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, he 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,
etc.; 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 antennæ; but not one excreted. Afterwards I
allowed an ant to visit them, and it immediately seemed, by its eager
way of running about, to be well aware what a rich flock it had
discovered; it then began to play with its antennæ on the abdomen first
of one aphis and then of another; and each aphis, as soon as it felt
the antennæ, immediately lifted up its abdomen and excreted a limpid
drop of sweet juice, which was eagerly devoured by the ant. Even the
quite young aphides behaved in this manner, showing that the action was
instinctive, and not the result of experience. 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 certainly 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 Gallinaceæ; 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
Sphegidæ (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 larvæ 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 larvæ. When the old nest is found
inconvenient, and they have to migrate, it is the slaves which
determine the migration, and actually carry their masters in their
jaws. So utterly helpless are the masters, that when Huber shut up
thirty of them without a slave, but with plenty of the food which they
like best, and with their larvæ and pupæ to stimulate them to work,
they did nothing; they could not even feed themselves, and many
perished of hunger. Huber then introduced a single slave (F. fusca),
and she instantly set to work, fed and saved the survivors; made some
cells and tended the larvæ, and put all to rights. What can be more
extraordinary than these well-ascertained facts? If we had not known of
any other slave-making ant, it would have been hopeless to 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 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 the nest: when the
nest is much disturbed and the larvæ and pupæ 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 pupæ to rear as slaves. I then dug up a
small parcel of the pupæ of F. fusca from another nest, and put them
down on a bare spot near the place of combat; they were eagerly seized,
and carried off by the tyrants, who perhaps fancied that, after all,
they had been victorious in their late combat.
At the same time I laid on the same place a small parcel of the pupæ of
another species, F. flava, with a few of these little yellow ants still
clinging to the fragments of 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 pupæ of F. fusca, which they habitually make into
slaves, from those of the little and furious F. flava, which they
rarely capture, and it was evident that they did at once distinguish
them: for we have seen that they eagerly and instantly seized the pupæ
of F. fusca, whereas they were much terrified when they came across the
pupæ, or even the earth from the nest of F. flava, and quickly ran
away; but in about a quarter of an hour, shortly after all the little
yellow ants had crawled away, they took heart and carried off the pupæ.
One evening I visited another community of F. sanguinea, and found a
number of these ants entering their nest, carrying the dead bodies of
F. fusca (showing that it was not a migration) and numerous pupæ. 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 larvæ,
and the masters alone go on slave-making expeditions. In Switzerland
the slaves and masters work together, making and bringing materials for
the nest: both, but chiefly the slaves, tend, and milk as it may be
called, their aphides; and thus both collect food for the community. In
England the masters alone usually leave the nest to collect building
materials and food for themselves, their slaves and larvæ. So that the
masters in this country receive much less service from their slaves
than they do in Switzerland.
By what steps the instinct of F. sanguinea originated I will not
pretend to conjecture. But as ants, which are not slave-makers, will,
as I have seen, carry off pupæ of other species, if scattered near
their nests, it is possible that pupæ originally stored as food might
become developed; and the ants thus unintentionally reared would then
follow their proper instincts, and do what work they could. If their
presence proved useful to the species which had seized them—if it were
more advantageous
to this species to capture workers than to procreate them—the habit of
collecting pupæ originally for food might by natural selection be
strengthened and rendered permanent for the very different purpose of
raising slaves. When the instinct was once acquired, if carried out to
a much less extent even than in our British F. sanguinea, which, as we
have seen, is less aided by its slaves than the same species in
Switzerland, 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 precious wax in their
construction. It has been remarked that a skilful workman, with fitting
tools and measures, would find it very difficult to make cells of wax
of the true form, though this is 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 his theory. Let us look to the great
principle of gradation, and see whether Nature does not reveal to us
her method of work. At one end of a short series we have humble-bees,
which use their old cocoons to hold honey, sometimes adding to them
short tubes of wax, and likewise making separate and very irregular
rounded cells of wax. At the other end of the series we have the cells
of the hive-bee, placed in a double layer: each cell, as is well known,
is an hexagonal prism, with the basal edges of its six sides bevelled
so as to join 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 the square root of 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 sides 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 effected
this by gnawing away the convex side; and I suspect that the bees in
such cases stand in the opposed cells and push and bend the ductile and
warm wax (which as I have tried is easily done) into its proper
intermediate plane, and thus flatten it.
From the experiment of the ridge of vermilion wax, we can clearly see
that if the bees were to build for themselves a thin wall of wax, they
could make their cells of the proper shape, by standing at the proper
distance from each other, by excavating at the same rate, and by
endeavouring to make equal spherical hollows, but never allowing the
spheres to break into each other. Now bees, as may be clearly seen by
examining the edge of a growing comb, do make a rough, circumferential
wall or rim all round the comb; and they gnaw into this from the
opposite sides, always working circularly as they deepen each cell.
They do not make the whole three-sided pyramidal base of any one cell
at the same time, but only the one rhombic plate which stands on the
extreme growing margin, or the two plates, as the case may be; and they
never complete the upper edges of the rhombic plates, until the
hexagonal walls are commenced. Some of these statements differ from
those made by the justly celebrated elder Huber, but I am convinced of
their accuracy; and if I had space, I could show that they are
conformable with my theory.
Huber’s statement that the very first cell is excavated out of a little
parallel-sided wall of wax, is not, as far as I have seen, strictly
correct; the first commencement having always been a little hood of
wax; but I will not here enter on these details. We see how important a
part excavation plays in the construction of the cells; but it would be
a great error to suppose that the bees cannot build up a rough wall of
wax in the proper
position—that is, along the plane of intersection between two adjoining
spheres. I have several specimens showing clearly that they can do
this. Even in the rude circumferential rim or wall of wax round a
growing comb, flexures may sometimes be observed, corresponding in
position to the planes of the rhombic basal plates of future cells. But
the rough wall of wax has in every case to be finished off, by being
largely gnawed away on both sides. The manner in which the bees build
is curious; they always make the first rough wall from ten to twenty
times thicker than the excessively thin finished wall of the cell,
which will ultimately be left. We shall understand how they work, by
supposing masons first to pile up a broad ridge of cement, and then to
begin cutting it away equally on both sides near the ground, till a
smooth, very thin wall is left in the middle; the masons always piling
up the cut-away cement, and adding fresh cement, on the summit of the
ridge. We shall thus have a thin wall steadily growing upward; but
always crowned by a gigantic coping. From all the cells, both those
just commenced and those completed, being thus crowned by a strong
coping of wax, the bees can cluster and crawl over the comb without
injuring the delicate hexagonal walls, which are only about one
four-hundredth of an inch in thickness; the plates of the pyramidal
basis being about twice as thick. By this singular manner of building,
strength is continually given to the comb, with the utmost ultimate
economy of wax.
It seems at first to add to the difficulty of understanding how the
cells are made, that a multitude of bees all work together; one bee
after working a short time at one cell going to another, so that, as
Huber has stated, a score of individuals work even at the commencement
of the first cell. I was able practically to show this fact, by
covering the edges of the hexagonal walls
of a single cell, or the extreme margin of the circumferential rim of a
growing comb, with an extremely thin layer of melted vermilion wax; and
I invariably found that the colour was most delicately diffused by the
bees—as delicately as a painter could have done with his brush—by atoms
of the coloured wax having been taken from the spot on which it had
been placed, and worked into the growing edges of the cells all round.
The work of construction seems to be a sort of balance struck between
many bees, all instinctively standing at the same relative distance
from each other, all trying to sweep equal spheres, and then building
up, or leaving ungnawed, the planes of intersection between these
spheres. It was really curious to note in cases of difficulty, as when
two pieces of comb met at an angle, how often the bees would entirely
pull down and rebuild in different ways the same cell, sometimes
recurring to a shape which they had at first rejected.
When bees have a place on which they can stand in their proper
positions for working,—for instance, on a slip of wood, placed directly
under the middle of a comb growing downwards so that the comb has to be
built over one face of the slip—in this case the bees can lay the
foundations of one wall of a new hexagon, in its strictly proper place,
projecting beyond the other completed cells. It suffices that the bees
should be enabled to stand at their proper relative distances from each
other and from the walls of the last completed cells, and then, by
striking imaginary spheres, they can build up a wall intermediate
between two adjoining spheres; but, as far as I have seen, they never
gnaw away and finish off the angles of a cell till a large part both of
that cell and of the adjoining cells has been built. This capacity in
bees of laying down under certain circumstances a rough wall in its
proper place between two just-commenced
cells, is important, as it bears on a fact, which seems at first quite
subversive of the foregoing theory; namely, that the cells on the
extreme margin of wasp-combs are sometimes strictly hexagonal; but I
have not space here to enter on this subject. Nor does there seem to me
any great difficulty in a single insect (as in the case of a
queen-wasp) making hexagonal cells, if she work alternately on the
inside and outside of two or three cells commenced at the same time,
always standing at the proper relative distance from the parts of the
cells just begun, sweeping spheres or cylinders, and building up
intermediate planes. It is even conceivable that an insect might, by
fixing on a point at which to commence a cell, and then moving outside,
first to one point, and then to five other points, at the proper
relative distances from the central point and from each other, strike
the planes of intersection, and so make an isolated hexagon: but I am
not aware that any such case has been observed; nor would any good be
derived from a single hexagon being built, as in its construction more
materials would be required than for a cylinder.
As natural selection acts only by the accumulation of slight
modifications of structure or instinct, each profitable to the
individual under its conditions of life, it may reasonably be asked,
how a long and graduated succession of modified architectural
instincts, all tending towards the present perfect plan of
construction, could have profited the progenitors of the hive-bee? I
think the answer is not difficult: it is known that bees are often hard
pressed to get sufficient nectar; and I am informed by Mr. Tegetmeier
that it has been experimentally found that no less than from twelve to
fifteen pounds of dry sugar are consumed by a hive of bees for the
secretion of each pound of wax; so that a prodigious quantity of fluid
nectar must be collected and consumed by the bees in a hive for
the secretion of the wax necessary for the construction of their combs.
Moreover, many bees have to remain idle for many days during the
process of secretion. A large store of honey is indispensable to
support a large stock of bees during the winter; and the security of
the hive is known mainly to depend on a large number of bees being
supported. Hence the saving of wax by largely saving honey must be a
most important element of success in any family of bees. Of course the
success of any species of bee may be dependent on the number of its
parasites or other enemies, or on quite distinct causes, and so be
altogether independent of the quantity of honey which the bees could
collect. But let us suppose that this latter circumstance determined,
as it probably often does determine, the numbers of a humble-bee which
could exist in a country; and let us further suppose that the community
lived throughout the winter, and consequently required a store of
honey: there can in this case be no doubt that it would be an advantage
to our humble-bee, if a slight modification of her instinct led her to
make her waxen cells near together, so as to intersect a little; for a
wall in common even to two adjoining cells, would save some little wax.
Hence it would continually be more and more advantageous to our
humble-bee, if she were to make her cells more and more regular, nearer
together, and aggregated into a mass, like the cells of the Melipona;
for in this case a large part of the bounding surface of each cell
would serve to bound other cells, and much wax would be saved. Again,
from the same cause, it would be advantageous to the Melipona, if she
were to make her cells closer together, and more regular in every way
than at present; for then, as we have seen, the spherical surfaces
would wholly disappear, and would all be replaced by plane surfaces;
and the Melipona
would make a comb as perfect as that of the hive-bee. Beyond this stage
of perfection in architecture, natural selection could not lead; for
the comb of the hive-bee, as far as we can see, is absolutely perfect
in economising wax.
Thus, as I believe, the most wonderful of all known instincts, that of
the hive-bee, can be explained by natural selection having taken
advantage of numerous, successive, slight modifications of simpler
instincts; natural selection having by slow degrees, more and more
perfectly, led the bees to sweep equal spheres at a given distance from
each other in a double layer, and to build up and excavate the wax
along the planes of intersection. The bees, of course, no more knowing
that they swept their spheres at one particular distance from each
other, than they know what are the several angles of the hexagonal
prisms and of the basal rhombic plates. The motive power of the process
of natural selection having been economy of wax; that individual swarm
which wasted least honey in the secretion of wax, having succeeded
best, and having transmitted by inheritance its newly acquired
economical instinct to new swarms, which in their turn will have had
the best chance of succeeding in the struggle for existence.
No doubt many instincts of very difficult explanation could be opposed
to the theory of natural selection,—cases, in which we cannot see how
an instinct could possibly have originated; cases, in which no
intermediate gradations are known to exist; cases of instinct of
apparently such trifling importance, that they could hardly have been
acted on by natural selection; cases of instincts almost identically
the same in animals so remote in the scale of nature, that we cannot
account
for their similarity by inheritance from a common parent, and must
therefore believe that they have been acquired by independent acts of
natural selection. I will not here enter on these several cases, but
will confine myself to one special difficulty, which at first appeared
to me insuperable, and actually fatal to my whole theory. I allude to
the neuters or sterile females in insect-communities: for these neuters
often differ widely in instinct and in structure from both the males
and fertile females, and yet, from being sterile, they cannot propagate
their kind.
The subject well deserves to be discussed at great length, but I will
here take only a single case, that of working or sterile ants. How the
workers have been rendered sterile is a difficulty; but not much
greater than that of any other striking modification of structure; for
it can be shown that some insects and other articulate animals in a
state of nature occasionally become sterile; and if such insects had
been social, and it had been profitable to the community that a number
should have been annually born capable of work, but incapable of
procreation, I can see no very great difficulty in this being effected
by natural selection. But I must pass over this preliminary difficulty.
The great difficulty lies in the working ants differing widely from
both the males and the fertile females in structure, as in the shape of
the thorax and in being destitute of wings and sometimes of eyes, and
in instinct. As far as instinct alone is concerned, the prodigious
difference in this respect between the workers and the perfect females,
would have been far better exemplified by the hive-bee. If a working
ant or other neuter insect had been an animal in the ordinary state, I
should have unhesitatingly assumed that all its characters had been
slowly acquired through natural selection; namely, by an individual
having been born with some slight profitable modification of structure,
this being inherited by its offspring, which again varied and were
again selected, and so onwards. But with the working ant we have an
insect differing greatly from its parents, yet absolutely sterile; so
that it could never have transmitted successively acquired
modifications of structure or instinct to its progeny. It may well be
asked how is it possible to reconcile this case with the theory of
natural selection?
First, let it be remembered that we have innumerable instances, both in
our domestic productions and in those in a state of nature, of all
sorts of differences of structure which have become correlated to
certain ages, and to either sex. We have differences correlated not
only to one sex, but to that short period alone when the reproductive
system is active, as in the nuptial plumage of many birds, and in the
hooked jaws of the male salmon. We have even slight differences in the
horns of different breeds of cattle in relation to an artificially
imperfect state of the male sex; for oxen of certain breeds have longer
horns than in other breeds, in comparison with the horns of the bulls
or cows of these same breeds. Hence I can see no real difficulty in any
character having become correlated with the sterile condition of
certain members of insect-communities: the difficulty lies in
understanding how such correlated modifications of structure could have
been slowly accumulated by natural selection.
This difficulty, though appearing insuperable, is lessened, or, as I
believe, disappears, when it is remembered that selection may be
applied to the family, as well as to the individual, and may thus gain
the desired end. Thus, a well-flavoured vegetable is cooked, and the
individual is destroyed; but the horticulturist sows seeds of the same
stock, and confidently expects to
get nearly the same variety; breeders of cattle wish the flesh and fat
to be well marbled together; the animal has been slaughtered, but the
breeder goes with confidence to the same family. I have such faith in
the powers of selection, that I do not doubt that a breed of cattle,
always yielding oxen with extraordinarily long horns, could be slowly
formed by carefully watching which individual bulls and cows, when
matched, produced oxen with the longest horns; and yet no one ox could
ever have propagated its kind. Thus I believe it has been with social
insects: a slight modification of structure, or instinct, correlated
with the sterile condition of certain members of the community, has
been advantageous to the community: consequently the fertile males and
females of the same community flourished, and transmitted to their
fertile offspring a tendency to produce sterile members having the same
modification. And I believe that this process has been repeated, until
that prodigious amount of difference between the fertile and sterile
females of the same species has been produced, which we see in many
social insects.
But we have not as yet touched on the climax of the difficulty; namely,
the fact that the neuters of several ants differ, not only from the
fertile females and males, but from each other, sometimes to an almost
incredible degree, and are thus divided into two or even three castes.
The castes, moreover, do not generally graduate into each other, but
are perfectly well defined; being as distinct from each other, as are
any two species of the same genus, or rather as any two genera of the
same family. Thus in Eciton, there are working and soldier neuters,
with jaws and instincts extraordinarily different: in Cryptocerus, the
workers of one caste alone carry a wonderful sort of shield on their
heads, the use of which is quite unknown: in the Mexican Myrmecocystus,
the workers of one caste never leave the nest; they are fed by the
workers of another caste, and they have an enormously developed abdomen
which secretes a sort of honey, supplying the place of that excreted by
the aphides, or the domestic cattle as they may be called, which our
European ants guard or imprison.
It will indeed be thought that I have an overweening confidence in the
principle of natural selection, when I do not admit that such wonderful
and well-established facts at once annihilate my theory. In the simpler
case of neuter insects all of one caste or of the same kind, which have
been rendered by natural selection, as I believe to be quite possible,
different from the fertile males and females,—in this case, we may
safely conclude from the analogy of ordinary variations, that each
successive, slight, profitable modification did not probably at first
appear in all the individual neuters in the same nest, but in a few
alone; and that by the long-continued selection of the fertile parents
which produced most neuters with the profitable modification, all the
neuters ultimately came to have the desired character. On this view we
ought occasionally to find neuter-insects of the same species, in the
same nest, presenting gradations of structure; and this we do find,
even often, considering how few neuter-insects out of Europe have been
carefully examined. Mr. F. Smith has shown how surprisingly the neuters
of several British ants differ from each other in size and sometimes in
colour; and that the extreme forms can sometimes be perfectly linked
together by individuals taken out of the same nest: I have myself
compared perfect gradations of this kind. It often happens that the
larger or the smaller sized workers are the most numerous; or that both
large and small are numerous, with those of an intermediate size scanty
in numbers. Formica flava has larger and
smaller workers, with some of intermediate size; and, in this species,
as Mr. F. Smith has observed, the larger workers have simple eyes
(ocelli), which though small can be plainly distinguished, whereas the
smaller workers have their ocelli rudimentary. Having carefully
dissected several specimens of these workers, I can affirm that the
eyes are far more rudimentary in the smaller workers than can be
accounted for merely by their proportionally lesser size; and I fully
believe, though I dare not assert so positively, that the workers of
intermediate size have their ocelli in an exactly intermediate
condition. So that we here have two bodies of sterile workers in the
same nest, differing not only in size, but in their organs of vision,
yet connected by some few members in an intermediate condition. I may
digress by adding, that if the smaller workers had been the most useful
to the community, and those males and females had been continually
selected, which produced more and more of the smaller workers, until
all the workers had come to be in this condition; we should then have
had a species of ant with neuters very nearly in the same condition
with those of Myrmica. For the workers of Myrmica have not even
rudiments of ocelli, though the male and female ants of this genus have
well-developed ocelli.
I may give one other case: so confidently did I expect to find
gradations in important points of structure between the different
castes of neuters in the same species, that I gladly availed myself of
Mr. F. Smith’s offer of numerous specimens from the same nest of the
driver ant (Anomma) of West Africa. The reader will perhaps best
appreciate the amount of difference in these workers, by my giving not
the actual measurements, but a strictly accurate illustration: the
difference was the same as if we were to see a set of workmen building
a house of whom many were five feet four inches high, and many sixteen
feet high; but we must suppose that the larger workmen had heads four
instead of three times as big as those of the smaller men, and jaws
nearly five times as big. The jaws, moreover, of the working ants of
the several sizes differed wonderfully in shape, and in the form and
number of the teeth. But the important fact for us is, that though the
workers can be grouped into castes of different sizes, yet they
graduate insensibly into each other, as does the widely-different
structure of their jaws. I speak confidently on this latter point, as
Mr. Lubbock made drawings for me with the camera lucida of the jaws
which I had dissected from the workers of the several sizes.
With these facts before me, I believe that natural selection, by acting
on the fertile parents, could form a species which should regularly
produce neuters, either all of large size with one form of jaw, or all
of small size with jaws having a widely different structure; or lastly,
and this is our climax of difficulty, one set of workers of one size
and structure, and simultaneously another set of workers of a different
size and structure;—a graduated series having been first formed, as in
the case of the driver ant, and then the extreme forms, from being the
most useful to the community, having been produced in greater and
greater numbers through the natural selection of the parents which
generated them; until none with an intermediate structure were
produced.
Thus, as I believe, the wonderful fact of two distinctly defined castes
of sterile workers existing in the same nest, both widely different
from each other and from their parents, has originated. We can see how
useful their production may have been to a social community of insects,
on the same principle that the division of
labour is useful to civilised man. As ants work by inherited instincts
and by inherited tools or weapons, and not by acquired knowledge and
manufactured instruments, a perfect division of labour could be
effected with them only by the workers being sterile; for had they been
fertile, they would have intercrossed, and their instincts and
structure would have become blended. And nature has, as I believe,
effected this admirable division of labour in the communities of ants,
by the means of natural selection. But I am bound to confess, that,
with all my faith in this principle, I should never have anticipated
that natural selection could have been efficient in so high a degree,
had not the case of these neuter insects convinced me of the fact. I
have, therefore, discussed this case, at some little but wholly
insufficient length, in order to show the power of natural selection,
and likewise because this is by far the most serious special
difficulty, which my theory has encountered. The case, also, is very
interesting, as it proves that with animals, as with plants, any amount
of modification in structure can be effected by the accumulation of
numerous, slight, and as we must call them accidental, variations,
which are in any manner profitable, without exercise or habit having
come into play. For no amount of exercise, or habit, or volition, in
the utterly sterile members of a community could possibly have affected
the structure or instincts of the fertile members, which alone leave
descendants. I am surprised that no one has advanced this demonstrative
case of neuter insects, against the well-known doctrine of Lamarck.
_Summary_.—I have endeavoured briefly in this chapter to show that the
mental qualities of our domestic animals vary, and that the variations
are inherited. Still more briefly I have attempted to show that
instincts
vary slightly in a state of nature. No one will dispute that instincts
are of the highest importance to each animal. Therefore I can see no
difficulty, under changing conditions of life, in natural selection
accumulating slight modifications of instinct to any extent, in any
useful direction. In some cases habit or use and disuse have probably
come into play. I do not pretend that the facts given in this chapter
strengthen in any great degree my theory; but none of the cases of
difficulty, to the best of my judgment, annihilate it. On the other
hand, the fact that instincts are not always absolutely perfect and are
liable to mistakes;—that no instinct has been produced for the
exclusive good of other animals, but that each animal takes advantage
of the instincts of others;—that the canon in natural history, of
“natura non facit saltum” is applicable to instincts as well as to
corporeal structure, and is plainly explicable on the foregoing views,
but is otherwise inexplicable,—all tend to corroborate the theory of
natural selection.
This theory is, also, strengthened by some few other facts in regard to
instincts; as by that common case of closely allied, but certainly
distinct, species, when inhabiting distant parts of the world and
living under considerably different conditions of life, yet often
retaining nearly the same instincts. For instance, we can understand on
the principle of inheritance, how it is that the thrush of South
America lines its nest with mud, in the same peculiar manner as does
our British thrush: how it is that the male wrens (Troglodytes) of
North America, build “cock-nests,” to roost in, like the males of our
distinct Kitty-wrens,—a habit wholly unlike that of any other known
bird. Finally, it may not be a logical deduction, but to my imagination
it is far more satisfactory to look at such instincts as the young
cuckoo ejecting its foster-brothers,—ants making slaves,—the larvæ of
ichneumonidæ feeding within the live bodies of caterpillars,—not as
specially endowed or created instincts, but as small consequences of
one general law, leading to the advancement of all organic beings,
namely, multiply, vary, let the strongest live and the weakest die.