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 T H E 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. Am 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 execretion 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 housetumblers, 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, bad 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 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 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 dowm 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 out- side, 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, froin 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 ;nsect-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 Myrruica 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 'cocknests,' 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.