The Origin of Species
Chapter 7: Instinct
by Charles Darwin
Instincts comparable with habits, but different in their origin -
Instincts graduated - Aphides and ants - Instincts variable - Domestic instincts, their
origin - Natural instincts of the cuckoo, ostrich, and parasitic bees - Slave-making ants
- Hive-bee, its cell-making instinct - Difficulties on the theory of the Natural Selection
of instincts - Neuter or sterile insects - Summary |
The subject of instinct might have been worked into the previous
chapters; but I have thought that it would be more convenient to treat the subject
separately, especially as so wonderful an instinct as that of the hive-bee making its
cells will probably have occurred to many readers, as a difficulty sufficient to overthrow
my whole theory. I must premise, that I have nothing to do with the origin of the primary
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 qualities of animals within 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 should require experience to enable
us to perform, when performed by an animal, more especially by a very young one, without
any 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 of instinct are universal. A little dose, as Pierre
Huber expresses it, of judgment or reason, often comes into play, even in animals very low
in the scale of nature.
Frederick Cuvier and several of the older metaphysicians have compared instinct with
habit. This comparison gives, I think, a remarkably accurate notion of the frame of mind
under which an instinctive action is performed, but not 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, and 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 feeling the
benefit of 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 I think 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 the
most 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 thus
acquired.
It will be universally admitted that instincts are as important as corporeal structure
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 may be 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 of quite subordinate importance to the effects of the natural
selection of what may be called accidental 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
amongst extinct species, how very generally gradations, leading to the most complex
instincts, can be discovered. The canon of 'Natura non facit saltum' applies with almost
equal force to instincts as to bodily organs. 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 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 with 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 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 antennae; 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 antennae on the abdomen first of one aphis and then of another; and
each aphis, as soon as it felt the antennae, 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. But as the excretion is extremely viscid, it is probably a
convenience to the aphides to have it removed; and therefore probably the aphides do not
instinctively excrete for the sole good of the ants. Although I do not believe that any
animal in the world performs an action for the exclusive good of another of a distinct
species, yet each species tries to take advantage of the instincts of others, as each
takes advantage of the weaker bodily structure of others. So again, in some few cases,
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 have been here 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 nests of the same species in the northern and
southern United States. 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. But fear of man is slowly acquired, as I
have elsewhere shown, by various animals inhabiting desert islands; and we may see an
instance of this, even in England, in the greater wildness of all our large birds than of
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 general disposition of individuals of the same species, born in a state of
nature, is extremely diversified, can be shown by a multitude of facts. Several cases
also, could be given, of occasional and strange habits in certain species, which might, if
advantageous to the species, give rise, through natural selection, to quite new instincts.
But I am well aware that these general statements, without facts given 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.
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 also be enabled to see the respective parts which habit and the selection of
so-called accidental variations have played in modifying the mental qualities of our
domestic animals. A number of curious and authentic instances could be given of the
inheritance of all shades of disposition and tastes, 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 several breeds of dogs: it cannot be doubted that young pointers (I have
myself seen a striking instance) 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 see 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 certify far less fixed or invariable 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, I think, 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. When the first tendency 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 at work, as
each man tries to procure, without intending to improve the breed, dogs which will stand
and hunt best. On the other hand, habit alone in some cases has sufficed; no 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 do not suppose that domestic rabbits have ever
been selected for tameness; and I presume that we must attribute the whole of the
inherited change from extreme wildness to extreme tameness, simply 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 universally and largely
the minds of our domestic animals have been modified by domestication. 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, with some degree of selection, has 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, in the same way as it is so plainly instinctive in young pheasants,
though reared 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 domestic 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 such 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, probably, habit and selection have acted together.
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, out of the
several which I shall have to discuss in my future work, namely, the instinct which leads
the cuckoo to lay her eggs in other birds' nests; the slave-making instinct of certain
ants; and the comb-making power of the hive-bee: these two latter instincts have
generally, and most justly, been ranked by naturalists as the most wonderful of all known
instincts.
It is now commonly admitted that the more immediate and final cause of the cuckoo's
instinct 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 has to migrate 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 asserted that the American
cuckoo occasionally lays her eggs in other birds' nests; but I hear on the high authority
of Dr. Brewer, that this is a mistake. Nevertheless, I could 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; but that occasionally she laid an egg in another bird's nest. If the
old bird profited by this occasional habit, or if the young were made more vigorous by
advantage having been taken of the mistaken maternal instinct of another bird, than by
their own mother's care, encumbered as she can 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 me 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 successful in
rearing their young. By a continued process of this nature, I believe that the strange
instinct of our cuckoo could be, and has been, generated. I may add that, according to Dr.
Gray and to some other observers, the European cuckoo has not utterly lost all maternal
love and care for her own offspring.
The occasional habit of birds laying their eggs in other birds' nests, either of the
same or of a distinct species, is not very uncommon with the Gallinaceae; and this perhaps
explains the origin of a singular instinct in the allied group of ostriches. For several
hen ostriches, at least in the case of the American species, 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 in the case of the cuckoo, at intervals of two or three days. This instinct, however,
of the American ostrich 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 always lay their eggs in the nests of bees of other kinds.
This case is more remarkable than that of the cuckoo; for these bees have not only their
instincts but their structure modified in accordance with their parasitic habits; for they
do not possess the pollen-collecting apparatus which would be necessary if they had to
store food for their own young. Some species, likewise, of Sphegidae (wasp-like insects)
are parasitic on other species; 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 larvae to feed on, 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 the supposed case of the 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 thus 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.
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
larvae. 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 larvae and pupae 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 larvae, 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 have speculated how so
wonderful an instinct could have been perfected.
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 truth of so extraordinary and odious an instinct as that of
making slaves. Hence I will give the observations which I have made myself 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 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 very great. When the nest is slightly disturbed, the slaves occasionally
come out, and like their masters are much agitated and defend their nest: when the nest is
much disturbed and the larvae and pupae are exposed, the slaves work energetically 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 have 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 present year, 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, in Switzerland the slaves 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 chanced to witness a migration from one nest to another, and it
was a most interesting spectacle to behold the masters carefully carrying, as Huber has
described, their slaves in their jaws. 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 pupae to rear as slaves. I then dug up a small parcel of the
pupae 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 pupae of another
species, F. flava, with a few of these little yellow ants still clinging to the fragments
of the 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 pupae 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 pupae of F. fusca, whereas they were much terrified when 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, shortly after all the little yellow ants had crawled away, they took
heart and carried off the pupae.
One evening I visited another community of F. sanguinea, and found a number of these
ants entering their nest, carrying the dead bodies of F. fusca (showing that it was not a
migration) and numerous pupae. I traced the returning file burthened with booty, for about
forty yards, to a very thick clump of heath. whence I saw the last individual of F.
sanguinea emerge, carrying a pupa; but I was not able to find 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 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 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 larvae, 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 larvae. 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 pupae of other
species, if scattered near their nests, it is possible that pupae originally stored as
food might become developed; and the 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 pupae 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, I can see no difficulty in natural selection
increasing and modifying 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
previous 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 perfectly effected by a crowd of bees working in a dark hive. Grant
whatever instincts you please, and 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 it at first appears: all this beautiful work can
be shown, I think, to follow from a few very 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 this 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 know, is an hexagonal prism,
with the basal edges of its six sides bevelled so as to join on to a pyramid, formed 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 perfectly flat surfaces, according as the cell adjoins two, three or more other
cells. When one cell comes into contact with 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 basis 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 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 probably 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.
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
make 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 in wood many insects can make, 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 largely; and then she unites the
points of intersection by perfectly flat surfaces. We have further to suppose, but this is
no difficulty, that after hexagonal prisms have been formed by the intersection of
adjoining spheres in the same layer, she can prolong the hexagon to any length requisite
to hold the stock of honey; in the same way as the rude humble-bee adds cylinders of wax
to the circular mouths of her old cocoons. By such modifications of instincts in
themselves 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, square 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 me 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 festooned 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, square 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 bottoms; and these flat bottoms, formed by thin little plates of the vermilion wax
having been left ungnawed, were situated, as far as the eye could judge, exactly along the
planes of imaginary intersection between the basins on the opposite sides of the ridge of
wax. In parts, only little bits, in other parts, large portions of a rhombic plate had
been 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 on
the opposite side of the ridge of vermilion wax, as they circularly gnawed away and
deepened the basins on both sides, in order to have succeeded in thus leaving flat plates
between the basins, by stopping work along the intermediate planes or 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 rhombic plate, that they
could have affected 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 clearly 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 into
this 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
the 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 these 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, which are only about one
four-hundredth of an inch in thickness; the plates of the pyramidal basis being about
twice as thick. By this 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 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 entirely 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 quite 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 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. It
is even conceivable that an insect might, by fixing on a point at which to commence a
cell, and then moving outside, first to one point, and then to five other points, at the
proper relative distances from the central point and from each other, strike the planes of
intersection, and so make an isolated hexagon: but I am not aware that any such case has
been observed; nor would any good be derived from a single hexagon being built, as in its
construction more materials would be required than for a cylinder.
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: 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 found that no less than from twelve to fifteen
pounds of dry sugar are consumed by a hive of bees for the secretion of each 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 must be a most important
element of success in any family of bees. Of course the success of any species of bee may
be dependent on the number of its parasites or other enemies, or on quite distinct causes,
and so be altogether independent of the quantity of honey which the bees could collect.
But let us suppose that this latter circumstance determined, as it probably often does
determine, the numbers of a humble-bee which could exist in a country; and let us further
suppose that the community lived throughout 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
humble-bee, if a slight modification of her instinct 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 wax. Hence it would continually be more and more advantageous to
our humble-bee, if she were to make her 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 other cells, and much 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 would all 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 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 economy of wax; that individual swarm which wasted least
honey in the secretion of wax, having succeeded best, and having transmitted by
inheritance its newly acquired economical instinct to new swarms, which in their turn will
have had the best chance of succeeding in the struggle for existence.
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 possibly have
originated; cases, in which no intermediate gradations are known to exist; cases of
instinct of apparently 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 parent, and must therefore believe that they have been acquired by independent acts
of 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 my 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 very great difficulty in this being
effected by 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 prodigious
difference in this respect between the workers and the perfect females, would have been
far better exemplified by the hive-bee. If a working ant or other neuter insect had been
an animal in the ordinary state, I should have unhesitatingly assumed that all its
characters had been slowly acquired through natural selection; namely, by an individual
having been born with some slight profitable modification of structure, this being
inherited by its offspring, which again varied and were again 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 is it
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 structure
which have become correlated to certain ages, and to either sex. We have differences
correlated not only to one sex, but to that short period alone 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 in other breeds, in comparison with the horns of the bulls or cows
of these same breeds. Hence I can see no real difficulty in any character having become
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. Thus, a well-flavoured vegetable is
cooked, and the individual is destroyed; but the horticulturist sows seeds of the same
stock, and confidently expects to get nearly the same variety; breeders of cattle wish the
flesh and fat to be well marbled together; the animal has been slaughtered, but the
breeder goes with confidence to the same family. I have such faith in the powers of
selection, that I do not doubt that a breed of cattle, always yielding oxen with
extraordinarily long horns, could be slowly formed by carefully watching which individual
bulls and cows, when matched, produced oxen with the longest horns; and yet no one ox
could ever have propagated its kind. Thus I believe it has been with social insects: a
slight modification of structure, or instinct, correlated with the sterile condition of
certain members of the community, has been advantageous to the community: consequently the
fertile males and females of the same community flourished, and transmitted to their
fertile offspring a tendency to produce sterile members having the same modification. And
I believe that this process has been repeated, 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 climax 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 or 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 my theory. In the simpler case of neuter insects all of one caste or of
the same kind, which have been rendered by natural selection, as I believe to be quite
possible, different from the fertile males and females, in this case, we may safely
conclude from the analogy of ordinary variations, that each successive, slight, profitable
modification did not probably at first appear in all the individual neuters in the same
nest, but in a few alone; and that by the long-continued selection of the fertile parents
which produced most neuters with the profitable modification, all the neuters ultimately
came to have the desired character. On this view we ought occasionally to find
neuter-insects of the same species, in the same nest, presenting gradations of structure;
and this we do find, even often, considering how few neuter-insects out of Europe have
been carefully examined. Mr F. Smith has shown how surprisingly the neuters of several
British ants differ from each other in size and sometimes in colour; and that the extreme
forms can sometimes be perfectly linked together by individuals taken out of the same
nest: I have myself compared perfect gradations of this kind. It often happens that the
larger or the smaller sized workers are the most numerous; or that both large and small
are numerous, with those of an intermediate size scanty in numbers. Formica flava has
larger and smaller workers, with some 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
proportionally 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 we here 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 had come to be
in this condition; we should then have had a species of ant with neuters very nearly in
the same condition with 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 to find gradations in important
points of structure 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 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 Mr Lubbock made drawings for me with the camera lucida of the jaws which I had
dissected from the workers of the several sizes.
With these facts before me, I believe that natural selection, by acting on the fertile
parents, could form a species which should regularly produce neuters, either all of large
size with one form of jaw, or all of small size with jaws having a widely different
structure; or lastly, and this is our climax of 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 been first formed, as in the case of the driver ant,
and then the extreme forms, from being the most useful to the community, having been
produced in greater and greater numbers through the natural selection of the parents which
generated them; until none with an intermediate structure were produced.
Thus, as 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 insects, on the same principle that the division of labour is useful to
civilised man. As ants work by inherited instincts and by inherited tools or weapons, and
not by acquired knowledge and manufactured instruments, a perfect division of labour could
be effected with them only by the workers being sterile; for had they been fertile, they
would have intercrossed, and their instincts and structure would have become blended. And
nature has, as I believe, effected this admirable division of labour in the communities of
ants, by the means of natural selection. But I am bound to confess, that, with all my
faith in this principle, I should never have anticipated that natural selection could have
been efficient in so high a degree, had not the case of these neuter insects convinced me
of the fact. 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 in structure can be effected by the accumulation of numerous,
slight, and as we must call them accidental, variations, which are in any manner
profitable, without exercise or habit having come into play. For no amount of exercise, or
habit, or volition, in the utterly sterile members of a community could possibly have
affected the structure or instincts of the fertile members, which alone leave descendants.
I am surprised that no one has advanced this demonstrative case of neuter insects, against
the well-known doctrine of Lamarck.
Summary. I have endeavoured briefly in this chapter 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 I can
see no difficulty, under changing conditions of life, in natural selection accumulating
slight modifications of instinct to any extent, in any useful direction. In some 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
has been produced for the exclusive good of other animals, but that each animal takes
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 certainly 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 South America lines its nest
with mud, in the same peculiar manner as does our British thrush: how it is that the male
wrens (Troglodytes) of North America, build 'cock-nests,' to roost in, like the males of
our distinct 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 larvae of ichneumonidae 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.