On the Origin of Species by Means of Natural Selection
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Chapter VII
Instinct
Instincts comparable with habits, but different in their origin --
Instincts graduated -- Aphides and ants -- Instincts variable -- Domestic
instincts, their origin -- Natural instincts of the cuckoo, ostrich, and
parasitic bees -- Slave-making ants -- Hive-bee, its cell-making instinct -
- Difficulties on the theory of the Natural Selection of instincts --
Neuter or sterile insects -- Summary.
The subject of instinct might have been worked into the previous chapters;
but I have thought that it would be more convenient to treat the subject
separately, especially as so wonderful an instinct as that of the hive-bee
making its cells will probably have occurred to many readers, as a
difficulty sufficient to overthrow my whole theory. I must premise, that I
have nothing to do with the origin of the primary mental powers, any more
than I have with that of life itself. We are concerned only with the
diversities of instinct and of the other mental qualities of animals within
the same class.
I will not attempt any definition of instinct. It would be easy to show
that several distinct mental actions are commonly embraced by this term;
but every one understands what is meant, when it is said that instinct
impels the cuckoo to migrate and to lay her eggs in other birds' nests. An
action, which we ourselves should require experience to enable us to
perform, when performed by an animal, more especially by a very young one,
without any experience, and when performed by many individuals in the same
way, without their knowing for what purpose it is performed, is usually
said to be instinctive. But I could show that none of these characters of
instinct are universal. A little dose, as Pierre Huber expresses it, of
judgment or reason, often comes into play, even in animals very low in the
scale of nature.
Frederick Cuvier and several of the older metaphysicians have compared
instinct with habit. This comparison gives, I think, a remarkably accurate
notion of the frame of mind under which an instinctive action is performed,
but not of its origin. How unconsciously many habitual actions are
performed, indeed not rarely in direct opposition to our conscious will!
yet they may be modified by the will or reason. Habits easily become
associated with other habits, and with certain periods of time and states
of the body. When once acquired, they often remain constant throughout
life. Several other points of resemblance between instincts and habits
could be pointed out. As in repeating a well-known song, so in instincts,
one action follows another by a sort of rhythm; if a person be interrupted
in a song, or in repeating anything by rote, he is generally forced to go
back to recover the habitual train of thought: so P. Huber found it was
with a caterpillar, which makes a very complicated hammock; for if he took
a caterpillar which had completed its hammock up to, say, the sixth stage
of construction, and put it into a hammock completed up only to the third
stage, the caterpillar simply re-performed the fourth, fifth, and sixth
stages of construction. If, however, a caterpillar were taken out of a
hammock made up, for instance, to the third stage, and were put into one
finished up to the sixth stage, so that much of its work was already done
for it, far from feeling the benefit of this, it was much embarrassed, and,
in order to complete its hammock, seemed forced to start from the third
stage, where it had left off, and thus tried to complete the already
finished work. If we suppose any habitual action to become inherited--and
I think it can be shown that this does sometimes happen--then the
resemblance between what originally was a habit and an instinct becomes so
close as not to be distinguished. If Mozart, instead of playing the
pianoforte at three years old with wonderfully little practice, had played
a tune with no practice at all, be might truly be said to have done so
instinctively. But it would be the most serious error to suppose that the
greater number of instincts have been acquired by habit in one generation,
and then transmitted by inheritance to succeeding generations. It can be
clearly shown that the most wonderful instincts with which we are
acquainted, namely, those of the hive-bee and of many ants, could not
possibly have been thus acquired.
It will be universally admitted that instincts are as important as
corporeal structure for the welfare of each species, under its present
conditions of life. Under changed conditions of life, it is at least
possible that slight modifications of instinct might be profitable to a
species; and if it can be shown that instincts do vary ever so little, then
I can see no difficulty in natural selection preserving and continually
accumulating variations of instinct to any extent that may be profitable.
It is thus, as I believe, that all the most complex and wonderful instincts
have originated. As modifications of corporeal structure arise from, and
are increased by, use or habit, and are diminished or lost by disuse, so I
do not doubt it has been with instincts. But I believe that the effects of
habit are of quite subordinate importance to the effects of the natural
selection of what may be called accidental variations of instincts;--that
is of variations produced by the same unknown causes which produce slight
deviations of bodily structure.
No complex instinct can possibly be produced through natural selection,
except by the slow and gradual accumulation of numerous, slight, yet
profitable, variations. Hence, as in the case of corporeal structures, we
ought to find in nature, not the actual transitional gradations by which
each complex instinct has been acquired--for these could be found only in
the lineal ancestors of each species--but we ought to find in the
collateral lines of descent some evidence of such gradations; or we ought
at least to be able to show that gradations of some kind are possible; and
this we certainly can do. I have been surprised to find, making allowance
for the instincts of animals having been but little observed except in
Europe and North America, and for no instinct being known amongst extinct
species, how very generally gradations, leading to the most complex
instincts, can be discovered. The canon of 'Natura non facit saltum'
applies with almost equal force to instincts as to bodily organs. Changes
of instinct may sometimes be facilitated by the same species having
different instincts at different periods of life, or at different seasons
of the year, or when placed under different circumstances, &c.; in which
case either one or the other instinct might be preserved by natural
selection. And such instances of diversity of instinct in the same species
can be shown to occur in nature.
Again as in the case of corporeal structure, and conformably with my
theory, the instinct of each species is good for itself, but has never, as
far as we can judge, been produced for the exclusive good of others. One
of the strongest instances of an animal apparently performing an action for
the sole good of another, with which I am acquainted, is that of aphides
voluntarily yielding their sweet excretion to ants: that they do so
voluntarily, the following facts show. I removed all the ants from a group
of about a dozen aphides on a dock-plant, and prevented their attendance
during several hours. After this interval, I felt sure that the aphides
would want to excrete. I watched them for some time through a lens, but
not one excreted; I then tickled and stroked them with a hair in the same
manner, as well as I could, as the ants do with their antennae; but not one
excreted. Afterwards I allowed an ant to visit them, and it immediately
seemed, by its eager way of running about, to be well aware what a rich
flock it had discovered; it then began to play with its antennae on the
abdomen first of one aphis and then of another; and each aphis, as soon as
it felt the antennae, immediately lifted up its abdomen and excreted a
limpid drop of sweet juice, which was eagerly devoured by the ant. Even
the quite young aphides behaved in this manner, showing that the action was
instinctive, and not the result of experience. But as the excretion is
extremely viscid, it is probably a convenience to the aphides to have it
removed; and therefore probably the aphides do not instinctively excrete
for the sole good of the ants. Although I do not believe that any animal
in the world performs an action for the exclusive good of another of a
distinct species, yet each species tries to take advantage of the instincts
of others, as each takes advantage of the weaker bodily structure of
others. So again, in some few cases, certain instincts cannot be
considered as absolutely perfect; but as details on this and other such
points are not indispensable, they may be here passed over.
As some degree of variation in instincts under a state of nature, and the
inheritance of such variations, are indispensable for the action of natural
selection, as many instances as possible ought to have been here given; but
want of space prevents me. I can only assert, that instincts certainly do
vary--for instance, the migratory instinct, both in extent and direction,
and in its total loss. So it is with the nests of birds, which vary partly
in dependence on the situations chosen, and on the nature and temperature
of the country inhabited, but often from causes wholly unknown to us:
Audubon has given several remarkable cases of differences in nests of the
same species in the northern and southern United States. Fear of any
particular enemy is certainly an instinctive quality, as may be seen in
nestling birds, though it is strengthened by experience, and by the sight
of fear of the same enemy in other animals. But fear of man is slowly
acquired, as I have elsewhere shown, by various animals inhabiting desert
islands; and we may see an instance of this, even in England, in the
greater wildness of all our large birds than of our small birds; for the
large birds have been most persecuted by man. We may safely attribute the
greater wildness of our large birds to this cause; for in uninhabited
islands large birds are not more fearful than small; and the magpie, so
wary in England, is tame in Norway, as is the hooded crow in Egypt.
That the general disposition of individuals of the same species, born in a
state of nature, is extremely diversified, can be shown by a multitude of
facts. Several cases also, could be given, of occasional and strange
habits in certain species, which might, if advantageous to the species,
give rise, through natural selection, to quite new instincts. But I am
well aware that these general statements, without facts given in detail,
can produce but a feeble effect on the reader's mind. I can only repeat my
assurance, that I do not speak without good evidence.
The possibility, or even probability, of inherited variations of instinct
in a state of nature will be strengthened by briefly considering a few
cases under domestication. We shall thus also be enabled to see the
respective parts which habit and the selection of so-called accidental
variations have played in modifying the mental qualities of our domestic
animals. A number of curious and authentic instances could be given of the
inheritance of all shades of disposition and tastes, and likewise of the
oddest tricks, associated with certain frames of mind or periods of time.
But let us look to the familiar case of the several breeds of dogs: it
cannot be doubted that young pointers (I have myself seen a striking
instance) will sometimes point and even back other dogs the very first time
that they are taken out; retrieving is certainly in some degree inherited
by retrievers; and a tendency to run round, instead of at, a flock of
sheep, by shepherd-dogs. I cannot see that these actions, performed
without experience by the young, and in nearly the same manner by each
individual, performed with eager delight by each breed, and without the end
being known,--for the young pointer can no more know that he points to aid
his master, than the white butterfly knows why she lays her eggs on the
leaf of the cabbage,--I cannot see that these actions differ essentially
from true instincts. If we were to see one kind of wolf, when young and
without any training, as soon as it scented its prey, stand motionless like
a statue, and then slowly crawl forward with a peculiar gait; and another
kind of wolf rushing round, instead of at, a herd of deer, and driving them
to a distant point, we should assuredly call these actions instinctive.
Domestic instincts, as they may be called, are 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.
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