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. {208} 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. {209}
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 {210} 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. 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
{211} 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 be 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
{212} 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 the
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 {213} 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 {214} 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: the act of
pointing is probably, as many have thought, only the exaggerated pause of
an animal preparing to spring on its prey. When the first tendency to point
was once displayed, methodical selection and the inherited effects of
compulsory training in each successive generation would soon complete the
{215} 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 {216} 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 {217} 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,
{218} 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
{219} 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 larvae 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.
Another species, 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 {220}
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 (F. fusca) 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,
through 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 {221} 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 witnessed a migration of F. sanguinea from one nest
to another, and it was a most interesting spectacle to behold the masters
carefully carrying (instead of being carried by, as in the case of F.
rufescens) 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. {222}
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 returning home and entering their nests, carrying the dead
bodies of F. fusca (showing that it was not a migration) and numerous pupæ.
I traced a long file of ants 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
{223} agitation, and one was perched motionless with its own pupa in its
mouth on the top of a spray of heath, an image of despair, 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
continental 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 such pupæ originally stored as food might become developed;
and the foreign ants thus unintentionally reared would then follow their
proper instincts, and do {224} 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 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 {225} 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 fit 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
{226} 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 bases 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 ×
[root]2, or radius × 1.41421 (or at some lesser distance), from the centres
of the six surrounding spheres in the same {227} 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 {228} 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 scalloped 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
{229} 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
{230} 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 {231} 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,
{232} 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 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 {233} 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 {234} are consumed by a hive of bees
for the secretion of each pound of wax; to 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 {235} 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 {236} 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 {237}
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 {238}
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 {239} 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 {240} 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 {241} 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 {242} 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 organs or tools, 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 affect 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 {243} 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 {244} 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 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.
* * * * *
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