CHAPTER VIII.

INSTINCT.

Instincts comparable with habits, but different in their

origin—Instincts graduated—Aphides and ants—Instincts variable—Domestic

instincts, their origin—Natural instincts of the cuckoo, molothrus,

ostrich, and parasitic bees—Slave-making ants—Hive-bee, its cell-making

instinct—Changes of instinct and structure not necessarily

simultaneous—Difficulties of the theory of the Natural Selection of

instincts—Neuter or sterile insects—Summary.

Many instincts are so wonderful that their development will probably

appear to the reader a difficulty sufficient to overthrow my whole

theory. I may here premise, that I have nothing to do with the origin

of the mental powers, any more than I have with that of life itself. We

are concerned only with the diversities of instinct and of the other

mental faculties in animals of the same class.

I will not attempt any definition of instinct. It would be easy to show

that several distinct mental actions are commonly embraced by this

term; but every one understands what is meant, when it is said that

instinct impels the cuckoo to migrate and to lay her eggs in other

birds’ nests. An action, which we ourselves require experience to

enable us to perform, when performed by an animal, more especially by a

very young one, without experience, and when performed by many

individuals in the same way, without their knowing for what purpose it

is performed, is usually said to be instinctive. But I could show that

none of these characters are universal. A little dose of judgment or

reason, as Pierre Huber expresses it, often comes into play, even with

animals low in the scale of nature.

Frederick Cuvier and several of the older metaphysicians have compared

instinct with habit. This comparison gives, I think, an accurate notion

of the frame of mind under which an instinctive action is performed,

but not necessarily of its origin. How unconsciously many habitual

actions are performed, indeed not rarely in direct opposition to our

conscious will! yet they may be modified by the will or reason. Habits

easily become associated with other habits, with certain periods of

time and states of the body. When once acquired, they often remain

constant throughout life. Several other points of resemblance between

instincts and habits could be pointed out. As in repeating a well-known

song, so in instincts, one action follows another by a sort of rhythm;

if a person be interrupted in a song, or in repeating anything by rote,

he is generally forced to go back to recover the habitual train of

thought: so P. Huber found it was with a caterpillar, which makes a

very complicated hammock; for if he took a caterpillar which had

completed its hammock up to, say, the sixth stage of construction, and

put it into a hammock completed up only to the third stage, the

caterpillar simply re-performed the fourth, fifth, and sixth stages of

construction. If, however, a caterpillar were taken out of a hammock

made up, for instance, to the third stage, and were put into one

finished up to the sixth stage, so that much of its work was already

done for it, far from deriving any benefit from this, it was much

embarrassed, and, in order to complete its hammock, seemed forced to

start from the third stage, where it had left off, and thus tried to

complete the already finished work.

If we suppose any habitual action to become inherited—and it can be

shown that this does sometimes happen—then the resemblance between what

originally was a habit and an instinct becomes so close as not to be

distinguished. If Mozart, instead of playing the pianoforte at three

years old with wonderfully little practice, had played a tune with no

practice at all, be might truly be said to have done so instinctively.

But it would be a serious error to suppose that the greater number of

instincts have been acquired by habit in one generation, and then

transmitted by inheritance to succeeding generations. It can be clearly

shown that the most wonderful instincts with which we are acquainted,

namely, those of the hive-bee and of many ants, could not possibly have

been acquired by habit.

It will be universally admitted that instincts are as important as

corporeal structures for the welfare of each species, under its present

conditions of life. Under changed conditions of life, it is at least

possible that slight modifications of instinct might be profitable to a

species; and if it can be shown that instincts do vary ever so little,

then I can see no difficulty in natural selection preserving and

continually accumulating variations of instinct to any extent that was

profitable. It is thus, as I believe, that all the most complex and

wonderful instincts have originated. As modifications of corporeal

structure arise from, and are increased by, use or habit, and are

diminished or lost by disuse, so I do not doubt it has been with

instincts. But I believe that the effects of habit are in many cases of

subordinate importance to the effects of the natural selection of what

may be called spontaneous variations of instincts;—that is of

variations produced by the same unknown causes which produce 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, yet

profitable, variations. Hence, as in the case of corporeal structures,

we ought to find in nature, not the actual transitional gradations by

which each complex instinct has been acquired—for these could be found

only in the lineal ancestors of each species—but we ought to find in

the collateral lines of descent some evidence of such gradations; or we

ought at least to be able to show that gradations of some kind are

possible; and this we certainly can do. I have been surprised to find,

making allowance for the instincts of animals having been but little

observed, except in Europe and North America, and for no instinct being

known among extinct species, how very generally gradations, leading to

the most complex instincts, can be discovered. Changes of instinct may

sometimes be facilitated by the same species having different instincts

at different periods of life, or at different seasons of the year, or

when placed under different circumstances, &c.; in which case either

the one or the other instinct might be preserved by natural selection.

And such instances of diversity 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 as we can judge, been produced for the exclusive good of others.

One of the strongest instances of an animal apparently performing an

action for the sole good of another, with which I am acquainted, is

that of aphides voluntarily yielding, as was first observed by Huber,

their sweet excretion to ants: that they do so voluntarily, the

following facts show. I removed all the ants from a group of about a

dozen aphides on a dock-plant, and prevented their attendance during

several hours. After this interval, I felt sure that the aphides would

want to excrete. I watched them for some time through a lens, but not

one excreted; I then tickled and stroked them with a hair in the same

manner, as well as I could, as the ants do with their antennæ; but not

one excreted. Afterwards, I allowed an ant to visit them, and it

immediately seemed, by its eager way of running about to be well aware

what a rich flock it had discovered; it then began to play with its

antennæ on the abdomen first of one aphis and then of another; and

each, as soon as it felt the antennæ, immediately lifted up its abdomen

and excreted a limpid drop of sweet juice, which was eagerly devoured

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 no dislike to the ants: if the latter be not present they

are at last compelled to eject their excretion. But as the excretion is

extremely viscid, 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 the exclusive good of another species, yet each tries to

take advantage of the instincts of others, as each takes advantage of

the weaker bodily structure of other species. So again certain

instincts cannot be considered as absolutely perfect; but as details on

this and other such points are not indispensable, they may be here

passed over.

As some degree of variation in instincts under a state of nature, and

the inheritance of such variations, are indispensable for the action of

natural selection, as many instances as possible ought to be given; but

want of space prevents me. I can only assert that instincts certainly

do vary—for instance, the migratory instinct, both in extent and

direction, and in its total loss. So it is with the nests of birds,

which vary partly in dependence on the situations chosen, and on the

nature and temperature of the country inhabited, but often from causes

wholly unknown to us. Audubon has given several remarkable cases of

differences in the nests of the same species in the northern and

southern United States. Why, it has been asked, if instinct be

variable, has it not granted to the bee “the ability to use some other

material when wax was deficient?” But what other natural material could

bees use? They will work, as I have seen, with wax hardened with

vermilion or softened with lard. Andrew Knight observed that his bees,

instead of laboriously collecting propolis, used a cement of wax and

turpentine, with which he had covered decorticated trees. It has lately

been shown that bees, instead of searching for pollen, will gladly use

a very different substance, namely, oatmeal. Fear of any particular

enemy is certainly an instinctive quality, as may be seen in nestling

birds, though it is strengthened by experience, and by the sight of

fear of the same enemy in other animals. The fear of man is slowly

acquired, as I have elsewhere shown, by the various animals which

inhabit desert islands; and we see an instance of this, even in

England, in the greater wildness of all our large birds in comparison

with our small birds; for the large birds have been most persecuted by

man. We may safely attribute the greater wildness of our large birds to

this cause; for in uninhabited islands large birds 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

of nature, vary much, could be shown by many facts. Several cases could

also be adduced of occasional and strange habits in wild animals,

which, if advantageous to the species, might have given rise, through

natural selection, to new instincts. But I am well aware that these

general statements, without the facts in detail, can produce but a

feeble effect on the reader’s mind. I can only repeat my assurance,

that I do not speak without good evidence.

_Inherited Changes of Habit or Instinct in Domesticated Animals._

The possibility, or even probability, of inherited variations of

instinct in a state of nature will be strengthened by briefly

considering a few cases under domestication. We shall thus be enabled

to see the part which habit and the selection of so-called spontaneous

variations have played in modifying the mental qualities of our

domestic animals. It is notorious how much domestic animals vary in

their mental qualities. With cats, for instance, one naturally takes to

catching rats, and another mice, and these tendencies are known to be

inherited. One cat, according to Mr. St. John, always brought home game

birds, another hares or rabbits, and another hunted on marshy ground

and almost nightly caught woodcocks or snipes. A number of curious and

authentic instances could be given of various shades of disposition and

taste, and likewise of the oddest tricks, associated with certain

frames of mind or periods of time. But let us look to the familiar case

of the breeds of dogs: it cannot be doubted that young pointers (I have

myself seen striking instances) will sometimes point and even back

other dogs the very first time that they are taken out; retrieving is

certainly in some degree inherited by retrievers; and a tendency 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 eager delight

by each breed, and without the end being known—for the young pointer

can no more know that he points to aid his master, than the white

butterfly knows why she lays her eggs on the leaf of the cabbage—I

cannot see that these actions differ essentially from true instincts.

If we were to behold one kind of wolf, when young and without any

training, as soon as it scented its prey, stand motionless like a

statue, and then slowly crawl forward 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, we should assuredly call these actions

instinctive. Domestic instincts, as they may be called, are certainly

far less fixed than natural instincts; but they have been acted on by

far less rigorous selection, and have been transmitted for an

incomparably shorter period, under less fixed conditions of life.

How strongly these domestic instincts, habits, and dispositions are

inherited, and how curiously they become mingled, is well shown when

different breeds of dogs are crossed. Thus it is known that a cross

with a bull-dog has affected for many generations the courage and

obstinacy of greyhounds; and a cross with a greyhound has given to a

whole family of shepherd-dogs a tendency to hunt hares. These domestic

instincts, when thus tested by crossing, resemble natural instincts,

which in a like manner become curiously blended together, and for a

long period exhibit traces of the instincts of either parent: for

example, Le Roy describes a dog, whose great-grandfather was a wolf,

and this dog showed a trace of its wild parentage only in one way, by

not coming in a straight line to his master, when called.

Domestic instincts are sometimes spoken of as actions which have become

inherited solely from long-continued and compulsory habit, but this is

not true. No one would ever have thought of teaching, or probably could

have taught, 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 may believe that some one pigeon showed a slight tendency to

this strange habit, and that the long-continued selection of the best

individuals in successive generations made tumblers what they now are;

and near Glasgow there are house-tumblers, as I hear from Mr. Brent,

which cannot fly eighteen inches high without going head over heels. It

may be doubted whether any one would have thought of training a dog to

point, had not some one dog naturally shown a tendency in this line;

and this is known occasionally to happen, as I once saw, in a pure

terrier: the act of pointing is probably, as many have thought, only

the exaggerated pause of an animal preparing to spring on its prey.

When the first tendency to point was once displayed, methodical

selection and the inherited effects of compulsory training in each

successive generation would soon complete the work; and unconscious

selection is still in progress, as each man tries to procure, without

intending to improve the breed, dogs which stand and hunt best. On the

other hand, habit alone in some cases has sufficed; hardly any 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 can hardly

suppose that domestic rabbits have often been selected for tameness

alone; so that we must attribute at least the greater part of the

inherited change from extreme wildness to extreme tameness, to habit

and long-continued close confinement.

Natural instincts are lost under domestication: a remarkable instance

of this is seen in those breeds of fowls which very rarely or never

become “broody,” that is, never wish to sit on their eggs. Familiarity

alone prevents our seeing how largely and how permanently the minds of

our domestic animals have been modified. It is scarcely possible to

doubt that the 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 to attack poultry, sheep and pigs; and this tendency has

been found incurable in dogs which have been brought home as puppies

from countries such as Tierra del Fuego and Australia, where the

savages do not keep these domestic 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 then beaten; and if not cured,

they are destroyed; so that habit and some degree of selection have

probably concurred in civilising by inheritance our dogs. On the other

hand, young chickens have lost wholly by habit, that fear of the dog

and cat which no doubt was originally instinctive in them, for I am

informed by Captain Hutton that the young chickens of the parent stock,

the Gallus bankiva, when reared in India under a hen, are at first

excessively wild. So it is with young pheasants reared in England under

a hen. It is not that chickens have lost all fear, but fear only of

dogs and cats, for if the hen gives the danger chuckle they will run

(more especially young turkeys) from under her and conceal themselves

in the surrounding grass or thickets; and this is evidently done for

the instinctive purpose of allowing, as we see in wild ground-birds,

their mother to fly away. But this instinct retained by our chickens

has become useless under domestication, for the mother-hen has almost

lost by disuse the power of flight.

Hence, we may conclude that under domestication instincts have been

acquired and natural instincts have been lost, partly by habit and

partly by man selecting and accumulating, during successive

generations, peculiar mental habits and actions, which at first

appeared from what we must in our ignorance call an accident. In some

cases compulsory habit alone has sufficed to produce inherited mental

changes; in other cases compulsory habit has done nothing, and all has

been the result of selection, pursued both methodically and

unconsciously; but in most cases habit and selection have probably

concurred.

_Special Instincts._

We shall, perhaps, best understand how instincts in a state of nature

have become modified by selection by considering a few cases. I will

select only three, namely, the instinct which leads the cuckoo to lay

her eggs in other birds’ nests; the slave-making instinct of certain

ants; and the cell-making power of the hive-bee: these two latter

instincts have generally and justly been ranked by naturalists as the

most wonderful of all known instincts.

_Instincts of the Cuckoo._—It is supposed by some naturalists that the

more immediate cause of the instinct of the cuckoo is that she lays her

eggs, not daily, but at intervals of two or three days; so that, if she

were to make her own nest and sit on her own eggs, those first laid

would have to be left for some time unincubated or there would be eggs

and young birds of different ages in the same nest. If this were the

case the process of laying and hatching might be inconveniently long,

more especially as she migrates at a very early period; and the first

hatched young would probably have to be fed by the male alone. But the

American cuckoo is in this predicament, for she makes her own nest and

has eggs and young successively hatched, all at the same time. It has

been both asserted and denied that the American cuckoo occasionally

lays her eggs in other birds’ nests; but I have lately heard from 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 their identification. I could also give several instances of

various birds which have been known occasionally to lay their eggs in

other birds’ nests. Now let us suppose that the ancient progenitor of

our European cuckoo had the habits of the American cuckoo, and that she

occasionally laid an egg in another bird’s nest. If the old bird

profited by this occasional habit through being enabled to emigrate

earlier or through any other cause; or if the young were made more

vigorous by advantage being taken of the mistaken instinct of another

species than when reared by their own mother, encumbered as she could

hardly fail to be by having eggs and young of different ages at the

same time, then the old birds or the fostered young would gain an

advantage. And analogy would lead us to believe, that the young thus

reared would be apt to follow by inheritance the occasional and

aberrant habit of their mother, and in their turn would be apt to lay

their eggs in other birds’ nests, and thus be more successful in

rearing their young. By a continued process of this nature, I believe

that the strange instinct of our cuckoo has been generated. It has,

also recently been ascertained on sufficient evidence, by Adolf Müller,

that the cuckoo occasionally lays her eggs on the bare ground, sits on

them and feeds her young. 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

and adaptations of structure in the cuckoo, which are spoken of as

necessarily co-ordinated. But in all cases, speculation on an instinct

known to us only in a single species, is useless, for we have hitherto

had no facts to guide us. Until recently the instincts of the European

and of the non-parasitic American cuckoo alone were known; now, owing

to Mr. Ramsay’s observations, we have learned something about three

Australian species, which lay their eggs in other birds’ nests. The

chief points to be referred to are three: first, that the common

cuckoo, with rare exceptions, lays only one egg in a nest, so that the

large and voracious young bird receives ample food. Secondly, that the

eggs are remarkably small, not exceeding those of the skylark—a bird

about one-fourth as large as the cuckoo. That the small size of the egg

is a real case of adaptation we may infer from the fact of the

mon-parasitic American cuckoo laying full-sized eggs. Thirdly, that the

young cuckoo, soon after birth, has the instinct, the strength and a

properly shaped back for ejecting its foster-brothers, which then

perish from cold and hunger. This has been boldly called a beneficent

arrangement, in order that the young cuckoo may get sufficient food,

and that its foster-brothers may perish before they had acquired much

feeling!

Turning now to the Australian species: though these birds generally lay

only one egg in a nest, it is not rare to find two and even three eggs

in the same nest. In the bronze cuckoo the eggs vary greatly in size,

from eight to ten lines in length. Now, if it had been of an advantage

to this species to have laid eggs even smaller than those now laid, so

as to have deceived certain foster-parents, or, as is more probable, to

have been hatched within a shorter period (for it is asserted that

there is a relation between the size of eggs and the period of their

incubation), then there is no difficulty in believing that a race or

species might have been formed which would have laid smaller and

smaller eggs; for these would have been more safely hatched and reared.

Mr. Ramsay remarks that two of the Australian cuckoos, when they lay

their eggs in an open nest, manifest a decided preference for nests

containing eggs similar in colour to their own. The European species

apparently manifests some tendency towards a similar instinct, but not

rarely departs from it, as is shown by her laying her dull and

pale-coloured eggs in the nest of the hedge-warbler with bright

greenish-blue eggs. Had our cuckoo invariably displayed the above

instinct, it would assuredly have been added to those which it is

assumed must all have been acquired together. The eggs of the

Australian bronze cuckoo vary, according to Mr. Ramsay, to an

extraordinary degree in colour; so that in this respect, as well as in

size, natural selection might have secured and fixed any advantageous

variation.

In the case of the European cuckoo, the offspring of the foster-parents

are commonly ejected from the nest within three days after the cuckoo

is hatched; 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 was performed by the foster-parents themselves. But he has now

received a trustworthy account of a young cuckoo which was actually

seen, while still blind and not able even to hold up its own head, in

the act of ejecting its foster-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 strange and odious instinct was acquired, if it

were of great importance for the young cuckoo, as is probably the case,

to receive as much food as possible soon after birth, I can see no

special difficulty in its having gradually acquired, during successive

generations, the blind desire, the strength, and structure necessary

for the work of ejection; for those cuckoos which had such habits and

structure best developed would be the most securely reared. The first

step towards the acquisition of the proper instinct might have been

mere unintentional restlessness on the part of the young bird, when

somewhat advanced in age and strength; the habit having been afterwards

improved, and transmitted to an earlier age. I can see no more

difficulty in this than in the unhatched young of other birds acquiring

the instinct to break through their own shells; or than in young snakes

acquiring in their upper jaws, as Owen has remarked, a transitory sharp

tooth for cutting through the tough egg-shell. For if each part is

liable to individual variations at all ages, and the variations tend to

be inherited at a corresponding or earlier age—propositions which

cannot be disputed—then the instincts and structure of the young could

be slowly modified as surely as those of the adult; and both cases must

stand or fall together 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 their instincts. The sexes of Molothrus badius are stated

by an excellent observer, Mr. Hudson, sometimes to live promiscuously

together in flocks, and sometimes to pair. They either build a nest of

their own or seize on one belonging to some other bird, occasionally

throwing out the nestlings of 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 usually sit on their own eggs and rear their own

young; but Mr. Hudson says it is probable that they are occasionally

parasitic, for he has seen the young of this species following old

birds of a distinct kind and clamouring to be fed by them. The

parasitic habits of another species of Molothrus, the M. bonariensis,

are much more highly developed than those of the last, but are still

far from perfect. This bird, as far as it is known, invariably lays its

eggs in the nests of strangers; but it is remarkable that several

together sometimes commence to build an irregular untidy nest of their

own, placed in singular ill-adapted situations, as on the leaves of a

large thistle. They never, however, as far as Mr. Hudson has

ascertained, complete a nest for themselves. They often lay so many

eggs—from fifteen to twenty—in the same foster-nest, that few or none

can possibly be hatched. They have, moreover, the extraordinary habit

of pecking holes in the eggs, whether of their own species or of their

foster parents, which they find in the appropriated nests. They drop

also many eggs on the bare ground, which are thus wasted. A third

species, the M. pecoris of North America, has acquired instincts as

perfect as those of the cuckoo, for it never lays more than one egg in

a foster-nest, so that the young bird is securely reared. Mr. Hudson is

a strong disbeliever in evolution, but he appears to have been so much

struck by the imperfect instincts of the Molothrus bonariensis that he

quotes my words, and asks, “Must we consider these habits, not as

especially endowed or created instincts, but as small consequences of

one general law, namely, transition?”

Various birds, as has already been remarked, occasionally lay their

eggs in the nests of other birds. This habit is not very uncommon with

the Gallinaceæ, 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 nest and then in another; and these are hatched by the

males. This instinct may probably be accounted for by the fact of the

hens laying a large number of eggs, but, as with the cuckoo, at

intervals of two or three days. The instinct, however, of the American

ostrich, as in the case of the Molothrus bonariensis, has not as yet

been perfected; for a surprising number of eggs lie strewed over the

plains, so that in one day’s hunting I picked up no less than twenty

lost and wasted eggs.

Many bees are parasitic, and regularly lay their eggs in the nests of

other kinds of bees. This case is more remarkable than that of the

cuckoo; for these bees have not only had their instincts but their

structure modified in accordance with their parasitic habits; for they

do not possess the pollen-collecting apparatus which would have been

indispensable if they had stored up food for their own young. Some

species of Sphegidæ (wasp-like insects) are likewise parasitic; and M.

Fabre has lately shown good reason for believing that, although the

Tachytes nigra generally makes its own burrow and stores it with

paralysed prey for its own larvæ, yet that, when this insect finds a

burrow already made and stored by another sphex, it takes advantage of

the prize, and becomes for the occasion parasitic. In this case, as

with that of the Molothrus or cuckoo, I can see no difficulty in

natural selection making an occasional habit permanent, if of advantage

to the species, and if the insect whose nest and stored food are

feloniously 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 celebrated father. This ant is absolutely dependent on

its slaves; without their aid, the species would certainly become

extinct in a single year. The males and fertile 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 own larvæ. When the old

nest is found inconvenient, and they have to migrate, it is the slaves

which determine the migration, and actually carry their masters in

their jaws. So utterly helpless are the masters, that when Huber shut

up thirty of them without a slave, but with plenty of the food which

they like best, and with their larvæ and pupæ to stimulate them to

work, they did nothing; they could not even feed themselves, and many

perished of hunger. Huber then introduced a single slave (F. fusca),

and she instantly set to work, fed and saved the survivors; made some

cells and tended the larvæ, and put all to rights. What can be more

extraordinary than these well-ascertained facts? If we had not known of

any other slave-making ant, it would have been hopeless to speculate

how so wonderful an 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 southern

parts of England, and its habits have been attended to by Mr. F. Smith,

of the British Museum, to whom I am much indebted for information on

this and other 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 any one may well be excused for doubting the

existence of so extraordinary an instinct as that of making slaves.

Hence, I will give the observations which 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 the slave-species (F. fusca) are found

only in their own proper communities, and have never been observed in

the nests of F. sanguinea. The slaves are black and not above half the

size of their red masters, so that the contrast in their appearance is

great. When the nest is slightly disturbed, the slaves occasionally

come out, and like their masters are much agitated and defend the nest:

when the nest is much disturbed, and the larvæ and pupæ are exposed,

the slaves work energetically together with their masters in carrying

them away to a place of safety. Hence, it is clear that the slaves feel

quite at home. During the months of June and July, on three successive

years, I watched for many hours several nests in Surrey 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 they might

behave differently when more numerous; but Mr. Smith informs me that he

has watched the nests at various hours during May, June and August,

both in Surrey and Hampshire, and has never seen the slaves, though

present in large numbers in August, either leave or enter the nest.

Hence, he considers them as strictly household slaves. The masters, on

the other hand, may be constantly seen bringing in materials for the

nest, and food of all kinds. During the year 1860, however, in the

month of July, I came across a community with an unusually large stock

of slaves, and I observed a few slaves mingled with their masters

leaving the nest, and marching along 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 slaves in Switzerland habitually

work with their masters in making the nest, and they alone open and

close the doors in the morning and evening; and, as Huber expressly

states, their principal office is to search for aphides. This

difference in the usual habits of the masters and slaves in the two

countries, probably depends merely on the slaves being captured in

greater numbers in Switzerland than in England.

One day I fortunately witnessed a migration of F. sanguinea from one

nest to another, and it was a most interesting spectacle to behold the

masters carefully carrying their slaves in their jaws instead of being

carried by them, as in the case of F. rufescens. Another day my

attention was struck by about a score of the slave-makers haunting the

same spot, and evidently not in search of food; they approached and

were vigorously repulsed by an independent community of the slave

species (F. fusca); sometimes as many as three of these ants clinging

to the legs of the slave-making F. sanguinea. The latter ruthlessly

killed their small opponents and carried their dead bodies as food to

their nest, twenty-nine yards distant; but they were prevented from

getting any pupæ to rear as slaves. I then dug up a small parcel of the

pupæ of F. fusca from another nest, and put them down on a bare spot

near the place of combat; they were eagerly seized 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 pupæ of

another species, F. flava, with a few of these little yellow ants still

clinging to the fragments of their nest. This species is sometimes,

though rarely, made into slaves, as has been described by Mr. Smith.

Although so small a species, it is very courageous, and I have seen it

ferociously attack other ants. In one instance I found to my surprise

an independent community of F. flava under a stone beneath a nest of

the slave-making F. sanguinea; and when I had accidentally disturbed

both nests, the little ants attacked their big neighbours with

surprising courage. Now I was curious to ascertain whether F. sanguinea

could distinguish the pupæ of F. fusca, which they habitually make into

slaves, from those of the little and furious F. flava, which they

rarely capture, and it was evident that they did at once distinguish

them; for we have seen that they eagerly and instantly seized the pupæ

of F. fusca, whereas they were much terrified when they came across the

pupæ, or even the earth from the nest, of F. flava, and quickly ran

away; but in about a quarter of an hour, shortly after all the little

yellow ants had crawled away, they took heart and carried off the pupæ.

One evening I visited another community of F. sanguinea, and found a

number of these ants returning home and entering their nests, carrying

the dead bodies of F. fusca (showing that it was not a migration) and

numerous pupæ. I traced a long file of ants burthened with booty, for

about forty yards back, 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 the desolated nest in the thick heath. The nest,

however, must have been close at hand, for two or three individuals of

F. fusca were rushing about in the greatest agitation, and one was

perched motionless with its own pupa in its mouth on the top of a spray

of heath, an image of despair over its ravaged home.

Such are the facts, though they did not need confirmation by me, in

regard to the wonderful instinct of making slaves. Let it be observed

what a contrast the instinctive habits of F. sanguinea present with

those of the continental F. rufescens. The latter does not build its

own nest, does not determine its own migrations, does not collect food

for itself or its young, and cannot even feed itself: it is absolutely

dependent on its numerous slaves. Formica sanguinea, on the other hand,

possesses much fewer slaves, and in the early part of the summer

extremely few. The masters determine when and where a new nest shall be

formed, and when they migrate, the masters carry the slaves. Both in

Switzerland and England the slaves seem to have the exclusive care of

the larvæ, and the masters alone go on slave-making expeditions. In

Switzerland the slaves and masters work together, making and bringing

materials for the nest: both, but chiefly the slaves, tend and milk as

it may be called, their aphides; and thus both collect food for the

community. In England the masters alone usually leave the nest to

collect building materials and food for themselves, their slaves and

larvæ. So that the masters in this country receive much less service

from their slaves than they do in Switzerland.

By what steps the instinct of F. sanguinea originated I will not

pretend to conjecture. But as ants which are not slave-makers, will, as

I have seen, carry off pupæ of other species, if scattered near their

nests, it is possible that such pupæ originally stored as food might

become developed; and the foreign ants thus unintentionally reared

would then follow their proper instincts, and do what work they could.

If their presence proved useful to the species which had seized them—if

it were more advantageous to this species, to capture workers than to

procreate them—the habit of collecting pupæ, originally for food, might

by natural selection be strengthened and rendered permanent for the

very different purpose of raising slaves. When the instinct was once

acquired, if carried out to a much less extent even than in our British

F. sanguinea, which, as we have seen, is less aided by its slaves than

the same species in Switzerland, natural selection might increase and

modify the instinct—always supposing each modification to be of use to

the species—until an ant was formed as abjectly 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 merely give an outline of the

conclusions at which I have arrived. He must be a dull man who can

examine the exquisite structure of a comb, so beautifully adapted to

its end, without enthusiastic admiration. We hear from mathematicians

that bees have practically solved a recondite problem, and have made

their cells of the proper shape to hold the greatest possible amount of

honey, with the least possible consumption of precious wax in their

construction. It has been remarked that a skilful workman, with fitting

tools and measures, would find it very difficult to make cells of wax

of the true form, though this is effected by a crowd of bees working in

a dark hive. Granting whatever instincts you please, it seems at first

quite inconceivable how they can make all the necessary angles and

planes, or even perceive when they are correctly made. But the

difficulty is not nearly 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

that the form of the cell stands in close relation to the presence of

adjoining cells; and the following view may, perhaps, be considered

only as a modification of his theory. Let us look to the great

principle of gradation, 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 old cocoons to hold honey, sometimes adding to them

short tubes of wax, and likewise making separate and very irregular

rounded cells of wax. At the other end of the series we have the cells

of the hive-bee, placed in a double layer: each cell, as is well known,

is an hexagonal prism, with the basal edges of its six sides bevelled

so as to join an inverted pyramid, of three rhombs. These rhombs have

certain angles, and the three which form the pyramidal base of a single

cell on one side of the comb, enter into the composition of the bases

of three adjoining cells on the opposite side. In the series between

the extreme perfection of the cells of the hive-bee and the simplicity

of those of the humble-bee, we have the cells of the Mexican Melipona

domestica, carefully described and figured by Pierre Huber. The

Melipona itself is intermediate in structure between the hive and

humble bee, but more nearly related to the latter: it forms a nearly

regular waxen comb of cylindrical cells, in which the young are

hatched, and, in addition, some large cells of wax for holding honey.

These latter cells are nearly spherical and of nearly equal sizes, and

are aggregated into an irregular mass. But the important point to

notice is, that these cells are always made at that degree of nearness

to each other that they would have intersected or broken into each

other if the spheres had been completed; but this is never permitted,

the bees building perfectly flat walls of wax between the spheres which

thus tend to intersect. Hence, each cell consists of an outer spherical

portion, and of two, three, or more flat surfaces, according as the

cell adjoins two, three or more other cells. When one cell rests on

three other cells, which, from the spheres being nearly of the same

size, is very frequently and necessarily the case, the three flat

surfaces are united into a pyramid; and this pyramid, as Huber has

remarked, is manifestly a gross imitation of the three-sided pyramidal

base of the cell of the hive-bee. As in the cells of the hive-bee, so

here, the three plane surfaces in any one cell necessarily enter into

the construction of three adjoining cells. It is obvious that the

Melipona saves 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 of the same thickness as the outer spherical portions,

and yet each flat portion forms a part of two cells.

Reflecting on this case, it occurred to me that if the Melipona had

made its spheres at some given distance from each other, and had made

them of equal sizes and had arranged them symmetrically in a double

layer, the resulting structure would have been as perfect as the comb

of the hive-bee. Accordingly I wrote to Professor Miller, of Cambridge,

and this geometer has kindly read over the following statement, drawn

up from his information, and tells me that it is strictly correct:—

If a number of equal spheres be described with their centres placed in

two parallel layers; with the centre of each sphere at the distance of

radius x sqrt(2) or radius x 1.41421 (or at some lesser distance), from

the centres of the six surrounding spheres in the same layer; and at

the same distance from the centres of the adjoining spheres in the

other and parallel layer; then, if planes of intersection between the

several spheres in both layers be formed, there will result a double

layer of hexagonal prisms united together by pyramidal bases formed of

three rhombs; and the rhombs and the sides of the hexagonal prisms will

have every angle identically the same with the best measurements which

have been made of the cells of the hive-bee. But I hear from Professor

Wyman, who has made numerous careful measurements, that the accuracy of

the workmanship of the bee has been greatly exaggerated; so much so,

that whatever the typical form of the cell may be, it is rarely, if

ever, realised.

Hence we may safely conclude that, if we could slightly modify the

instincts already possessed by the Melipona, and in themselves not very

wonderful, this bee would make a structure as wonderfully perfect as

that of the hive-bee. We must suppose the Melipona to have the power of

forming her cells truly spherical, and of equal sizes; and this would

not be very surprising, seeing that she already does so to a certain

extent, and seeing what perfectly cylindrical burrows many insects make

in wood, apparently by turning round on a fixed point. We must suppose

the Melipona to arrange her cells in level layers, as she already does

her cylindrical cells; and we must further suppose, and this is the

greatest difficulty, that she can somehow judge accurately at what

distance to stand from her fellow-labourers when several are making

their spheres; but she is already so far enabled to judge of distance,

that she always describes her spheres so as to intersect to a certain

extent; and then she unites the points of intersection by perfectly

flat surfaces. By such modifications of instincts which in themselves

are not very wonderful—hardly more wonderful than those which guide a

bird to make its nest—I believe that the hive-bee has acquired, through

natural selection, her inimitable architectural powers.

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

minute circular pits in it; and as they deepened these little pits,

they made them wider and wider until they were converted into shallow

basins, appearing to the eye perfectly true or parts of a sphere, and

of about the diameter of a cell. It was most interesting to observe

that, wherever several bees had begun to excavate these basins near

together, they had begun their work at such a distance from each other

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 about one 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 soon as this

occurred, the bees ceased to excavate, and began to build up flat walls

of wax on the lines of intersection between the basins, so that each

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 of ordinary cells.

I then put into the hive, instead of a thick, rectangular piece of wax,

a thin and narrow, knife-edged ridge, coloured with vermilion. The bees

instantly 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 the bottoms of the basins, if they had been excavated to the same

depth as in the former experiment, would have broken into each other

from the opposite sides. The bees, however, did not suffer this to

happen, and they stopped their excavations in due time; so that the

basins, as soon as they had been a little deepened, came to have flat

bases; and these flat bases, formed by thin little plates of the

vermilion wax left ungnawed, were situated, as far as the eye could

judge, exactly along the planes of imaginary intersection between the

basins on the opposite side of the ridge of wax. In some parts, only

small portions, in other parts, large portions of a rhombic plate were

thus left between the opposed basins, but the work, from the unnatural

state of things, had not been neatly performed. The bees must have

worked at very nearly the same rate in circularly gnawing away and

deepening the basins on both sides of the ridge of vermilion wax, in

order to have thus succeeded in leaving flat plates between the basins,

by stopping work at the planes of intersection.

Considering how flexible thin wax is, I do not see that there is any

difficulty 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, and then stopping their work. In ordinary combs it has

appeared to me that the bees do not always succeed in working at

exactly the same rate from the opposite sides; for I have noticed

half-completed rhombs at the base of a just-commenced cell, which were

slightly concave on one side, where I suppose that the bees had

excavated too quickly, and convex on the opposed side 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 working for a short

time, and again examined the cell, and I found that the rhombic plate

had been completed, and had become _perfectly flat:_ it was absolutely

impossible, from the extreme thinness of the little plate, that they

could have effected this by gnawing away the convex side; and I suspect

that the bees in such cases stand in the opposed cells and push and

bend the ductile and warm wax (which as I have tried is easily done)

into its proper intermediate plane, and thus flatten it.

From the experiment of the ridge of vermilion wax we can see that, if

the bees were to build for themselves a thin wall of wax, they could

make their cells of the proper shape, by standing at the proper

distance from each other, by excavating at the same rate, and by

endeavouring to make equal spherical 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 they gnaw this away from the

opposite sides, always working circularly as they deepen each cell.

They do not make the whole three-sided pyramidal base of any one cell

at the same time, but only that one rhombic plate which stands on the

extreme growing margin, or the two plates, as the case may be; and they

never complete the upper edges of the rhombic plates, until the

hexagonal walls are commenced. Some of these statements differ from

those made by the justly celebrated elder Huber, but I am convinced of

their accuracy; and if I had space, I could show that they are

conformable with my theory.

Huber’s statement, that the very first cell is excavated out of a

little parallel-sided wall of wax, is not, as far as I have seen,

strictly correct; the first commencement having always been a little

hood of wax; but I will not here enter on details. We see how important

a part excavation plays in the construction of the cells; but it would

be a great error to suppose that the bees cannot build up a rough wall

of wax in the proper position—that is, along the plane of intersection

between two adjoining spheres. I have several specimens showing clearly

that they can do this. Even in the rude circumferential rim or wall of

wax round a growing comb, flexures may sometimes be observed,

corresponding in position to the planes of the rhombic basal plates of

future cells. But the rough wall of wax has in every case to be

finished off, by being largely gnawed away on both sides. The manner in

which the bees build is curious; they always make the first rough wall

from ten to twenty times thicker than the excessively thin finished

wall of the cell, which will ultimately be left. We shall understand

how they work, by supposing masons first to pile up a broad ridge of

cement, and then to begin cutting it away equally on both sides 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 on

the summit of the ridge. We shall thus have a thin wall steadily

growing upward but always crowned by a gigantic coping. From all the

cells, both those just commenced and those completed, being thus

crowned by a strong coping of wax, the bees can cluster and crawl over

the comb without injuring the delicate hexagonal walls. These walls, as

Professor Miller has kindly ascertained for me, vary greatly in

thickness; being, on an average of twelve measurements made near the

border of the comb, 1/353 of an inch in thickness; whereas the basal

rhomboidal plates are thicker, nearly in the proportion of three to

two, having a mean thickness, from twenty-one measurements, of 1/229 of

an inch. By the above singular manner of building, strength is

continually given to the comb, with the utmost ultimate economy of wax.

It seems at first to add to the difficulty of understanding how the

cells are made, that a multitude of bees all work together; one bee

after working a short time at one cell going to another, so that, as

Huber has stated, a score of individuals work even at the commencement

of the first cell. I was able practically to show this fact, by

covering the edges of the hexagonal walls of a single cell, or the

extreme margin of the circumferential rim of a growing comb, with an

extremely thin layer of melted vermilion wax; and I invariably found

that the colour was most delicately diffused by the bees—as delicately

as a painter could have done it with his brush—by atoms of the coloured

wax having been taken from the spot on which it had been placed, and

worked into the growing edges of the cells all round. The work of

construction seems to be a sort of balance struck between many bees,

all instinctively standing at the same relative distance from each

other, all trying to sweep equal spheres, and then building up, or

leaving ungnawed, the planes of intersection between these spheres. It

was really curious to note in cases of difficulty, as when two pieces

of comb met at an angle, how often the bees would pull down and rebuild

in different ways the same cell, sometimes recurring to a shape which

they had at first rejected.

When bees have a place on which they can stand in their proper

positions for working—for instance, on a slip of wood, placed directly

under the middle of a comb growing downwards, so that the comb has to

be built over one face of the slip—in this case the bees can lay the

foundations of one wall of a new hexagon, in its strictly proper place,

projecting beyond the other completed cells. It suffices that the bees

should be enabled to stand at their proper relative distances from each

other and from the walls of the last completed cells, and then, by

striking imaginary spheres, they can build up a wall intermediate

between two adjoining spheres; but, as far as I have seen, they never

gnaw away and finish off the angles of a cell till a large part both of

that cell and of the adjoining cells has been built. This capacity in

bees of laying down under certain circumstances a rough wall in its

proper place between two just-commenced cells, is important, as it

bears on a fact, which seems at first subversive of the foregoing

theory; namely, that the cells on the extreme margin of wasp-combs are

sometimes strictly hexagonal; but I have not space here to enter on

this subject. Nor does there seem to me any great difficulty in a

single insect (as in the case of a queen-wasp) making hexagonal cells,

if she were to work alternately on the inside and outside of two or

three cells commenced at the same time, always standing at the proper

relative distance from the parts of the cells just begun, sweeping

spheres or cylinders, and building up intermediate planes.

As natural selection acts only by the accumulation of slight

modifications of structure or instinct, each profitable to the

individual under its conditions of life, it may reasonably be asked,

how a long and graduated succession of modified architectural

instincts, all tending towards the present perfect plan of

construction, could have profited the progenitors of the hive-bee? I

think the answer is not difficult: cells constructed like those of the

bee or the wasp gain in strength, and save much in labour and space,

and in the materials of which they are constructed. With respect to the

formation of wax, it is known that bees are often hard pressed to get

sufficient nectar; and I am informed by Mr. Tegetmeier that it has been

experimentally proved that from twelve to fifteen pounds of dry sugar

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 and

consumed by the bees in a hive for the secretion of the wax necessary

for the construction of their combs. Moreover, many bees have to remain

idle for many days during the process of secretion. A large store of

honey is indispensable to support a large stock of bees during the

winter; and the security of the hive is known mainly to depend on a

large number of bees being supported. Hence the saving of wax by

largely saving honey, and the time consumed in collecting the honey,

must be an important element of success any family of bees. Of course

the success of the species may be dependent on the number of its

enemies, or parasites, or on quite distinct causes, and so be

altogether independent of the quantity of honey which the bees can

collect. But let us suppose that this latter circumstance determined,

as it probably often has determined, whether a bee allied to our

humble-bees could exist in large numbers in any country; and let us

further suppose that the community lived through the winter, and

consequently required a store of honey: there can in this case be no

doubt that it would be an advantage to our imaginary humble-bee if a

slight modification of her instincts led her to make her waxen cells

near together, so as to intersect a little; for a wall in common even

to two adjoining cells would save some little labour and wax. Hence, it

would continually be more and more advantageous to our humble-bees, if

they were to make their cells more and more regular, nearer together,

and aggregated into a mass, like the cells of the Melipona; for in this

case a large part of the bounding surface of each cell would serve to

bound the adjoining cells, 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 cells closer together, and more regular in

every way than at present; for then, as we have seen, the spherical

surfaces would wholly disappear and be replaced by plane surfaces; and

the Melipona would make a comb as perfect as 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 as we can see, is

absolutely perfect in economising labour and wax.

Thus, as I believe, the most wonderful of all known instincts, that of

the hive-bee, can be explained by natural selection having taken

advantage of numerous, successive, slight modifications of simpler

instincts; natural selection having, by slow degrees, more and more

perfectly led the bees to sweep 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. The bees, of course, no more knowing

that they swept their spheres at one particular distance from each

other, than they know what are the several angles of the hexagonal

prisms and of the basal rhombic plates; the motive power of the process

of natural selection having been the construction of cells of due

strength and of the proper size and shape for the larvæ, this being

effected with the greatest possible economy of labour and wax; that

individual swarm which thus made the best cells with least labour, and

least waste of honey in the secretion of wax, having succeeded best,

and having transmitted their newly-acquired economical instincts to new

swarms, which in their turn will have had the best chance of succeeding

in the struggle 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

simultaneous and accurately adjusted to each other, as a modification

in the one without an immediate corresponding change in the other would

have been fatal.” The force of this objection rests entirely on the

assumption that the changes in the instincts and structure are abrupt.

To take as an illustration the case of the larger titmouse, (Parus

major) alluded to in a previous chapter; this bird often holds the

seeds of the yew between its feet on a branch, and hammers with its

beak till it gets at the kernel. Now what special difficulty would

there be in natural selection preserving all the slight individual

variations in the shape of the beak, which were better and better

adapted to break open the seeds, until a beak was formed, as well

constructed for this purpose as that of the nuthatch, at the same time

that habit, or compulsion, or spontaneous variations of taste, led the

bird to become more and more of a seed-eater? In this case the beak is

supposed to be slowly modified by natural selection, subsequently to,

but in accordance with, slowly changing habits or taste; but let the

feet of the titmouse vary and grow larger from correlation with the

beak, or from any other unknown cause, and it is not improbable that

such larger feet would lead the bird to climb more and more until it

acquired the remarkable climbing instinct and power of the nuthatch. In

this case a gradual change of structure is supposed to lead to changed

instinctive habits. To take one more case: few instincts are more

remarkable than that which leads the swift 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) of sticks

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

with instincts leading it to neglect other materials and to make its

nest exclusively of inspissated saliva? And so in other cases. It must,

however, be admitted that in many instances we cannot conjecture

whether it was instinct or structure which first varied.

No doubt many instincts of very difficult explanation could be opposed

to the theory of natural selection—cases, in which we cannot see how an

instinct could have originated; cases, in which no intermediate

gradations are known to exist; cases of instincts of such trifling

importance, that they could hardly have been acted on by natural

selection; cases of instincts almost identically the same in animals so

remote in the scale of nature that we cannot account for their

similarity by inheritance from a common progenitor, and consequently

must believe that they were independently acquired through natural

selection. I will not here enter on these several cases, but will

confine myself to one special difficulty, which at first appeared to me

insuperable, and actually fatal to the whole theory. I allude to the

neuters or sterile females in insect communities: for these neuters

often differ widely in instinct and in structure from both the males

and fertile females, and yet, from being sterile, they cannot propagate

their kind.

The subject well deserves to be discussed at great length, but I will

here take only a single case, that of working or sterile ants. How the

workers have been rendered sterile is a difficulty; but not much

greater than that of any other striking modification of structure; for

it can be shown that some insects and other articulate animals in a

state of nature occasionally become sterile; and if such insects had

been social, and it had been profitable to the community that a number

should have been annually born capable of work, but incapable of

procreation, I can see no especial difficulty in this having been

effected through natural selection. But I must pass over this

preliminary difficulty. The great difficulty lies in the working ants

differing widely from both the males and the fertile females in

structure, as in the shape of the thorax, and in being destitute of

wings and sometimes of eyes, and in instinct. As far as instinct alone

is concerned, the wonderful difference in this respect between the

workers and 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, I should have unhesitatingly assumed that all its

characters had been slowly acquired through natural selection; namely,

by individuals having been born with slight profitable modifications,

which were inherited by the offspring, and that these again varied and

again were selected, and so onwards. But with the working ant we have

an insect differing greatly from its parents, yet absolutely sterile;

so that it could never have transmitted successively acquired

modifications of structure or instinct to its progeny. It may well be

asked how it is possible to reconcile this case with the theory of

natural selection?

First, let it be remembered that we have innumerable instances, both in

our domestic productions and in those in a state of nature, of all

sorts of differences of inherited structure which are correlated with

certain ages and with either sex. We have differences correlated not

only with one sex, but with that short period when the reproductive

system is active, as in the nuptial plumage of many birds, and in the

hooked jaws of the male salmon. We have even slight differences in the

horns of different breeds of 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 these same breeds. Hence, I can see

no great difficulty in any character becoming correlated with the

sterile condition of certain members of insect communities; the

difficulty lies in understanding how such correlated modifications of

structure could have been slowly accumulated by natural selection.

This difficulty, though appearing insuperable, is lessened, or, as I

believe, disappears, when it is remembered that selection may be

applied to the family, as well as to the individual, and may thus gain

the desired end. Breeders of cattle wish the flesh and fat to be well

marbled together. An animal thus characterized has been slaughtered,

but the breeder has gone with confidence to the same stock and has

succeeded. Such faith may be placed in the power of selection that a

breed of cattle, always yielding oxen with extraordinarily long horns,

could, it is probable, be formed by carefully watching which individual

bulls and cows, when matched, produced oxen with the longest horns; and

yet no one ox would ever have propagated its kind. Here is a better and

real illustration: According to M. Verlot, some varieties of the double

annual stock, from having been long and carefully selected to the right

degree, always produce a large proportion of seedlings bearing double

and quite sterile flowers, but they likewise yield some single and

fertile plants. These latter, by which alone the variety can be

propagated, may be compared with the fertile male and female ants, and

the double sterile plants with the neuters of the same community. As

with the varieties of the stock, so with social insects, selection has

been applied to the family, and not to the individual, for the sake of

gaining a serviceable end. Hence, we may conclude that slight

modifications of structure or of instinct, correlated with the sterile

condition of certain members of the community, have proved

advantageous; consequently the fertile males and females have

flourished, and transmitted to their fertile offspring a tendency to

produce sterile members with the same modifications. This process must

have been repeated many times, until that prodigious amount of

difference between the fertile and sterile females of the same species

has been produced which we see in many social insects.

But we have not as yet touched on the acme of the difficulty; namely,

the fact that the neuters of several ants differ, not only from the

fertile females and males, but from each other, sometimes to an almost

incredible degree, and are thus divided into two or even three castes.

The castes, moreover, do not generally graduate into each other, but

are perfectly well defined; being as distinct from each other as are

any two species of the same genus, or rather as any two genera of the

same family. Thus, in Eciton, there are working and soldier neuters,

with jaws and instincts extraordinarily different: in Cryptocerus, the

workers of one caste alone carry a wonderful sort of shield on their

heads, the use of which is quite unknown: in the Mexican Myrmecocystus,

the workers of one caste never leave the nest; they are fed by the

workers of another caste, and they have an enormously developed abdomen

which secretes a sort of honey, supplying the place of that excreted by

the aphides, or the domestic cattle as they may be called, which our

European ants guard and imprison.

It will indeed be thought that I have an overweening confidence in the

principle of natural selection, when I do not admit that such wonderful

and well-established facts at once annihilate the theory. In the

simpler case of neuter insects all of one caste, which, as I believe,

have been rendered different from the fertile males and females through

natural selection, we may conclude from the analogy of ordinary

variations, that the successive, slight, profitable modifications did

not first arise in all the neuters in the same nest, but in some few

alone; and that by the survival of the communities with females which

produced most neuters having the advantageous modification, all the

neuters ultimately came to be thus characterized. According to this

view we ought occasionally to find in the same nest neuter-insects,

presenting gradations of structure; and this we do find, even not

rarely, considering how few neuter-insects out of Europe have been

carefully examined. Mr. F. Smith has shown that the neuters of several

British ants differ surprisingly from each other in size and sometimes

in colour; and that the extreme forms can be linked together by

individuals taken out of the same nest: I have myself compared perfect

gradations of this kind. It sometimes happens that the larger or the

smaller sized workers are the most numerous; or that both large and

small are numerous, while those of an intermediate size are scanty in

numbers. Formica flava has larger and smaller workers, with some few of

intermediate size; and, in this species, as Mr. F. Smith has observed,

the larger workers have simple eyes (ocelli), which, though small, can

be plainly distinguished, whereas the smaller workers have their ocelli

rudimentary. Having carefully dissected several specimens of these

workers, I can affirm that the eyes are far more rudimentary in the

smaller workers than can be accounted for merely by their

proportionately lesser size; and I fully believe, though I dare not

assert so positively, that the workers of intermediate 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 by some few members in an

intermediate condition. I may digress by adding, that if the smaller

workers had been the most useful to the community, and those males and

females had been continually selected, which produced more and more of

the smaller workers, until all the workers were in this condition; we

should then have had a species of ant with neuters in nearly the same

condition as those of Myrmica. For the workers of Myrmica have not even

rudiments of ocelli, though the male and female ants of this genus have

well-developed ocelli.

I may give one other case: so confidently did I expect occasionally to

find gradations of important structures between the different castes of

neuters in the same species, that I gladly availed myself of Mr. F.

Smith’s offer of numerous specimens from the same nest of the driver

ant (Anomma) of West Africa. The reader will perhaps best appreciate

the amount of difference in these workers by my giving, not the actual

measurements, but a strictly accurate illustration: the difference was

the same as if we were to see a set of workmen building a house, of

whom many were five feet four inches high, and many sixteen feet high;

but we must in addition suppose that the larger workmen had heads four

instead of three times as big as those of the smaller men, and jaws

nearly five times as big. The jaws, moreover, of the working ants of

the several sizes differed wonderfully in shape, and in the form and

number of the teeth. But the important fact for us is that, though the

workers can be grouped into castes of different sizes, yet they

graduate insensibly into each other, as does the widely-different

structure of their jaws. I speak confidently on this latter point, as

Sir J. Lubbock made drawings for me, with the camera lucida, of the

jaws which I dissected from the workers of the several sizes. Mr.

Bates, in his interesting “Naturalist on the Amazons,” has described

analogous cases.

With these facts before me, I believe that natural selection, by acting

on the fertile ants or parents, could form a species which should

regularly produce neuters, all of large size with one form of jaw, or

all of small size with widely different jaws; or lastly, and this is

the greatest difficulty, one set of workers of one size and structure,

and simultaneously another set of workers of a different size and

structure; a graduated series having first been formed, as in the case

of the driver ant, and then the extreme forms having been produced in

greater and greater numbers, through the survival of the parents which

generated them, until none with an intermediate structure were

produced.

An analogous explanation has been given by Mr. Wallace, of the equally

complex case, of certain Malayan butterflies regularly appearing under

two or even three distinct female forms; and by Fritz Müller, of

certain Brazilian crustaceans likewise appearing under two widely

distinct male forms. But this subject need not here be discussed.

I have now explained how, I believe, the wonderful fact of two

distinctly defined castes of sterile workers existing in the same nest,

both widely different from each other and from their parents, has

originated. We can see how useful their production may have been to a

social community of ants, on the same principle that the division of

labour is useful to civilised man. Ants, however, work by inherited

instincts and by inherited organs or tools, while man works by acquired

knowledge and manufactured instruments. But I must confess, that, with

all my faith in natural selection, I should never have anticipated that

this principle could have been efficient in so high a degree, had not

the case of these neuter insects led me to this conclusion. I have,

therefore, discussed this case, at some little but wholly insufficient

length, in order to show the power of natural selection, and likewise

because this is by far the most serious special difficulty which my

theory has encountered. The case, also, is very interesting, as it

proves that with animals, as with plants, any amount of modification

may be effected by the accumulation of numerous, slight, spontaneous

variations, which are in any way profitable, without exercise or habit

having been brought into play. For peculiar habits, confined to the

workers of sterile females, however long they might be followed, could

not possibly affect the males and fertile females, which alone leave

descendants. I am surprised that no one has advanced this demonstrative

case of neuter insects, against the well-known doctrine of inherited

habit, as advanced by Lamarck.

_Summary._

I have endeavoured in this chapter briefly to show that the mental

qualities of our domestic animals vary, and that the variations are

inherited. Still more briefly I have attempted to show that instincts

vary slightly in a state of nature. No one will dispute that instincts

are of the highest importance to each animal. Therefore, there is no

real difficulty, under changing conditions of life, in natural

selection accumulating to any extent slight modifications of instinct

which are in any way useful. In many cases habit or use and disuse have

probably come into play. I do not pretend that the facts given in this

chapter strengthen in any great degree my theory; but none of the cases

of difficulty, to the best of my judgment, annihilate it. On the other

hand, the fact that instincts are not always absolutely perfect and are

liable to mistakes;—that no instinct can be shown to have been produced

for the good of other animals, though animals take advantage of the

instincts of others;—that the canon in natural history, of “Natura non

facit saltum,” is applicable to instincts as well as to corporeal

structure, and is plainly explicable on the foregoing views, but is

otherwise inexplicable—all tend to corroborate the theory of natural

selection.

This theory is also strengthened by some few other facts in regard to

instincts; as by that common case of closely allied, but distinct,

species, when inhabiting distant parts of the world and living under

considerably different conditions of life, yet often retaining nearly

the same instincts. For instance, we can understand, on the principle

of inheritance, how it is that the thrush of tropical South America

lines its nest with mud, in the same peculiar manner as does our

British thrush; how it is that the Hornbills of Africa and India have

the same extraordinary instinct 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 feed them and their young when hatched; how it

is that the male wrens (Troglodytes) of North America, build

“cock-nests,” to roost in, like the males of our Kitty-wrens,—a habit

wholly unlike that of any other known bird. Finally, it may not be a

logical deduction, but to my imagination it is far more satisfactory to

look at such instincts as the young cuckoo ejecting its

foster-brothers, ants making slaves, the larvæ of ichneumonidæ feeding

within the live bodies of caterpillars, not as specially endowed or

created instincts, but as small consequences of one general law leading

to the advancement of all organic beings—namely, multiply, vary, let

the strongest live and the weakest die.