On the Origin of Species by Means of Natural Selection (1859)
by Charles Darwin
CHAPTER VII.
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, 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.
HE 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 pupæ
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 ´ Ö 2, or radius ´ 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 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.
[ Charles Darwin,
On
the Origin Of Species: A Facsimile of the First Edition,
Cambridge, Massachusetts: Harvard University Press, 1964, pp. 207-244. ]
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