for itself or itsyoung, and cannot even feed itself: it is absolutely dependent on itsnumerous slaves. Formica sanguinea, on the other hand, possesses muchfewer slaves, and in the early part of the summer extremely few. Themasters determine when and where a new nest shall be formed, and when theymigrate, the masters carry the slaves. Both in Switzerland and England theslaves seem to have the exclusive care of the larvae, and the masters alonego on slave-making expeditions. In Switzerland the slaves and masters worktogether, making and bringing materials for the nest: both, but chieflythe slaves, tend and milk as it may be called, their aphides; and thus bothcollect food for the community. In England the masters alone usually leavethe nest to collect building materials and food for themselves, theirslaves and larvae. So that the masters in this country receive much lessservice from their slaves than they do in Switzerland.

By what steps the instinct of F. sanguinea originated I will not pretend toconjecture. But as ants which are not slave-makers, will, as I have seen,carry off pupae of other species, if scattered near their nests, it ispossible that such pupae originally stored as food might become developed;and the foreign ants thus unintentionally reared would then follow theirproper instincts, and do what work they could. If their presence proveduseful to the species which had seized them--if it were more advantageousto this species, to capture workers than to procreate them--the habit ofcollecting pupae, originally for food, might by natural selection bestrengthened and rendered permanent for the very different purpose ofraising slaves. When the instinct was once acquired, if carried out to amuch less extent even than in our British F. sanguinea, which, as we haveseen, is less aided by its slaves than the same species in Switzerland,natural selection might increase and modify the instinct--always supposingeach modification to be of use to the species--until an ant was formed asabjectly dependent on its slaves as is the Formica rufescens.

CELL-MAKING INSTINCT OF THE HIVE-BEE.

I will not here enter on minute details on this subject, but will merelygive an outline of the conclusions at which I have arrived. He must be adull man who can examine the exquisite structure of a comb, so beautifullyadapted to its end, without enthusiastic admiration. We hear frommathematicians that bees have practically solved a recondite problem, andhave made their cells of the proper shape to hold the greatest possibleamount of honey, with the least possible consumption of precious wax intheir construction. It has been remarked that a skilful workman, withfitting tools and measures, would find it very difficult to make cells ofwax of the true form, though this is effected by a crowd of bees working ina dark hive. Granting whatever instincts you please, it seems at firstquite inconceivable how they can make all the necessary angles and planes,or even perceive when they are correctly made. But the difficulty is notnearly so great as at first appears: all this beautiful work can be shown,I think, to follow from a few simple instincts.

I was led to investigate this subject by Mr. Waterhouse, who has shown thatthe form of the cell stands in close relation to the presence of adjoiningcells; and the following view may, perhaps, be considered only as amodification of his theory. Let us look to the great principle ofgradation, and see whether Nature does not reveal to us her method of work. At one end of a short series we have humble-bees, which use their oldcocoons to hold honey, sometimes adding to them short tubes of wax, andlikewise making separate and very irregular rounded cells of wax. At theother end of the series we have the cells of the hive-bee, placed in adouble layer: each cell, as is well known, is an hexagonal prism, with thebasal edges of its six sides bevelled so as to join an inverted pyramid, ofthree rhombs. These rhombs have certain angles, and the three which formthe pyramidal base of a single cell on one side of the comb, enter into thecomposition of the bases of three adjoining cells on the opposite side. Inthe series between the extreme perfection of the cells of the hive-bee andthe simplicity of those of the humble-bee, we have the cells of the MexicanMelipona domestica, carefully described and figured by Pierre Huber. TheMelipona itself is intermediate in structure between the hive and humblebee, but more nearly related to the latter: it forms a nearly regularwaxen comb of cylindrical cells, in which the young are hatched, and, inaddition, some large cells of wax for holding honey. These latter cellsare nearly spherical and of nearly equal sizes, and are aggregated into anirregular mass. But the important point to notice is, that these cells arealways made at that degree of nearness to each other that they would haveintersected or broken into each other if the spheres had been completed;but this is never permitted, the bees building perfectly flat walls of waxbetween the spheres which thus tend to intersect. Hence, each cellconsists of an outer spherical portion, and of two, three, or more flatsurfaces, according as the cell adjoins two, three or more other cells. When one cell rests on three other cells, which, from the spheres beingnearly of the same size, is very frequently and necessarily the case, thethree flat surfaces are united into a pyramid; and this pyramid, as Huberhas remarked, is manifestly a gross imitation of the three-sided pyramidalbase of the cell of the hive-bee. As in the cells of the hive-bee, sohere, the three plane surfaces in any one cell necessarily enter into theconstruction of three adjoining cells. It is obvious that the Meliponasaves wax, and what is more important, labour, by this manner of building;for the flat walls between the adjoining cells are not double, but are ofthe same thickness as the outer spherical portions, and yet each flatportion forms a part of two cells.

Reflecting on this case, it occurred to me that if the Melipona had madeits spheres at some given distance from each other, and had made them ofequal sizes and had arranged them symmetrically in a double layer, theresulting structure would have been as perfect as the comb of the hive-bee. Accordingly I wrote to Professor Miller, of Cambridge, and this geometerhas kindly read over the following statement, drawn up from hisinformation, and tells me that it is strictly correct:-

If a number of equal spheres be described with their centres placed in twoparallel layers; with the centre of each sphere at the distance of radius xsqrt(2) or radius x 1.41421 (or at some lesser distance), from the centresof the six surrounding spheres in the same layer; and at the same distancefrom the centres of the adjoining spheres in the other and parallel layer;then, if planes of intersection between the several spheres in both layersbe formed, there will result a double layer of hexagonal prisms unitedtogether by pyramidal bases formed of three rhombs; and the rhombs and thesides of the hexagonal prisms will have every angle identically the samewith the best measurements which have been made of the cells of thehive-bee. But I hear from Professor Wyman, who has made numerous carefulmeasurements, that the accuracy of the workmanship of the bee has beengreatly exaggerated; so much so, that whatever the typical form of the cellmay be, it is rarely, if ever, realised.

Hence we may safely conclude that, if we could slightly modify theinstincts already possessed by the Melipona, and in themselves not verywonderful, this bee would make a structure as wonderfully perfect as thatof the hive-bee. We must suppose the Melipona to have the power of formingher cells truly spherical, and of equal sizes; and this would not be verysurprising, seeing that she already does so to a certain extent, and seeingwhat perfectly cylindrical burrows many insects make in wood, apparently byturning round on a fixed point. We must suppose the Melipona to arrangeher cells in level layers, as she already does her cylindrical cells; andwe must further suppose, and this is the greatest difficulty, that she cansomehow judge accurately at what distance to stand from herfellow-labourers when several are making their spheres; but she is alreadyso far enabled to judge of distance, that she always describes her spheresso as to intersect to a certain extent; and then she unites the points ofintersection by perfectly flat surfaces. By such modifications ofinstincts which in themselves are not very wonderful--hardly more wonderfulthan those which guide a bird to make its nest--I believe that the hive-beehas acquired, through natural selection, her inimitable architecturalpowers.

But this theory can be tested by experiment. Following the example of Mr.Tegetmeier, I separated two combs, and put between them a long, thick,rectangular strip of wax: the bees instantly began to excavate minutecircular pits in it; and as they deepened these little pits, they made themwider and wider until they were converted into shallow basins, appearing tothe eye perfectly true or parts of a sphere, and of about the diameter of acell. It was most interesting to observe that, wherever several bees hadbegun to excavate these basins near together, they had begun their work atsuch a distance from each other that by the time the basins had acquiredthe above stated width (i.e. about the width of an ordinary cell), and werein depth about one sixth of the diameter of the sphere of which they formeda part, the rims of the basins intersected or broke into each other. Assoon as this occurred, the bees ceased to excavate, and began to build upflat walls of wax on the lines of intersection between the basins, so thateach hexagonal prism was built upon the scalloped edge of a smooth basin,instead of on the straight edges of a three-sided pyramid as in the case ofordinary cells.

I then put into the hive, instead of a thick, rectangular piece of wax, athin and narrow, knife-edged ridge, coloured with vermilion. The beesinstantly began on both sides to excavate little basins near to each other,in the same way as before; but the ridge of wax was so thin, that thebottoms of the basins, if they had been excavated to the same depth as inthe former experiment, would have broken into each other from the oppositesides. The bees, however, did not suffer this to happen, and they stoppedtheir excavations in due time; so that the basins, as soon as they had beena little deepened, came to have flat bases; and these flat bases, formed bythin little plates of the vermilion wax left ungnawed, were situated, asfar as the eye could judge, exactly along the planes of imaginaryintersection between the basins on the opposite side of the ridge of wax. In some parts, only small portions, in other parts, large portions of arhombic plate were thus left between the opposed basins, but the work, fromthe unnatural state of things, had not been neatly performed. The beesmust have worked at very nearly the same rate in circularly gnawing awayand deepening the basins on both sides of the ridge of vermilion wax, inorder to have thus succeeded in leaving flat plates between the basins, bystopping work at the 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 of wax,perceiving when they have gnawed the wax away to the proper thinness, andthen stopping their work. In ordinary combs it has appeared to me that thebees do not always succeed in working at exactly the same rate from theopposite sides; for I have noticed half-completed rhombs at the base of ajust-commenced cell, which were slightly concave on one side, where Isuppose that the bees had excavated too quickly, and convex on the opposedside where the bees had worked less quickly. In one well-marked instance,I put the comb back into the hive, and allowed the bees to go on workingfor a short time, and again examined the cell, and I found that the rhombicplate had been completed, and had become PERFECTLY FLAT: it was absolutelyimpossible, from the extreme thinness of the little plate, that they couldhave effected this by gnawing away the convex side; and I suspect that thebees in such cases stand in the opposed cells and push and bend the ductileand warm wax (which as I have tried is easily done) into its properintermediate plane, and thus flatten it.

>From the experiment of the ridge of vermilion wax we can see that, if thebees were to build for themselves a thin wall of wax, they could make theircells of the proper shape, by standing at the proper distance from eachother, by excavating at the same rate, and by endeavouring to make equalspherical hollows, but never allowing the spheres to break into each other. Now bees, as may be clearly seen by examining the edge of a growing comb,do make a rough, circumferential wall or rim all round the comb; and theygnaw this away from the opposite sides, always working circularly as theydeepen each cell. They do not make the whole three-sided pyramidal base ofany one cell at the same time, but only that one rhombic plate which standson the extreme 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 from thosemade by the justly celebrated elder Huber, but I am convinced of theiraccuracy; and if I had space, I could show that they are conformable withmy theory.

Huber's statement, that the very first cell is excavated out of a littleparallel-sided wall of wax, is not, as far as I have seen, strictlycorrect; the first commencement having always been a little hood of wax;but I will not here enter on details. We see how important a partexcavation plays in the construction of the cells; but it would be a greaterror to suppose that the bees cannot build up a rough wall of wax in theproper position--that is, along the plane of intersection between twoadjoining spheres. I have several specimens showing clearly that they cando this. Even in the rude circumferential rim or wall of wax round agrowing comb, flexures may sometimes be observed, corresponding in positionto the planes of the rhombic basal plates of future cells. But the roughwall of wax has in every case to be finished off, by being largely gnawedaway on both sides. The manner in which the bees build is curious; theyalways make the first rough wall from ten to twenty times thicker than theexcessively thin finished wall of the cell, which will ultimately be left. We shall understand how they work, by supposing masons first to pile up abroad ridge of cement, and then to begin cutting it away equally on bothsides near the ground, till a smooth, very thin wall is left in the middle;the masons always piling up the cut-away cement, and adding fresh cement onthe summit of the ridge. We shall thus have a thin wall steadily growingupward but always crowned by a gigantic coping. From all the cells, boththose just commenced and those completed, being thus crowned by a strongcoping of wax, the bees can cluster and crawl over the comb withoutinjuring the delicate hexagonal walls. These walls, as Professor Millerhas kindly ascertained for me, vary greatly in thickness; being, on anaverage of twelve measurements made near the border of the comb, 1/352 ofan inch in thickness; whereas the basal rhomboidal plates are thicker,nearly in the proportion of three to two, having a mean thickness, fromtwenty-one measurements, of 1/229 of an inch. By the above singular mannerof building, strength is continually given to the comb, with the utmostultimate economy of wax.

It seems at first to add to the difficulty of understanding how the cellsare made, that a multitude of bees all work together; one bee after workinga short time at one cell going to another, so that, as Huber has stated, ascore of individuals work even at the commencement of the first cell. Iwas able practically to show this fact, by covering the edges of thehexagonal walls of a single cell, or the extreme margin of thecircumferential rim of a growing comb, with an extremely thin layer ofmelted vermilion wax; and I invariably found that the colour was mostdelicately diffused by the bees--as delicately as a painter could have doneit with his brush--by atoms of the coloured wax having been taken from thespot on which it had been placed, and worked into the growing edges of thecells all round. The work of construction seems to be a sort of balancestruck between many bees, all instinctively standing at the same relativedistance from each other, all trying to sweep equal spheres, and thenbuilding up, or leaving ungnawed, the planes of intersection between thesespheres. It was really curious to note in cases of difficulty, as when twopieces of comb met at an angle, how often the bees would pull down andrebuild in different ways the same cell, sometimes recurring to a shapewhich they had at first rejected.

When bees have a place on which they can stand in their proper positionsfor working--for instance, on a slip of wood, placed directly under themiddle of a comb growing downwards, so that the comb has to be built overone face of the slip--in this case the bees can lay the foundations of onewall of a new hexagon, in its strictly proper place, projecting beyond theother completed cells. It suffices that the bees should be enabled tostand at their proper relative distances from each other and from the wallsof the last completed cells, and then, by striking imaginary spheres, theycan build up a wall intermediate between two adjoining spheres; but, as faras I have seen, they never gnaw away and finish off the angles of a celltill a large part both of that cell and of the adjoining cells has beenbuilt. This capacity in bees of laying down under certain circumstances arough wall in its proper place between two just-commenced cells, isimportant, as it bears on a fact, which seems at first subversive of theforegoing theory; namely, that the cells on the extreme margin ofwasp-combs are sometimes strictly hexagonal; but I have not space here toenter on this subject. Nor does there seem to me any great difficulty in asingle insect (as in the case of a queen-wasp) making hexagonal cells, ifshe were to work alternately on the inside and outside of two or threecells commenced at the same time, always standing at the proper relativedistance from the parts of the cells just begun, sweeping spheres orcylinders, and building up intermediate planes.

As natural selection acts only by the accumulation of slight modificationsof structure or instinct, each profitable to the individual under itsconditions of life, it may reasonably be asked, how a long and graduatedsuccession of modified architectural instincts, all tending towards thepresent perfect plan of construction, could have profited the progenitorsof the hive-bee? I think the answer is not difficult: cells constructedlike those of the bee or the wasp gain in strength, and save much in labourand space, and in the materials of which they are constructed. Withrespect to the formation of wax, it is known that bees are often hardpressed to get sufficient nectar; and I am informed by Mr. Tegetmeier thatit has been experimentally proved that from twelve to fifteen pounds of drysugar are consumed by a hive of bees for the secretion of a pound of wax;so that a prodigious quantity of fluid nectar must be collected andconsumed by the bees in a hive for the secretion of the wax necessary forthe construction of their combs. Moreover, many bees have to remain idlefor many days during the process of secretion. A large store of honey isindispensable to support a large stock of bees during the winter; and thesecurity of the hive is known mainly to depend on a large number of beesbeing supported. Hence the saving of wax by largely saving honey, and thetime consumed in collecting the honey, must be an important element ofsuccess any family of bees. Of course the success of the species may bedependent on the number of its enemies, or parasites, or on quite distinctcauses, and so be altogether independent of the quantity of honey which thebees can collect. But let us suppose that this latter circumstancedetermined, as it probably often has determined, whether a bee allied toour humble-bees could exist in large numbers in any country; and let usfurther suppose that the community lived through the winter, andconsequently required a store of honey: there can in this case be no doubtthat it would be an advantage to our imaginary humble-bee if a slightmodification of her instincts led her to make her waxen cells neartogether, so as to intersect a little; for a wall in common even to twoadjoining cells would save some little labour and wax. Hence, it wouldcontinually be more and more advantageous to our humble-bees, if they wereto make their cells more and more regular, nearer together, and aggregatedinto a mass, like the cells of the Melipona; for in this case a large partof the bounding surface of each cell would serve to bound the adjoiningcells, and much labour and wax would be saved. Again, from the same cause,it would be advantageous to the Melipona, if she were to make her cellscloser together, and more regular in every way than at present; for then,as we have seen, the spherical surfaces would wholly disappear and bereplaced by plane surfaces; and the Melipona would make a comb as perfectas that of the hive-bee. Beyond this stage of perfection in architecture,natural selection could not lead; for the comb of the hive-bee, as far aswe can see, is absolutely perfect in economising labour and wax.

Thus, as I believe, the most wonderful of all known instincts, that of thehive-bee, can be explained by natural selection having taken advantage ofnumerous, successive, slight modifications of simpler instincts; naturalselection having, by slow degrees, more and more perfectly led the bees tosweep equal spheres at a given distance from each other in a double layer,and to build up and excavate the wax along the planes of intersection. Thebees, of course, no more knowing that they swept their spheres at oneparticular distance from each other, than they know what are the severalangles of the hexagonal prisms and of the basal rhombic plates; the motivepower of the process of natural selection having been the construction ofcells of due strength and of the proper size and shape for the larvae, thisbeing effected with the greatest possible economy of labour and wax; thatindividual swarm which thus made the best cells with least labour, andleast waste of honey in the secretion of wax, having succeeded best, andhaving transmitted their newly-acquired economical instincts to new swarms,which in their turn will have had the best chance of succeeding in thestruggle for existence.

OBJECTIONS TO THE THEORY OF NATURAL SELECTION AS APPLIED TO INSTINCTS: NEUTER AND STERILE INSECTS.

It has been objected to the foregoing view of the origin of instincts that"the variations of structure and of instinct must have been simultaneousand accurately adjusted to each other, as a modification in the one withoutan immediate corresponding change in the other would have been fatal." Theforce of this objection rests entirely on the assumption that the changesin the instincts and structure are abrupt. To take as an illustration thecase of the larger titmouse, (Parus major) alluded to in a previouschapter; this bird often holds the seeds of the yew between its feet on abranch, and hammers with its beak till it gets at the kernel. Now whatspecial difficulty would there be in natural selection preserving all theslight individual variations in the shape of the beak, which were betterand better adapted to break open the seeds, until a beak was formed, aswell constructed for this purpose as that of the nuthatch, at the same timethat habit, or compulsion, or spontaneous variations of taste, led the birdto become more and more of a seed-eater? In this case the beak is supposedto be slowly modified by natural selection, subsequently to, but inaccordance with, slowly changing habits or taste; but let the feet of thetitmouse vary and grow larger from correlation with the beak, or from anyother unknown cause, and it is not improbable that such larger feet wouldlead the bird to climb more and more until it acquired the remarkableclimbing instinct and power of the nuthatch. In this case a gradual changeof structure is supposed to lead to changed instinctive habits. To takeone more case: few instincts are more remarkable than that which leads theswift of the Eastern Islands to make its nest wholly of inspissated saliva.Some birds build their nests of mud, believed to be moistened with saliva;and one of the swifts of North America makes its nest (as I have seen) ofsticks agglutinated with saliva, and even with flakes of this substance. Is it then very improbable that the natural selection of individual swifts,which secreted more and more saliva, should at last produce a species withinstincts leading it to neglect other materials and to make its nestexclusively of inspissated saliva? And so in other cases. It must,however, be admitted that in many instances we cannot conjecture whether itwas instinct or structure which first varied.

No doubt many instincts of very difficult explanation could be opposed tothe theory of natural selection--cases, in which we cannot see how aninstinct could have originated; cases, in which no intermediate gradationsare known to exist; cases of instincts of such trifling importance, thatthey could hardly have been acted on by natural selection; cases ofinstincts almost identically the same in animals so remote in the scale ofnature that we cannot account for their similarity by inheritance from acommon progenitor, and consequently must believe that they wereindependently acquired through natural selection. I will not here enter onthese several cases, but will confine myself to one special difficulty,which at first appeared to me insuperable, and actually fatal to the wholetheory. I allude to the neuters or sterile females in insect communities: for these neuters often differ widely in instinct and in structure fromboth the males and fertile females, and yet, from being sterile, theycannot propagate their kind.

The subject well deserves to be discussed at great length, but I will heretake only a single case, that of working or sterile ants. How the workershave been rendered sterile is a difficulty; but not much greater than thatof any other striking modification of structure; for it can be shown thatsome insects and other articulate animals in a state of nature occasionallybecome sterile; and if such insects had been social, and it had beenprofitable to the community that a number should have been annually borncapable of work, but incapable of procreation, I can see no especialdifficulty in this having been effected through natural selection. But Imust pass over this preliminary difficulty. The great difficulty lies inthe working ants differing widely from both the males and the fertilefemales in structure, as in the shape of the thorax, and in being destituteof wings and sometimes of eyes, and in instinct. As far as instinct aloneis concerned, the wonderful difference in this respect between the workersand the perfect females would have been better exemplified by the hive-bee. If a working ant or other neuter insect had been an ordinary animal, Ishould have unhesitatingly assumed that all its characters had been slowlyacquired through natural selection; namely, by individuals having been bornwith slight profitable modifications, which were inherited by theoffspring, and that these again varied and again were selected, and soonwards. But with the working ant we have an insect differing greatly fromits parents, yet absolutely sterile; so that it could never havetransmitted successively acquired modifications of structure or instinct toits progeny. It may well be asked how it is possible to reconcile thiscase with the theory of natural selection?

First, let it be remembered that we have innumerable instances, both in ourdomestic productions and in those in a state of nature, of all sorts ofdifferences of inherited structure which are correlated with certain agesand with either sex. We have differences correlated not only with one sex,but with that short period when the reproductive system is active, as inthe nuptial plumage of many birds, and in the hooked jaws of the malesalmon. We have even slight differences in the horns of different breedsof cattle in relation to an artificially imperfect state of the male sex;for oxen of certain breeds have longer horns than the oxen of other breeds,relatively to the length of the horns in both the bulls and cows of thesesame breeds. Hence, I can see no great difficulty in any characterbecoming correlated with the sterile condition of certain members of insectcommunities; the difficulty lies in understanding how such correlatedmodifications of structure could have been slowly accumulated by naturalselection.

This difficulty, though appearing insuperable, is lessened, or, as Ibelieve, disappears, when it is remembered that selection may be applied tothe family, as well as to the individual, and may thus gain the desiredend. Breeders of cattle wish the flesh and fat to be well marbledtogether. An animal thus characterized has been slaughtered, but thebreeder has gone with confidence to the same stock and has succeeded. Suchfaith may be placed in the power of selection that a breed of cattle,always yielding oxen with extraordinarily long horns, could, it isprobable, be formed by carefully watching which individual bulls and cows,when matched, produced oxen with the longest horns; and yet no one ox wouldever have propagated its kind. Here is a better and real illustration: According to M. Verlot, some varieties of the double annual stock, fromhaving been long and carefully selected to the right degree, always producea large proportion of seedlings bearing double and quite sterile flowers,but they likewise yield some single and fertile plants. These latter, bywhich alone the variety can be propagated, may be compared with the fertilemale and female ants, and the double sterile plants with the neuters of thesame community. As with the varieties of the stock, so with socialinsects, selection has been applied to the family, and not to theindividual, for the sake of gaining a serviceable end. Hence, we mayconclude that slight modifications of structure or of instinct, correlatedwith the sterile condition of certain members of the community, have provedadvantageous; consequently the fertile males and females have flourished,and transmitted to their fertile offspring a tendency to produce sterilemembers with the same modifications. This process must have been repeatedmany times, until that prodigious amount of difference between the fertileand sterile females of the same species has been produced which we see inmany social insects.

But we have not as yet touched on the acme of the difficulty; namely, thefact that the neuters of several ants differ, not only from the fertilefemales and males, but from each other, sometimes to an almost incredibledegree, and are thus divided into two or even three castes. The castes,moreover, do not generally graduate into each other, but are perfectly welldefined; being as distinct from each other as are any two species of thesame genus, or rather as any two genera of the same family. Thus, inEciton, there are working and soldier neuters, with jaws and instinctsextraordinarily different: in Cryptocerus, the workers of one caste alonecarry a wonderful sort of shield on their heads, the use of which is quiteunknown: in the Mexican Myrmecocystus, the workers of one caste neverleave the nest; they are fed by the workers of another caste, and they havean enormously developed abdomen which secretes a sort of honey, supplyingthe place of that excreted by the aphides, or the domestic cattle as theymay be called, which our European ants guard and imprison.

It will indeed be thought that I have an overweening confidence in theprinciple of natural selection, when I do not admit that such wonderful andwell-established facts at once annihilate the theory. In the simpler caseof neuter insects all of one caste, which, as I believe, have been rendereddifferent from the fertile males and females through natural selection, wemay conclude from the analogy of ordinary variations, that the successive,slight, profitable modifications did not first arise in all the neuters inthe same nest, but in some few alone; and that by the survival of thecommunities with females which produced most neuters having theadvantageous modification, all the neuters ultimately came to be thuscharacterized. According to this view we ought occasionally to find in thesame nest neuter-insects, presenting gradations of structure; and this wedo find, even not rarely, considering how few neuter-insects out of Europehave been carefully examined. Mr. F. Smith has shown that the neuters ofseveral British ants differ surprisingly from each other in size andsometimes in colour; and that the extreme forms can be linked together byindividuals taken out of the same nest: I have myself compared perfectgradations of this kind. It sometimes happens that the larger or thesmaller sized workers are the most numerous; or that both large and smallare numerous, while those of an intermediate size are scanty in numbers. Formica flava has larger and smaller workers, with some few of intermediatesize; and, in this species, as Mr. F. Smith has observed, the largerworkers have simple eyes (ocelli), which, though small, can be plainlydistinguished, whereas the smaller workers have their ocelli rudimentary. Having carefully dissected several specimens of these workers, I can affirmthat the eyes are far more rudimentary in the smaller workers than can beaccounted for merely by their proportionately lesser size; and I fullybelieve, though I dare not assert so positively, that the workers ofintermediate size have their ocelli in an exactly intermediate condition. So that here we have two bodies of sterile workers in the same nest,differing not only in size, but in their organs of vision, yet connected bysome few members in an intermediate condition. I may digress by adding,that if the smaller workers had been the most useful to the community, andthose males and females had been continually selected, which produced moreand more of the smaller workers, until all the workers were in thiscondition; we should then have had a species of ant with neuters in nearlythe same condition as those of Myrmica. For the workers of Myrmica havenot even rudiments of ocelli, though the male and female ants of this genushave well-developed ocelli.

I may give one other case: so confidently did I expect occasionally tofind gradations of important structures between the different castes ofneuters in the same species, that I gladly availed myself of Mr. F. Smith'soffer of numerous specimens from the same nest of the driver ant (Anomma)of West Africa. The reader will perhaps best appreciate the amount ofdifference in these workers by my giving, not the actual measurements, buta strictly accurate illustration: the difference was the same as if wewere to see a set of workmen building a house, of whom many were five feetfour inches high, and many sixteen feet high; but we must in additionsuppose that the larger workmen had heads four instead of three times asbig as those of the smaller men, and jaws nearly five times as big. Thejaws, moreover, of the working ants of the several sizes differedwonderfully in shape, and in the form and number of the teeth. But theimportant fact for us is that, though the workers can be grouped intocastes of different sizes, yet they graduate insensibly into each other, asdoes the widely-different structure of their jaws. I speak confidently onthis latter point, as Sir J. Lubbock made drawings for me, with the cameralucida, of the jaws which I dissected from the workers of the severalsizes. Mr. Bates, in his interesting "Naturalist on the Amazons," hasdescribed analogous cases.

With these facts before me, I believe that natural selection, by acting onthe fertile ants or parents, could form a species which should regularlyproduce neuters, all of large size with one form of jaw, or all of smallsize with widely different jaws; or lastly, and this is the greatestdifficulty, one set of workers of one size and structure, andsimultaneously another set of workers of a different size and structure; agraduated series having first been formed, as in the case of the driverant, and then the extreme forms having been produced in greater and greaternumbers, through the survival of the parents which generated them, untilnone with an intermediate structure were produced.

An analogous explanation has been given by Mr. Wallace, of the equallycomplex case, of certain Malayan butterflies regularly appearing under twoor even three distinct female forms; and by Fritz Muller, of certainBrazilian crustaceans likewise appearing under two widely distinct maleforms. But this subject need not here be discussed.

I have now explained how, I believe, the wonderful fact of two distinctlydefined castes of sterile workers existing in the same nest, both widelydifferent from each other and from their parents, has originated. We cansee how useful their production may have been to a social community ofants, on the same principle that the division of labour is useful tocivilised man. Ants, however, work by inherited instincts and by inheritedorgans or tools, while man works by acquired knowledge and manufacturedinstruments. But I must confess, that, with all my faith in naturalselection, I should never have anticipated that this principle could havebeen efficient in so high a degree, had not the case of these neuterinsects led me to this conclusion. I have, therefore, discussed this case,at some little but wholly insufficient length, in order to show the powerof natural selection, and likewise because this is by far the most seriousspecial difficulty which my theory has encountered. The case, also, isvery interesting, as it proves that with animals, as with plants, anyamount of modification may be effected by the accumulation of numerous,slight, spontaneous variations, which are in any way profitable, withoutexercise or habit having been brought into play. For peculiar habits,confined to the workers of sterile females, however long they might befollowed, could not possibly affect the males and fertile females, whichalone leave descendants. I am surprised that no one has advanced thisdemonstrative case of neuter insects, against the well-known doctrine ofinherited habit, as advanced by Lamarck.

SUMMARY.

I have endeavoured in this chapter briefly to show that the mentalqualities of our domestic animals vary, and that the variations areinherited. Still more briefly I have attempted to show that instincts varyslightly in a state of nature. No one will dispute that instincts are ofthe highest importance to each animal. Therefore, there is no realdifficulty, under changing conditions of life, in natural selectionaccumulating to any extent slight modifications of instinct which are inany way useful. In many cases habit or use and disuse have probably comeinto play. I do not pretend that the facts given in this chapterstrengthen in any great degree my theory; but none of the cases ofdifficulty, to the best of my judgment, annihilate it. On the other hand,the fact that instincts are not always absolutely perfect and are liable tomistakes; that no instinct can be shown to have been produced for the goodof other animals, though animals take advantage of the instincts of others;that the canon in natural history, of "Natura non facit saltum," isapplicable to instincts as well as to corporeal structure, and is plainlyexplicable on the foregoing views, but is otherwise inexplicable--all tendto corroborate the theory of natural selection.

This theory is also strengthened by some few other facts in regard toinstincts; as by that common case of closely allied, but distinct, species,when inhabiting distant parts of the world and living under considerablydifferent conditions of life, yet often retaining nearly the sameinstincts. For instance, we can understand, on the principle ofinheritance, how it is that the thrush of tropical South America lines itsnest with mud, in the same peculiar manner as does our British thrush; howit is that the Hornbills of Africa and India have the same extraordinaryinstinct of plastering up and imprisoning the females in a hole in a tree,with only a small hole left in the plaster through which the males feedthem and their young when hatched; how it is that the male wrens(Troglodytes) of North America, build "cock-nests," to roost in, like themales of our Kitty-wrens,--a habit wholly unlike that of any other knownbird. Finally, it may not be a logical deduction, but to my imagination itis far more satisfactory to look at such instincts as the young cuckooejecting its foster-brothers, ants making slaves, the larvae ofichneumonidae feeding within the live bodies of caterpillars, not asspecially endowed or created instincts, but as small consequences of onegeneral law leading to the advancement of all organic beings--namely,multiply, vary, let the strongest live and the weakest die.