 Chapter
7 - 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
THE subject of instinct might have been worked into the
previous chapters; but I have thought that it would be more
convenient to treat the subject separately, especially as
so wonderful an instinct as that of the hive-bee making
its cells will probably have occurred to many readers, as
a difficulty sufficient to overthrow my whole theory. I
must premise, that I have nothing to do with the origin
of the primary mental powers, any more than I have with
that of life itself. We are concerned only with the diversities
of instinct and of the other mental qualities of animals
within the same class.
I will not attempt any definition of instinct. It would
be easy to show that several distinct mental actions are
commonly embraced by this term; but every one understands
what is meant, when it is said that instinct impels the
cuckoo to migrate and to lay her eggs in other birds' nests.
An action, which we ourselves should require experience
to enable us to perform, when performed by an animal, more
especially by a very young one, without any experience,
and when performed by many individuals in the same way,
without their knowing for what purpose it is performed,
is usually said to be instinctive. But I could show that
none of these characters of instinct are universal. A little
dose, as Pierre Huber expresses it, of judgment or reason,
often comes into play, even in animals very low in the scale
of nature.
Frederick Cuvier and several of the older metaphysicians
have compared instinct with habit. This comparison gives,
I think, a remarkably accurate notion of the frame of mind
under which an instinctive action is performed, but not
of its origin. How unconsciously many habitual actions are
performed, indeed not rarely in direct opposition to our
conscious will! yet they may be modified by the will or
reason. Habits easily become associated with other habits,
and with certain periods of time and states of the body.
When once acquired, they often remain constant throughout
life. Several other points of resemblance between instincts
and habits could be pointed out. As in repeating a well-known
song, so in instincts, one action follows another by a sort
of rhythm; if a person be interrupted in a song, or in repeating
anything by rote, he is generally forced to go back to recover
the habitual train of thought: so P. Huber found it was
with a caterpillar, which makes a very complicated hammock;
for if he took a caterpillar which had completed its hammock
up to, say, the sixth stage of construction, and put it
into a hammock completed up only to the third stage, the
caterpillar simply re-performed the fourth, fifth, and sixth
stages of construction. If, however, a caterpillar were
taken out of a hammock made up, for instance, to the third
stage, and were put into one finished up to the sixth stage,
so that much of its work was already done for it, far from
feeling the benefit of this, it was much embarrassed, and,
in order to complete its hammock, seemed forced to start
from the third stage, where it had left off, and thus tried
to complete the already finished work.
If we suppose any habitual action to become inherited
and I think it can be shown that this does sometimes happen
then the resemblance between what originally was a habit
and an instinct becomes so close as not to be distinguished.
If Mozart, instead of playing the pianoforte at three years
old with wonderfully little practice, had played a tune
with no practice at all, be might truly be said to have
done so instinctively. But it would be the most serious
error to suppose that the greater number of instincts have
been acquired by habit in one generation, and then transmitted
by inheritance to succeeding generations. It can be clearly
shown that the most wonderful instincts with which we are
acquainted, namely, those of the hive-bee and of many ants,
could not possibly have been thus acquired.
It will be universally admitted that instincts are as
important as corporeal structure for the welfare of each
species, under its present conditions of life. Under changed
conditions of life, it is at least possible that slight
modifications of instinct might be profitable to a species;
and if it can be shown that instincts do vary ever so little,
then I can see no difficulty in natural selection preserving
and continually accumulating variations of instinct to any
extent that may be profitable. It is thus, as I believe,
that all the most complex and wonderful instincts have originated.
As modifications of corporeal structure arise from, and
are increased by, use or habit, and are diminished or lost
by disuse, so I do not doubt it has been with instincts.
But I believe that the effects of habit are of quite subordinate
importance to the effects of the natural selection of what
may be called accidental variations of instincts; that is
of variations produced by the same unknown causes which
produce slight deviations of bodily structure.
No complex instinct can possibly be produced through natural
selection, except by the slow and gradual accumulation of
numerous, slight, yet profitable, variations. Hence, as
in the case of corporeal structures, we ought to find in
nature, not the actual transitional gradations by which
each complex instinct has been acquired for these could
be found only in the lineal ancestors of each species but
we ought to find in the collateral lines of descent some
evidence of such gradations; or we ought at least to be
able to show that gradations of some kind are possible;
and this we certainly can do. I have been surprised to find,
making allowance for the instincts of animals having been
but little observed except in Europe and North America,
and for no instinct being known amongst extinct species,
how very generally gradations, leading to the most complex
instincts, can be discovered. The canon of 'Natura non facit
saltum' applies with almost equal force to instincts as
to bodily organs. Changes of instinct may sometimes be facilitated
by the same species having different instincts at different
periods of life, or at different seasons of the year, or
when placed under different circumstances, &c.; in which
case either one or the other instinct might be preserved
by natural selection. And such instances of diversity of
instinct in the same species can be shown to occur in nature.
Again as in the case of corporeal structure, and conformably
with my theory, the instinct of each species is good for
itself, but has never, as far as we can judge, been produced
for the exclusive good of others. One of the strongest instances
of an animal apparently performing an action for the sole
good of another, with which I am acquainted, is that of
aphides voluntarily yielding their sweet excretion to ants:
that they do so voluntarily, the following facts show. I
removed all the ants from a group of about a dozen aphides
on a dock-plant, and prevented their attendance during several
hours. After this interval, I felt sure that the aphides
would want to excrete. I watched them for some time through
a lens, but not one excreted; I then tickled and stroked
them with a hair in the same manner, as well as I could,
as the ants do with their antennae; but not one excreted.
Afterwards I allowed an ant to visit them, and it immediately
seemed, by its eager way of running about, to be well aware
what a rich flock it had discovered; it then began to play
with its antennae on the abdomen first of one aphis and
then of another; and each aphis, as soon as it felt the
antennae, immediately lifted up its abdomen and excreted
a limpid drop of sweet juice, which was eagerly devoured
by the ant. Even the quite young aphides behaved in this
manner, showing that the action was instinctive, and not
the result of experience. But as the excretion is extremely
viscid, it is probably a convenience to the aphides to have
it removed; and therefore probably the aphides do not instinctively
excrete for the sole good of the ants. Although I do not
believe that any animal in the world performs an action
for the exclusive good of another of a distinct species,
yet each species tries to take advantage of the instincts
of others, as each takes advantage of the weaker bodily
structure of others. So again, in some few cases, certain
instincts cannot be considered as absolutely perfect; but
as details on this and other such points are not indispensable,
they may be here passed over.
As some degree of variation in instincts under a state
of nature, and the inheritance of such variations, are indispensable
for the action of natural selection, as many instances as
possible ought to have been here given; but want of space
prevents me. I can only assert, that instincts certainly
do vary for instance, the migratory instinct, both in extent
and direction, and in its total loss. So it is with the
nests of birds, which vary partly in dependence on the situations
chosen, and on the nature and temperature of the country
inhabited, but often from causes wholly unknown to us: Audubon
has given several remarkable cases of differences in nests
of the same species in the northern and southern United
States. Fear of any particular enemy is certainly an instinctive
quality, as may be seen in nestling birds, though it is
strengthened by experience, and by the sight of fear of
the same enemy in other animals. But fear of man is slowly
acquired, as I have elsewhere shown, by various animals
inhabiting desert islands; and we may see an instance of
this, even in England, in the greater wildness of all our
large birds than of our small birds; for the large birds
have been most persecuted by man. We may safely attribute
the greater wildness of our large birds to this cause; for
in uninhabited islands large birds are not more fearful
than small; and the magpie, so wary in England, is tame
in Norway, as is the hooded crow in Egypt.
That the general disposition of individuals of the same
species, born in a state of nature, is extremely diversified,
can be shown by a multitude of facts. Several cases also,
could be given, of occasional and strange habits in certain
species, which might, if advantageous to the species, give
rise, through natural selection, to quite new instincts.
But I am well aware that these general statements, without
facts given in detail, can produce but a feeble effect on
the reader's mind. I can only repeat my assurance, that
I do not speak without good evidence.
The possibility, or even probability, of inherited variations
of instinct in a state of nature will be strengthened by
briefly considering a few cases under domestication. We
shall thus also be enabled to see the respective parts which
habit and the selection of so-called accidental variations
have played in modifying the mental qualities of our domestic
animals. A number of curious and authentic instances could
be given of the inheritance of all shades of disposition
and tastes, and likewise of the oddest tricks, associated
with certain frames of mind or periods of time. But let
us look to the familiar case of the several breeds of dogs:
it cannot be doubted that young pointers (I have myself
seen a striking instance) will sometimes point and even
back other dogs the very first time that they are taken
out; retrieving is certainly in some degree inherited by
retrievers; and a tendency to run round, instead of at,
a flock of sheep, by shepherd-dogs. I cannot see that these
actions, performed without experience by the young, and
in nearly the same manner by each individual, performed
with eager delight by each breed, and without the end being
known, for the young pointer can no more know that he points
to aid his master, than the white butterfly knows why she
lays her eggs on the leaf of the cabbage, I cannot see that
these actions differ essentially from true instincts. If
we were to see one kind of wolf, when young and without
any training, as soon as it scented its prey, stand motionless
like a statue, and then slowly crawl forward with a peculiar
gait; and another kind of wolf rushing round, instead of
at, a herd of deer, and driving them to a distant point,
we should assuredly call these actions instinctive. Domestic
instincts, as they may be called, are certify far less fixed
or invariable than natural instincts; but they have been
acted on by far less rigorous selection, and have been transmitted
for an incomparably shorter period, under less fixed conditions
of life.
How strongly these domestic instincts, habits, and dispositions
are inherited, and how curiously they become mingled, is
well shown when different breeds of dogs are crossed. Thus
it is known that a cross with a bull-dog has affected for
many generations the courage and obstinacy of greyhounds;
and a cross with a greyhound has given to a whole family
of shepherd-dogs a tendency to hunt hares. These domestic
instincts, when thus tested by crossing, resemble natural
instincts, which in a like manner become curiously blended
together, and for a long period exhibit traces of the instincts
of either parent: for example, Le Roy describes a dog, whose
great-grandfather was a wolf, and this dog showed a trace
of its wild parentage only in one way, by not coming in
a straight line to his master when called.
Domestic instincts are sometimes spoken of as actions
which have become inherited solely from long-continued and
compulsory habit, but this, I think, is not true. No one
would ever have thought of teaching, or probably could have
taught, the tumbler-pigeon to tumble, an action which, as
I have witnessed, is performed by young birds, that have
never seen a pigeon tumble. We may believe that some one
pigeon showed a slight tendency to this strange habit, and
that the long-continued selection of the best individuals
in successive generations made tumblers what they now are;
and near Glasgow there are house-tumblers, as I hear from
Mr Brent, which cannot fly eighteen inches high without
going head over heels. It may be doubted whether any one
would have thought of training a dog to point, had not some
one dog naturally shown a tendency in this line; and this
is known occasionally to happen, as I once saw in a pure
terrier. When the first tendency was once displayed, methodical
selection and the inherited effects of compulsory training
in each successive generation would soon complete the work;
and unconscious selection is still at work, as each man
tries to procure, without intending to improve the breed,
dogs which will stand and hunt best. On the other hand,
habit alone in some cases has sufficed; no animal is more
difficult to tame than the young of the wild rabbit; scarcely
any animal is tamer than the young of the tame rabbit; but
I do not suppose that domestic rabbits have ever been selected
for tameness; and I presume that we must attribute the whole
of the inherited change from extreme wildness to extreme
tameness, simply to habit and long-continued close confinement.
Natural instincts are lost under domestication: a remarkable
instance of this is seen in those breeds of fowls which
very rarely or never become 'broody,' that is, never wish
to sit on their eggs. Familiarity alone prevents our seeing
how universally and largely the minds of our domestic animals
have been modified by domestication. It is scarcely possible
to doubt that the love of man has become instinctive in
the dog. All wolves, foxes, jackals, and species of the
cat genus, when kept tame, are most eager to attack poultry,
sheep, and pigs; and this tendency has been found incurable
in dogs which have been brought home as puppies from countries,
such as Tierra del Fuego and Australia, where the savages
do not keep these domestic animals. How rarely, on the other
hand, do our civilised dogs, even when quite young, require
to be taught not to attack poultry, sheep, and pigs! No
doubt they occasionally do make an attack, and are then
beaten; and if not cured, they are destroyed; so that habit,
with some degree of selection, has probably concurred in
civilising by inheritance our dogs. On the other hand, young
chickens have lost, wholly by habit, that fear of the dog
and cat which no doubt was originally instinctive in them,
in the same way as it is so plainly instinctive in young
pheasants, though reared under a hen. It is not that chickens
have lost all fear, but fear only of dogs and cats, for
if the hen gives the danger-chuckle, they will run (more
especially young turkeys) from under her, and conceal themselves
in the surrounding grass or thickets; and this is evidently
done for the instinctive purpose of allowing, as we see
in wild ground-birds, their mother to fly away. But this
instinct retained by our chickens has become useless under
domestication, for the mother-hen has almost lost by disuse
the power of flight.
Hence, we may conclude, that domestic instincts have been
acquired and natural instincts have been lost partly by
habit, and partly by man selecting and accumulating during
successive generations, peculiar mental habits and actions,
which at first appeared from what we must in our ignorance
call an accident. In some cases compulsory habit alone has
sufficed to produce such inherited mental changes; in other
cases compulsory habit has done nothing, and all has been
the result of selection, pursued both methodically and unconsciously;
but in most cases, probably, habit and selection have acted
together.
We shall, perhaps, best understand how instincts in a
state of nature have become modified by selection, by considering
a few cases. I will select only three, out of the several
which I shall have to discuss in my future work, namely,
the instinct which leads the cuckoo to lay her eggs in other
birds' nests; the slave-making instinct of certain ants;
and the comb-making power of the hive-bee: these two latter
instincts have generally, and most justly, been ranked by
naturalists as the most wonderful of all known instincts.
It is now commonly admitted that the more immediate and
final cause of the cuckoo's instinct is, that she lays her
eggs, not daily, but at intervals of two or three days;
so that, if she were to make her own nest and sit on her
own eggs, those first laid would have to be left for some
time unincubated, or there would be eggs and young birds
of different ages in the same nest. If this were the case,
the process of laying and hatching might be inconveniently
long, more especially as she has to migrate at a very early
period; and the first hatched young would probably have
to be fed by the male alone. But the American cuckoo is
in this predicament; for she makes her own nest and has
eggs and young successively hatched, all at the same time.
It has been asserted that the American cuckoo occasionally
lays her eggs in other birds' nests; but I hear on the high
authority of Dr. Brewer, that this is a mistake. Nevertheless,
I could give several instances of various birds which have
been known occasionally to lay their eggs in other birds'
nests. Now let us suppose that the ancient progenitor of
our European cuckoo had the habits of the American cuckoo;
but that occasionally she laid an egg in another bird's
nest. If the old bird profited by this occasional habit,
or if the young were made more vigorous by advantage having
been taken of the mistaken maternal instinct of another
bird, than by their own mother's care, encumbered as she
can hardly fail to be by having eggs and young of different
ages at the same time; then the old birds or the fostered
young would gain an advantage. And analogy would lead me
to believe, that the young thus reared would be apt to follow
by inheritance the occasional and aberrant habit of their
mother, and in their turn would be apt to lay their eggs
in other birds' nests, and thus be successful in rearing
their young. By a continued process of this nature, I believe
that the strange instinct of our cuckoo could be, and has
been, generated. I may add that, according to Dr. Gray and
to some other observers, the European cuckoo has not utterly
lost all maternal love and care for her own offspring.
The occasional habit of birds laying their eggs in other
birds' nests, either of the same or of a distinct species,
is not very uncommon with the Gallinaceae; and this perhaps
explains the origin of a singular instinct in the allied
group of ostriches. For several hen ostriches, at least
in the case of the American species, unite and lay first
a few eggs in one nest and then in another; and these are
hatched by the males. This instinct may probably be accounted
for by the fact of the hens laying a large number of eggs;
but, as in the case of the cuckoo, at intervals of two or
three days. This instinct, however, of the American ostrich
has not as yet been perfected; for a surprising number of
eggs lie strewed over the plains, so that in one day's hunting
I picked up no less than twenty lost and wasted eggs.
Many bees are parasitic, and always lay their eggs in
the nests of bees of other kinds. This case is more remarkable
than that of the cuckoo; for these bees have not only their
instincts but their structure modified in accordance with
their parasitic habits; for they do not possess the pollen-collecting
apparatus which would be necessary if they had to store
food for their own young. Some species, likewise, of Sphegidae
(wasp-like insects) are parasitic on other species; and
M. Fabre has lately shown good reason for believing that
although the Tachytes nigra generally makes its own burrow
and stores it with paralysed prey for its own larvae to
feed on, yet that when this insect finds a burrow already
made and stored by another sphex, it takes advantage of
the prize, and becomes for the occasion parasitic. In this
case, as with the supposed case of the cuckoo, I can see
no difficulty in natural selection making an occasional
habit permanent, if of advantage to the species, and if
the insect whose nest and stored food are thus feloniously
appropriated, be not thus exterminated.
Slave-making instinct. This remarkable instinct
was first discovered in the Formica (Polyerges) rufescens
by Pierre Huber, a better observer even than his celebrated
father. This ant is absolutely dependent on its slaves;
without their aid, the species would certainly become extinct
in a single year. The males and fertile females do no work.
The workers or sterile females, though most energetic and
courageous in capturing slaves, do no other work. They are
incapable of making their own nests, or of feeding their
own larvae. When the old nest is found inconvenient, and
they have to migrate, it is the slaves which determine the
migration, and actually carry their masters in their jaws.
So utterly helpless are the masters, that when Huber shut
up thirty of them without a slave, but with plenty of the
food which they like best, and with their larvae and pupae
to stimulate them to work, they did nothing; they could
not even feed themselves, and many perished of hunger. Huber
then introduced a single slave (F. fusca), and she instantly
set to work, fed and saved the survivors; made some cells
and tended the larvae, and put all to rights. What can be
more extraordinary than these well-ascertained facts? If
we had not known of any other slave-making ant, it would
have been hopeless to have speculated how so wonderful an
instinct could have been perfected.
Formica sanguinea was likewise first discovered by P.
Huber to be a slave-making ant. This species is found in
the southern parts of England, and its habits have been
attended to by Mr. F. Smith, of the British Museum, to whom
I am much indebted for information on this and other subjects.
Although fully trusting to the statements of Huber and Mr.
Smith, I tried to approach the subject in a sceptical frame
of mind, as any one may well be excused for doubting the
truth of so extraordinary and odious an instinct as that
of making slaves. Hence I will give the observations which
I have made myself made, in some little detail. I opened
fourteen nests of F. sanguinea, and found a few slaves in
all. Males and fertile females of the slave-species are
found only in their own proper communities, and have never
been observed in the nests of F. sanguinea. The slaves are
black and not above half the size of their red masters,
so that the contrast in their appearance is very great.
When the nest is slightly disturbed, the slaves occasionally
come out, and like their masters are much agitated and defend
their nest: when the nest is much disturbed and the larvae
and pupae are exposed, the slaves work energetically with
their masters in carrying them away to a place of safety.
Hence, it is clear, that the slaves feel quite at home.
During the months of June and July, on three successive
years, I have watched for many hours several nests in Surrey
and Sussex, and never saw a slave either leave or enter
a nest. As, during these months, the slaves are very few
in number, I thought that they might behave differently
when more numerous; but Mr. Smith informs me that he has
watched the nests at various hours during May, June and
August, both in Surrey and Hampshire, and has never seen
the slaves, though present in large numbers in August, either
leave or enter the nest. Hence he considers them as strictly
household slaves. The masters, on the other hand, may be
constantly seen bringing in materials for the nest, and
food of all kinds. During the present year, however, in
the month of July, I came across a community with an unusually
large stock of slaves, and I observed a few slaves mingled
with their masters leaving the nest, and marching along
the same road to a tall Scotch-fir-tree, twenty-five yards
distant, which they ascended together, probably in search
of aphides or cocci. According to Huber, who had ample opportunities
for observation, in Switzerland the slaves habitually work
with their masters in making the nest, and they alone open
and close the doors in the morning and evening; and, as
Huber expressly states, their principal office is to search
for aphides. This difference in the usual habits of the
masters and slaves in the two countries, probably depends
merely on the slaves being captured in greater numbers in
Switzerland than in England.
One day I fortunately chanced to witness a migration from
one nest to another, and it was a most interesting spectacle
to behold the masters carefully carrying, as Huber has described,
their slaves in their jaws. Another day my attention was
struck by about a score of the slave-makers haunting the
same spot, and evidently not in search of food; they approached
and were vigorously repulsed by an independent community
of the slave species (F. fusca); sometimes as many as three
of these ants clinging to the legs of the slave-making F.
sanguinea. The latter ruthlessly killed their small opponents,
and carried their dead bodies as food to their nest, twenty-nine
yards distant; but they were prevented from getting any
pupae to rear as slaves. I then dug up a small parcel of
the pupae of F. fusca from another nest, and put them down
on a bare spot near the place of combat; they were eagerly
seized, and carried off by the tyrants, who perhaps fancied
that, after all, they had been victorious in their late
combat.
At the same time I laid on the same place a small parcel
of the pupae of another species, F. flava, with a few of
these little yellow ants still clinging to the fragments
of the nest. This species is sometimes, though rarely, made
into slaves, as has been described by Mr Smith. Although
so small a species, it is very courageous, and I have seen
it ferociously attack other ants. In one instance I found
to my surprise an independent community of F. flava under
a stone beneath a nest of the slave-making F. sanguinea;
and when I had accidentally disturbed both nests, the little
ants attacked their big neighbours with surprising courage.
Now I was curious to ascertain whether F. sanguinea could
distinguish the pupae of F. fusca, which they habitually
make into slaves, from those of the little and furious F.
flava, which they rarely capture, and it was evident that
they did at once distinguish them: for we have seen that
they eagerly and instantly seized the pupae of F. fusca,
whereas they were much terrified when they came across the
pupae, or even the earth from the nest of F. flava, and
quickly ran away; but in about a quarter of an hour, shortly
after all the little yellow ants had crawled away, they
took heart and carried off the pupae.
One evening I visited another community of F. sanguinea,
and found a number of these ants entering their nest, carrying
the dead bodies of F. fusca (showing that it was not a migration)
and numerous pupae. I traced the returning file burthened
with booty, for about forty yards, to a very thick clump
of heath. whence I saw the last individual of F. sanguinea
emerge, carrying a pupa; but I was not able to find the
desolated nest in the thick heath. The nest, however, must
have been close at hand, for two or three individuals of
F. fusca were rushing about in the greatest agitation, and
one was perched motionless with its own pupa in its mouth
on the top of a spray of heath over its ravaged home.
Such are the facts, though they did not need confirmation
by me, in regard to the wonderful instinct of making slaves.
Let it be observed what a contrast the instinctive habits
of F. sanguinea present with those of the F. rufescens.
The latter does not build its own nest, does not determine
its own migrations, does not collect food for itself or
its young, and cannot even feed itself: it is absolutely
dependent on its numerous slaves. Formica sanguinea, on
the other hand, possesses much fewer slaves, and in the
early part of the summer extremely few. The masters determine
when and where a new nest shall be formed, and when they
migrate, the masters carry the slaves. Both in Switzerland
and England the slaves seem to have the exclusive care of
the larvae, and the masters alone go on slave-making expeditions.
In Switzerland the slaves and masters work together, making
and bringing materials for the nest: both, but chiefly the
slaves, tend, and milk as it may be called, their aphides;
and thus both collect food for the community. In England
the masters alone usually leave the nest to collect building
materials and food for themselves, their slaves and larvae.
So that the masters in this country receive much less service
from their slaves than they do in Switzerland.
By what steps the instinct of F. sanguinea originated
I will not pretend to conjecture. But as ants, which are
not slave-makers, will, as I have seen, carry off pupae
of other species, if scattered near their nests, it is possible
that pupae originally stored as food might become developed;
and the ants thus unintentionally reared would then follow
their proper instincts, and do what work they could. If
their presence proved useful to the species which had seized
them if it were more advantageous to this species to capture
workers than to procreate them the habit of collecting pupae
originally for food might by natural selection be strengthened
and rendered permanent for the very different purpose of
raising slaves. When the instinct was once acquired, if
carried out to a much less extent even than in our British
F. sanguinea, which, as we have seen, is less aided by its
slaves than the same species in Switzerland, I can see no
difficulty in natural selection increasing and modifying
the instinct always supposing each modification to be of
use to the species until an ant was formed as abjectly dependent
on its slaves as is the Formica rufescens.
Cell-making instinct of the Hive-Bee. I will not
here enter on minute details on this subject, but will merely
give an outline of the conclusions at which I have arrived.
He must be a dull man who can examine the exquisite structure
of a comb, so beautifully adapted to its end, without enthusiastic
admiration. We hear from mathematicians that bees have practically
solved a recondite problem, and have made their cells of
the proper shape to hold the greatest possible amount of
honey, with the least possible consumption of previous wax
in their construction. It has been remarked that a skilful
workman, with fitting tools and measures, would find it
very difficult to make cells of wax of the true form, though
this is perfectly effected by a crowd of bees working in
a dark hive. Grant whatever instincts you please, and it
seems at first quite inconceivable how they can make all
the necessary angles and planes, or even perceive when they
are correctly made. But the difficulty is not nearly so
great as it at first appears: all this beautiful work can
be shown, I think, to follow from a few very simple instincts.
I was led to investigate this subject by Mr. Waterhouse,
who has shown that the form of the cell stands in close
relation to the presence of adjoining cells; and the following
view may, perhaps, be considered only as a modification
of this theory. Let us look to the great principle of gradation,
and see whether Nature does not reveal to us her method
of work. At one end of a short series we have humble-bees,
which use their old cocoons to hold honey, sometimes adding
to them short tubes of wax, and likewise making separate
and very irregular rounded cells of wax. At the other end
of the series we have the cells of the hive-bee, placed
in a double layer: each cell, as is well know, is an hexagonal
prism, with the basal edges of its six sides bevelled so
as to join on to a pyramid, formed of three rhombs. These
rhombs have certain angles, and the three which form the
pyramidal base of a single cell on one side of the comb,
enter into the composition of the bases of three adjoining
cells on the opposite side. In the series between the extreme
perfection of the cells of the hive-bee and the simplicity
of those of the humble-bee, we have the cells of the Mexican
Melipona domestica, carefully described and figured by Pierre
Huber. The Melipona itself is intermediate in structure
between the hive and humble bee, but more nearly related
to the latter: it forms a nearly regular waxen comb of cylindrical
cells, in which the young are hatched, and, in addition,
some large cells of wax for holding honey. These latter
cells are nearly spherical and of nearly equal sizes, and
are aggregated into an irregular mass. But the important
point to notice, is that these cells are always made at
that degree of nearness to each other, that they would have
intersected or broken into each other, if the spheres had
been completed; but this is never permitted, the bees building
perfectly flat walls of wax between the spheres which thus
tend to intersect. Hence each cell consists of an outer
spherical portion and of two, three, or more perfectly flat
surfaces, according as the cell adjoins two, three or more
other cells. When one cell comes into contact with three
other cells, which, from the spheres being nearly of the
same size, is very frequently and necessarily the case,
the three flat surfaces are united into a pyramid; and this
pyramid, as Huber has remarked, is manifestly a gross imitation
of the three-sided pyramidal basis of the cell of the hive-bee.
As in the cells of the hive-bee, so here, the three plane
surfaces in any one cell necessarily enter into the construction
of three adjoining cells. It is obvious that the Melipona
saves wax by this manner of building; for the flat walls
between the adjoining cells are not double, but are of the
same thickness as the outer spherical portions, and yet
each flat portion forms a part of two cells.
Reflecting on this case, it occurred to me that if the
Melipona had made its spheres at some given distance from
each other, and had made them of equal sizes and had arranged
them symmetrically in a double layer, the resulting structure
would probably have been as perfect as the comb of the hive-bee.
Accordingly I wrote to Professor Miller, of Cambridge, and
this geometer has kindly read over the following statement,
drawn up from his information, and tells me that it is strictly
correct:-
If a number of equal spheres be described with their centres
placed in two parallel layers; with the centre of each sphere
at the distance of radius X /sqrt[2] or radius X 1.41421
(or at some lesser distance), from the centres of the six
surrounding spheres in the same layer; and at the same distance
from the centres of the adjoining spheres in the other and
parallel layer; then, if planes of intersection between
the several spheres in both layers be formed, there will
result a double layer of hexagonal prisms united together
by pyramidal bases formed of three rhombs; and the rhombs
and the sides of the hexagonal prisms will have every angle
identically the same with the best measurements which have
been made of the cells of the hive-bee.
Hence we may safely conclude that if we could slightly
modify the instincts already possessed by the Melipona,
and in themselves not very wonderful, this bee would make
a structure as wonderfully perfect as that of the hive-bee.
We must suppose the Melipona to make her cells truly spherical,
and of equal sizes; and this would not be very surprising,
seeing that she already does so to a certain extent, and
seeing what perfectly cylindrical burrows in wood many insects
can make, apparently by turning round on a fixed point.
We must suppose the Melipona to arrange her cells in level
layers, as she already does her cylindrical cells; and we
must further suppose, and this is the greatest difficulty,
that she can somehow judge accurately at what distance to
stand from her fellow-labourers when several are making
their spheres; but she is already so far enabled to judge
of distance, that she always describes her spheres so as
to intersect largely; and then she unites the points of
intersection by perfectly flat surfaces. We have further
to suppose, but this is no difficulty, that after hexagonal
prisms have been formed by the intersection of adjoining
spheres in the same layer, she can prolong the hexagon to
any length requisite to hold the stock of honey; in the
same way as the rude humble-bee adds cylinders of wax to
the circular mouths of her old cocoons. By such modifications
of instincts in themselves not very wonderful, hardly more
wonderful than those which guide a bird to make its nest,
I believe that the hive-bee has acquired, through natural
selection, her inimitable architectural powers.
But this theory can be tested by experiment. Following
the example of Mr Tegetmeier, I separated two combs, and
put between them a long, thick, square strip of wax: the
bees instantly began to excavate minute circular pits in
it; and as they deepened these little pits, they made them
wider and wider until they were converted into shallow basins,
appearing to the eye perfectly true or parts of a sphere,
and of about the diameter of a cell. It was most interesting
to me to observe that wherever several bees had begun to
excavate these basins near together, they had begun their
work at such a distance from each other, that by the time
the basins had acquired the above stated width (i.e.
about the width of an ordinary cell), and were in depth
about one sixth of the diameter of the sphere of which they
formed a part, the rims of the basins intersected or broke
into each other. As soon as this occurred, the bees ceased
to excavate, and began to build up flat walls of wax on
the lines of intersection between the basins, so that each
hexagonal prism was built upon the festooned edge of a smooth
basin, instead of on the straight edges of a three-sided
pyramid as in the case of ordinary cells.
I then put into the hive, instead of a thick, square piece
of wax, a thin and narrow, knife-edged ridge, coloured with
vermilion. The bees instantly began on both sides to excavate
little basins near to each other, in the same way as before;
but the ridge of wax was so thin, that the bottoms of the
basins, if they had been excavated to the same depth as
in the former experiment, would have broken into each other
from the opposite sides. The bees, however, did not suffer
this to happen, and they stopped their excavations in due
time; so that the basins, as soon as they had been a little
deepened, came to have flat bottoms; and these flat bottoms,
formed by thin little plates of the vermilion wax having
been left ungnawed, were situated, as far as the eye could
judge, exactly along the planes of imaginary intersection
between the basins on the opposite sides of the ridge of
wax. In parts, only little bits, in other parts, large portions
of a rhombic plate had been left between the opposed basins,
but the work, from the unnatural state of things, had not
been neatly performed. The bees must have worked at very
nearly the same rate on the opposite side of the ridge of
vermilion wax, as they circularly gnawed away and deepened
the basins on both sides, in order to have succeeded in
thus leaving flat plates between the basins, by stopping
work along the intermediate planes or planes of intersection.
Considering how flexible thin wax is, I do not see that
there is any difficulty in the bees, whilst at work on the
two sides of a strip of wax, perceiving when they have gnawed
the wax away to the proper thinness, and then stopping their
work. In ordinary combs it has appeared to me that the bees
do not always succeed in working at exactly the same rate
from the opposite sides; for I have noticed half-completed
rhombs at the base of a just-commenced cell, which were
slightly concave on one side, where I suppose that the bees
had excavated too quickly, and convex on the opposed side,
where the bees had worked less quickly. In one well-marked
instance, I put the comb back into the hive and allowed
the bees to go on working for a short time and again examined
the cell, and I found that the rhombic plate had been completed,
and had become perfectly flat: it was absolutely
impossible, from the extreme thinness of the little rhombic
plate, that they could have affected this by gnawing away
the convex side; and I suspect that the bees in such cases
stand in the opposed cells and push and bend the ductile
and warm wax (which as I have tried is easily done) into
its proper intermediate plane, and thus flatten it.
From the experiment of the ridge of vermilion wax, we
can clearly see that if the bees were to build for themselves
a thin wall of wax, they could make their cells of the proper
shape, by standing at the proper distance from each other,
by excavating at the same rate, and by endeavouring to make
equal spherical hollows, but never allowing the spheres
to break into each other. Now bees, as may be clearly seen
by examining the edge of a growing comb, do make a rough,
circumferential wall or rim all round the comb; and they
gnaw into this from the opposite sides, always working circularly
as they deepen each cell. They do not make the whole three-sided
pyramidal base of any one cell at the same time, but only
the one rhombic plate which stands on the extreme growing
margin, or the two plates, as the case may be; and they
never complete the upper edges of the rhombic plates, until
the hexagonal walls are commenced. Some of these statements
differ from those made by the justly celebrated elder Huber,
but I am convinced of their accuracy; and if I had space,
I could show that they are conformable with my theory.
Huber's statement that the very first cell is excavated
out of a little parallel-sided wall of wax, is not, as far
as I have seen, strictly correct; the first commencement
having always been a little hood of wax; but I will not
here enter on these details. We see how important a part
excavation plays in the construction of the cells; but it
would be a great error to suppose that the bees cannot build
up a rough wall of wax in the proper position that is, along
the plane of intersection between two adjoining spheres.
I have several specimens showing clearly that they can do
this. Even in the rude circumferential rim or wall of wax
round a growing comb, flexures may sometimes be observed,
corresponding in position to the planes of the rhombic basal
plates of future cells. But the rough wall of wax has in
every case to be finished off, by being largely gnawed away
on both sides. The manner in which the bees build is curious;
they always make the first rough wall from ten to twenty
times thicker than the excessively thin finished wall of
the cell, which will ultimately be left. We shall understand
how they work, by supposing masons first to pile up a broad
ridge of cement, and then to begin cutting it away equally
on both sides near the ground, till a smooth, very thin
wall is left in the middle; the masons always piling up
the cut-away cement, and adding fresh cement, on the summit
of the ridge. We shall thus have a thin wall steadily growing
upward; but always crowned by a gigantic coping. From all
the cells, both those just commenced and those completed,
being thus crowned by a strong coping of wax, the bees can
cluster and crawl over the comb without injuring the delicate
hexagonal walls, which are only about one four-hundredth
of an inch in thickness; the plates of the pyramidal basis
being about twice as thick. By this singular manner of building,
strength is continually given to the comb, with the utmost
ultimate economy of wax.
It seems at first to add to the difficulty of understanding
how the cells are made, that a multitude of bees all work
together; one bee after working a short time at one cell
going to another, so that, as Huber has stated, a score
of individuals work even at the commencement of the first
cell. I was able practically to show this fact, by covering
the edges of the hexagonal walls of a single cell, or the
extreme margin of the circumferential rim of a growing comb,
with an extremely thin layer of melted vermilion wax; and
I invariably found that the colour was most delicately diffused
by the bees as delicately as a painter could have done with
his brush by atoms of the coloured wax having been taken
from the spot on which it had been placed, and worked into
the growing edges of the cells all round. The work of construction
seems to be a sort of balance struck between many bees,
all instinctively standing at the same relative distance
from each other, all trying to sweep equal spheres, and
then building up, or leaving ungnawed, the planes of intersection
between these spheres. It was really curious to note in
cases of difficulty, as when two pieces of comb met at an
angle, how often the bees would entirely pull down and rebuild
in different ways the same cell, sometimes recurring to
a shape which they had at first rejected.
When bees have a place on which they can stand in their
proper positions for working, for instance, on a slip of
wood, placed directly under the middle of a comb growing
downwards so that the comb has to be built over one face
of the slip in this case the bees can lay the foundations
of one wall of a new hexagon, in its strictly proper place,
projecting beyond the other completed cells. It suffices
that the bees should be enabled to stand at their proper
relative distances from each other and from the walls of
the last completed cells, and then, by striking imaginary
spheres, they can build up a wall intermediate between two
adjoining spheres; but, as far as I have seen, they never
gnaw away and finish off the angles of a cell till a large
part both of that cell and of the adjoining cells has been
built. This capacity in bees of laying down under certain
circumstances a rough wall in its proper place between two
just-commenced cells, is important, as it bears on a fact,
which seems at first quite subversive of the foregoing theory;
namely, that the cells on the extreme margin of wasp-combs
are sometimes strictly hexagonal; but I have not space here
to enter on this subject. Nor does there seem to me any
great difficulty in a single insect (as in the case of a
queen-wasp) making hexagonal cells, if she work alternately
on the inside and outside of two or three cells commenced
at the same time, always standing at the proper relative
distance from the parts of the cells just begun, sweeping
spheres or cylinders, and building up intermediate planes.
It is even conceivable that an insect might, by fixing on
a point at which to commence a cell, and then moving outside,
first to one point, and then to five other points, at the
proper relative distances from the central point and from
each other, strike the planes of intersection, and so make
an isolated hexagon: but I am not aware that any such case
has been observed; nor would any good be derived from a
single hexagon being built, as in its construction more
materials would be required than for a cylinder.
As natural selection acts only by the accumulation of
slight modifications of structure or instinct, each profitable
to the individual under its conditions of life, it may reasonably
be asked, how a long and graduated succession of modified
architectural instincts, all tending towards the present
perfect plan of construction, could have profited the progenitors
of the hive-bee? I think the answer is not difficult: it
is known that bees are often hard pressed to get sufficient
nectar; and I am informed by Mr. Tegetmeier that it has
been experimentally found that no less than from twelve
to fifteen pounds of dry sugar are consumed by a hive of
bees for the secretion of each pound of wax; so that a prodigious
quantity of fluid nectar must be collected and consumed
by the bees in a hive for the secretion of the wax necessary
for the construction of their combs. Moreover, many bees
have to remain idle for many days during the process of
secretion. A large store of honey is indispensable to support
a large stock of bees during the winter; and the security
of the hive is known mainly to depend on a large number
of bees being supported. Hence the saving of wax by largely
saving honey must be a most important element of success
in any family of bees. Of course the success of any species
of bee may be dependent on the number of its parasites or
other enemies, or on quite distinct causes, and so be altogether
independent of the quantity of honey which the bees could
collect. But let us suppose that this latter circumstance
determined, as it probably often does determine, the numbers
of a humble-bee which could exist in a country; and let
us further suppose that the community lived throughout the
winter, and consequently required a store of honey: there
can in this case be no doubt that it would be an advantage
to our humble-bee, if a slight modification of her instinct
led her to make her waxen cells near together, so as to
intersect a little; for a wall in common even to two adjoining
cells, would save some little wax. Hence it would continually
be more and more advantageous to our humble-bee, if she
were to make her cells more and more regular, nearer together,
and aggregated into a mass, like the cells of the Melipona;
for in this case a large part of the bounding surface of
each cell would serve to bound other cells, and much wax
would be saved. Again, from the same cause, it would be
advantageous to the Melipona, if she were to make her cells
closer together, and more regular in every way than at present;
for then, as we have seen, the spherical surfaces would
wholly disappear, and would all be replaced by plane surfaces;
and the Melipona would make a comb as perfect as that of
the hive-bee. Beyond this stage of perfection in architecture,
natural selection could not lead; for the comb of the hive-bee,
as far as we can see, is absolutely perfect in economising
wax.
Thus, as I believe, the most wonderful of all known instincts,
that of the hive-bee, can be explained by natural selection
having taken advantage of numerous, successive, slight modifications
of simpler instincts; natural selection having by slow degrees,
more and more perfectly, led the bees to sweep equal spheres
at a given distance from each other in a double layer, and
to build up and excavate the wax along the planes of intersection.
The bees, of course, no more knowing that they swept their
spheres at one particular distance from each other, than
they know what are the several angles of the hexagonal prisms
and of the basal rhombic plates. The motive power of the
process of natural selection having been economy of wax;
that individual swarm which wasted least honey in the secretion
of wax, having succeeded best, and having transmitted by
inheritance its newly acquired economical instinct to new
swarms, which in their turn will have had the best chance
of succeeding in the struggle for existence.
No doubt many instincts of very difficult explanation
could be opposed to the theory of natural selection, cases,
in which we cannot see how an instinct could possibly have
originated; cases, in which no intermediate gradations are
known to exist; cases of instinct of apparently such trifling
importance, that they could hardly have been acted on by
natural selection; cases of instincts almost identically
the same in animals so remote in the scale of nature, that
we cannot account for their similarity by inheritance from
a common parent, and must therefore believe that they have
been acquired by independent acts of natural selection.
I will not here enter on these several cases, but will confine
myself to one special difficulty, which at first appeared
to me insuperable, and actually fatal to my whole theory.
I allude to the neuters or sterile females in insect-communities:
for these neuters often differ widely in instinct and in
structure from both the males and fertile females, and yet,
from being sterile, they cannot propagate their kind.
The subject well deserves to be discussed at great length,
but I will here take only a single case, that of working
or sterile ants. How the workers have been rendered sterile
is a difficulty; but not much greater than that of any other
striking modification of structure; for it can be shown
that some insects and other articulate animals in a state
of nature occasionally become sterile; and if such insects
had been social, and it had been profitable to the community
that a number should have been annually born capable of
work, but incapable of procreation, I can see no very great
difficulty in this being effected by natural selection.
But I must pass over this preliminary difficulty. The great
difficulty lies in the working ants differing widely from
both the males and the fertile females in structure, as
in the shape of the thorax and in being destitute of wings
and sometimes of eyes, and in instinct. As far as instinct
alone is concerned, the prodigious difference in this respect
between the workers and the perfect females, would have
been far better exemplified by the hive-bee. If a working
ant or other neuter insect had been an animal in the ordinary
state, I should have unhesitatingly assumed that all its
characters had been slowly acquired through natural selection;
namely, by an individual having been born with some slight
profitable modification of structure, this being inherited
by its offspring, which again varied and were again selected,
and so onwards. But with the working ant we have an insect
differing greatly from its parents, yet absolutely sterile;
so that it could never have transmitted successively acquired
modifications of structure or instinct to its progeny. It
may well be asked how is it possible to reconcile this case
with the theory of natural selection?
First, let it be remembered that we have innumerable instances,
both in our domestic productions and in those in a state
of nature, of all sorts of differences of structure which
have become correlated to certain ages, and to either sex.
We have differences correlated not only to one sex, but
to that short period alone when the reproductive system
is active, as in the nuptial plumage of many birds, and
in the hooked jaws of the male salmon. We have even slight
differences in the horns of different breeds of cattle in
relation to an artificially imperfect state of the male
sex; for oxen of certain breeds have longer horns than in
other breeds, in comparison with the horns of the bulls
or cows of these same breeds. Hence I can see no real difficulty
in any character having become correlated with the sterile
condition of certain members of insect-communities: the
difficulty lies in understanding how such correlated modifications
of structure could have been slowly accumulated by natural
selection.
This difficulty, though appearing insuperable, is lessened,
or, as I believe, disappears, when it is remembered that
selection may be applied to the family, as well as to the
individual, and may thus gain the desired end. Thus, a well-flavoured
vegetable is cooked, and the individual is destroyed; but
the horticulturist sows seeds of the same stock, and confidently
expects to get nearly the same variety; breeders of cattle
wish the flesh and fat to be well marbled together; the
animal has been slaughtered, but the breeder goes with confidence
to the same family. I have such faith in the powers of selection,
that I do not doubt that a breed of cattle, always yielding
oxen with extraordinarily long horns, could be slowly formed
by carefully watching which individual bulls and cows, when
matched, produced oxen with the longest horns; and yet no
one ox could ever have propagated its kind. Thus I believe
it has been with social insects: a slight modification of
structure, or instinct, correlated with the sterile condition
of certain members of the community, has been advantageous
to the community: consequently the fertile males and females
of the same community flourished, and transmitted to their
fertile offspring a tendency to produce sterile members
having the same modification. And I believe that this process
has been repeated, until that prodigious amount of difference
between the fertile and sterile females of the same species
has been produced, which we see in many social insects.
But we have not as yet touched on the climax of the difficulty;
namely, the fact that the neuters of several ants differ,
not only from the fertile females and males, but from each
other, sometimes to an almost incredible degree, and are
thus divided into two or even three castes. The castes,
moreover, do not generally graduate into each other, but
are perfectly well defined; being as distinct from each
other, as are any two species of the same genus, or rather
as any two genera of the same family. Thus in Eciton, there
are working and soldier neuters, with jaws and instincts
extraordinarily different: in Cryptocerus, the workers of
one caste alone carry a wonderful sort of shield on their
heads, the use of which is quite unknown: in the Mexican
Myrmecocystus, the workers of one caste never leave the
nest; they are fed by the workers of another caste, and
they have an enormously developed abdomen which secretes
a sort of honey, supplying the place of that excreted by
the aphides, or the domestic cattle as they may be called,
which our European ants guard or imprison.
It will indeed be thought that I have an overweening confidence
in the principle of natural selection, when I do not admit
that such wonderful and well-established facts at once annihilate
my theory. In the simpler case of neuter insects all of
one caste or of the same kind, which have been rendered
by natural selection, as I believe to be quite possible,
different from the fertile males and females, in this case,
we may safely conclude from the analogy of ordinary variations,
that each successive, slight, profitable modification did
not probably at first appear in all the individual neuters
in the same nest, but in a few alone; and that by the long-continued
selection of the fertile parents which produced most neuters
with the profitable modification, all the neuters ultimately
came to have the desired character. On this view we ought
occasionally to find neuter-insects of the same species,
in the same nest, presenting gradations of structure; and
this we do find, even often, considering how few neuter-insects
out of Europe have been carefully examined. Mr F. Smith
has shown how surprisingly the neuters of several British
ants differ from each other in size and sometimes in colour;
and that the extreme forms can sometimes be perfectly linked
together by individuals taken out of the same nest: I have
myself compared perfect gradations of this kind. It often
happens that the larger or the smaller sized workers are
the most numerous; or that both large and small are numerous,
with those of an intermediate size scanty in numbers. Formica
flava has larger and smaller workers, with some of intermediate
size; and, in this species, as Mr F. Smith has observed,
the larger workers have simple eyes (ocelli), which though
small can be plainly distinguished, whereas the smaller
workers have their ocelli rudimentary. Having carefully
dissected several specimens of these workers, I can affirm
that the eyes are far more rudimentary in the smaller workers
than can be accounted for merely by their proportionally
lesser size; and I fully believe, though I dare not assert
so positively, that the workers of intermediate size have
their ocelli in an exactly intermediate condition. So that
we here have two bodies of sterile workers in the same nest,
differing not only in size, but in their organs of vision,
yet connected by some few members in an intermediate condition.
I may digress by adding, that if the smaller workers had
been the most useful to the community, and those males and
females had been continually selected, which produced more
and more of the smaller workers, until all the workers had
come to be in this condition; we should then have had a
species of ant with neuters very nearly in the same condition
with those of Myrmica. For the workers of Myrmica have not
even rudiments of ocelli, though the male and female ants
of this genus have well-developed ocelli.
I may give one other case: so confidently did I expect
to find gradations in important points of structure between
the different castes of neuters in the same species, that
I gladly availed myself of Mr F. Smith's offer of numerous
specimens from the same nest of the driver ant (Anomma)
of West Africa. The reader will perhaps best appreciate
the amount of difference in these workers, by my giving
not the actual measurements, but a strictly accurate illustration:
the difference was the same as if we were to see a set of
workmen building a house of whom many were five feet four
inches high, and many sixteen feet high; but we must suppose
that the larger workmen had heads four instead of three
times as big as those of the smaller men, and jaws nearly
five times as big. The jaws, moreover, of the working ants
of the several sizes differed wonderfully in shape, and
in the form and number of the teeth. But the important fact
for us is, that though the workers can be grouped into castes
of different sizes, yet they graduate insensibly into each
other, as does the widely-different structure of their jaws.
I speak confidently on this latter point, as Mr Lubbock
made drawings for me with the camera lucida of the jaws
which I had dissected from the workers of the several sizes.
With these facts before me, I believe that natural selection,
by acting on the fertile parents, could form a species which
should regularly produce neuters, either all of large size
with one form of jaw, or all of small size with jaws having
a widely different structure; or lastly, and this is our
climax of difficulty, one set of workers of one size and
structure, and simultaneously another set of workers of
a different size and structure; a graduated series having
been first formed, as in the case of the driver ant, and
then the extreme forms, from being the most useful to the
community, having been produced in greater and greater numbers
through the natural selection of the parents which generated
them; until none with an intermediate structure were produced.
Thus, as I believe, the wonderful fact of two distinctly
defined castes of sterile workers existing in the same nest,
both widely different from each other and from their parents,
has originated. We can see how useful their production may
have been to a social community of insects, on the same
principle that the division of labour is useful to civilised
man. As ants work by inherited instincts and by inherited
tools or weapons, and not by acquired knowledge and manufactured
instruments, a perfect division of labour could be effected
with them only by the workers being sterile; for had they
been fertile, they would have intercrossed, and their instincts
and structure would have become blended. And nature has,
as I believe, effected this admirable division of labour
in the communities of ants, by the means of natural selection.
But I am bound to confess, that, with all my faith in this
principle, I should never have anticipated that natural
selection could have been efficient in so high a degree,
had not the case of these neuter insects convinced me of
the fact. I have, therefore, discussed this case, at some
little but wholly insufficient length, in order to show
the power of natural selection, and likewise because this
is by far the most serious special difficulty, which my
theory has encountered. The case, also, is very interesting,
as it proves that with animals, as with plants, any amount
of modification in structure can be effected by the accumulation
of numerous, slight, and as we must call them accidental,
variations, which are in any manner profitable, without
exercise or habit having come into play. For no amount of
exercise, or habit, or volition, in the utterly sterile
members of a community could possibly have affected the
structure or instincts of the fertile members, which alone
leave descendants. I am surprised that no one has advanced
this demonstrative case of neuter insects, against the well-known
doctrine of Lamarck.
Summary. I have endeavoured briefly in this chapter
to show that the mental qualities of our domestic animals
vary, and that the variations are inherited. Still more
briefly I have attempted to show that instincts vary slightly
in a state of nature. No one will dispute that instincts
are of the highest importance to each animal. Therefore
I can see no difficulty, under changing conditions of life,
in natural selection accumulating slight modifications of
instinct to any extent, in any useful direction. In some
cases habit or use and disuse have probably come into play.
I do not pretend that the facts given in this chapter strengthen
in any great degree my theory; but none of the cases of
difficulty, to the best of my judgment, annihilate it. On
the other hand, the fact that instincts are not always absolutely
perfect and are liable to mistakes; that no instinct has
been produced for the exclusive good of other animals, but
that each animal takes advantage of the instincts of others;
that the canon in natural history, of 'natura non facit
saltum' is applicable to instincts as well as to corporeal
structure, and is plainly explicable on the foregoing views,
but is otherwise inexplicable, all tend to corroborate the
theory of natural selection.
This theory is, also, strengthened by some few other facts
in regard to instincts; as by that common case of closely
allied, but certainly distinct, species, when inhabiting
distant parts of the world and living under considerably
different conditions of life, yet often retaining nearly
the same instincts. For instance, we can understand on the
principle of inheritance, how it is that the thrush of South
America lines its nest with mud, in the same peculiar manner
as does our British thrush: how it is that the male wrens
(Troglodytes) of North America, build 'cock-nests,' to roost
in, like the males of our distinct Kitty-wrens, a habit
wholly unlike that of any other known bird. Finally, it
may not be a logical deduction, but to my imagination it
is far more satisfactory to look at such instincts as the
young cuckoo ejecting its foster-brothers, ants making slaves,
-- the larvae of ichneumonidae feeding within the live bodies
of caterpillars, not as specially endowed or created instincts,
but as small consequences of one general law, leading to
the advancement of all organic beings, namely, multiply,
vary, let the strongest live and the weakest die.
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