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be called spontaneous variations of instincts — that is of variations produced
by the same unknown causes which produce slight deviations of bodily
, structure.
4n|ij 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 among extinct species, how very
generally gradations, leading to the most complex instincts, can be dis-
covered. 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 the 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 to 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, as was first observed by Huber, 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. Afterward, 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,
as soon as it felt the antennae, immediately lifted up its abdomen and ex-
creted 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. It is certain,
from the observations of Huber, that the aphides show no dislike to the
ants: if the latter be not present they are at last compelled to eject their
excretion. But as the excretion is extremely viscid, it is no doubt a con-
venience to the aphides to have it removed; therefore probably they do not
excrete solely for the good of the ants. Although there is no evidence that
any animal performs an action for the exclusive good of another species,
yet each tries to take advantage of the instincts of others as each takes
advantage of the weaker bodily structure of other species. So again 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 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 m its
total loss. So it is with the nests of birds, which vary partly in independence
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. Why, it has been asked, if instinct be
variable, has it not granted to the bee "the ability to use some other
material when wax was deficient?" But what other natural material could
bees use? They will work, as I have seen, with wax hardened with vermilion
or softened with lard. Andrew Knight observed that his bees, instead of


laboriously collecting propolis, used a cement of wax and turpentine, with
which he had covered decorated trees. It has lately been shown that bees,
instead of searching for pollen, will gladly use a very different substance,
namely, oatmeal. Fear of any particular enemy is certainly an instinctive
quality, as may be seen in nestling birds; though it is strengthened by ex-
perience, and by the sight of fear of the same enemy in other animals. The
fear of man is slowly acquired, as I have elsewhere shown, by the various
animals which inhabit desert islands; and we see an instance of this even
in England, in the greater wildness of all our large birds in comparison with
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 mental qualities of animals of the same kind, born in a state
of nature, vary much, could be shown by many facts. Several cases could
also be adduced of occasional and strange habits in wild animals, which,
if advantageous to the species, might have given rise, through natural
selection, to new instincts. But I am well aware that these general state-
ments, without the facts in detail, will 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 be enabled to see the part which habit
and the selection of so-called spontaneous variations have played in modi-
fying the mental qualities of our domestic animals. It is notorious how
much domestic animals vary in their mental qualities. With cats, for in-
stance, one naturally takes to catching rats, and another mice, and these
tendencies are known to be inherited. One cat, according to Mr. St. John,
always brought home game birds, another hares or rabbits, and another
hunted on marshy ground and almost nightly caught woodcocks or snipes.
A number of curious and authentic instances could be given of various
shades of disposition and of taste, and likewise of the oddest tricks, associated
with certain frames of minds or periods of time, being inherited. But let us
look to the familiar case of the breeds of the dogs: it cannot be doubted
that young pointers (I have myself seen striking instances) 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 behold 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 than natural instincts; but they have been acted on by far
less rigorous selections, and have been transmitted for an incomparably
shorter period, under less fixed conditions of life.

How strongly these domestic instincts, habits, and dispositions are in-
herited, and how curiously they become mingled, is well shown when
diff'erent 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 be-
come inherited solely from long-continued and compulsory habit; but this
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 in-
dividuals 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 com-
pulsory training in each successive generation would soon complete the
work; and unconscious selection is still in progress, as each man tries to
procure, without intending to improve the breed, dogs which stand and
hunt best. On the other hand, habit alone in some cases has sufficed ; hardly
any 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 can
Jiardly suppose that domestic rabbits have often been selected for tameness


alone; so that we must attribute at least the greater part of the inherited
change from extreme wildness to extreme tameness, 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 largely and how permanently the minds of our domestic
animals have been modified. 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 civilized 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 and some degree of selection
have probably concurred in civilizing 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, for I am in-
formed by Captain Hutton that the young chickens of the parent stock,
the Gallus bankiva, when reared in India under a hen, are at first exces-
sively wild. So it is with young pheasants reared in England 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 under domestication 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 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 habit and selection
have probably concurred.


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, namely, the instinct which leads the cuckoo to lay her eggs in other
birds' nests; the slave-making instinct of certain ants; and the cell-making


power of the hive-bee. These two latter instincts have generally and justly-
been ranked by naturalists as the most wonderful of all known instincts.


It is supposed by some naturalists that the more immediate cause of the
instinct of the cuckoo 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 unin-
cubated, 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 migrates 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 both asserted and denied that the American cuckoo occa-
sionally lays her eggs in other birds' nests; but I have lately heard from Dr.
Merrill, of Iowa, that he once found in Illinois a young cuckoo, together with a
young jay, in the nest of a blue jay (Garrulus cristatus) ; and as both were
nearly full feathered, there could be no mistake in their identification. I
could also 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, and that she occasionally laid an egg in another bird's
nest. If the old bird profited by this occasional habit through being enabled
to emigrate earlier or through any other cause; or if the young were made
more vigorous by advantage being taken of the mistaken instinct of another
species than when reared by their own mother, encumbered as she could
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 us 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 more successful in rearing their young. By a continued
process of this nature, I believe that the strange instinct of our cuckoo has
been generated. It has, also, recently been ascertained on sufficient evidence,
by Adolf Miiller, that the cuckoo occasionally lays her eggs on the bare
ground, sits on them and feeds her young. This rare event is probably the
case of reversion to the long-lost, aboriginal instinct of nidification.

It has been objected that I have not noticed other related instincts and
adaptations of structure in the cuckoo, which are spoken of as necessarily
co-ordinated. But in all cases, speculation on an instinct known to us only
in a single species, is useless, for we have hitherto had no facts to guide us.
Until recently the instincts of the European and of the non-parasitic Ameri-
can cuckoo alone were known; now, owing to Mr. Ramsay's observations,
we have learned something about three Australian species, which lay their


eggs in other birds' nests. The chief points to be referred to are three:
first, that the common cuckoo, with rare exceptions, lays only one egg in a
nest, so that the large and voracious young bird receives ample food. Sec-
ondly, that the eggs are remarkably small, not exceeding those of the sky-
lark — a bird about one-fourth as large as the cuckoo. That the small size
of the egg is a real case of adaptation we may infer from the fact of the
non-parasitic American cuckoo laying full-sized eggs. Thirdly, that the
young cuckoo, soon after birth, has the instinct, the strength, and a properly
shaped back for ejecting its foster-brothers, which then perish from cold
and hunger. This has been boldly called a beneficent arrangement, in order
that the young cuckoo may get sufficient food, and that its foster-brothers
may perish before they had acquired much feeling!

Turning now to the Australian species: though these birds generally lay
only one egg in a nest, it is not rare to find two and even three eggs in the
same nest. In the bronze cuckoo the eggs vary greatly in size, from eight to
ten lines in length. Now, if it had been of an advantage to this species to
have laid eggs even smaller than those now laid, so as to have deceived
certain foster-parents, or, as is more probable, to have been hatched within
a shorter period (for it is asserted that there is a relation between the size
of eggs and the period of their incubation), then there is no difficulty in
believing that a race or species might have been formed which would have
laid smaller and smaller eggs; for these would have been more safely
hatched and reared. Mr. Ramsay remarks that two of the Australian
cuckoos, when they lay their eggs in an open nest, manifest a decided prefer-
ence for nests containing eggs similar in color to their own. The European
species apparently manifests some tendency toward a similar instinct, but
not rarely departs from it, as is shown by her laying her dull and pale-
colored eggs in the nest of the hedge-warbler with bright greenish-blue eggs.
Had our cuckoo invariably displayed the above instinct, it would assuredly
have been added to those which it is assumed must all have been acquired
together. The eggs of the Australian bronze cuckoo vary, according to Mr.
Ramsay, to an extraordinary degree in color; so that in this respect, as well
as in size, natural selection might have secured and fixed any advantageous

In the case of the European cuckoo, the offspring of the foster-parents
are commonly ejected from the nest within three days after the cuckoo is
hatched; and as the latter at this age is in a most helpless condition, Mr.
Gould was formerly inclined to believe that the act of ejection was per-
formed by the foster-parents themselves. But he has now received a trust-
worthy account of a young cuckoo which was actually seen, while still blind
and not able even to hold up its own head, in the act of ejecting its foster-
brothers. One of these was replaced in the nest by the observer, and was
again thrown out. With respect to the means by which this strange and
odious instinct was acquired, if it were of great importance for the young
cuckoo, as is probably the case, to receive as much food as possible soon
after birth, I can see no special difficulty in its having gradually acquired,


during successive generations, the blind desire, the strength, and structure
necessary for the work of ejection; for those cuckoos which had such habits
and structure best developed would be the most securely reared. The first
step toward the acquisition of the proper instinct might have been mere
unintentional restlessness on the part of the young bird, when somewhat
advanced in age and strength; the habit having been afterward improved,
and transmitted to an earlier age. I can see no more difficulty in this than
in the unhatched young of other birds acquiring the instinct to break
through their own shells; or than in young snakes acquiring in their upper
jaws, as Owen has remarked, a transitory sharp tooth for cutting through
the tough egg-shell. For if each part is liable to individual variations at all
ages, and the variations tend to be inherited at a corresponding or earlier
age — propositions which cannot be disputed — then the instincts and struc-
ture of the young could be slowly modified as surely as those of the adult;
and both cases must stand or fall together with the whole theory of natural

Some species of Molothrus, a widely distinct genus of American birds,
allied to our starlings, have parasitic habits like those of the cuckoo; and
the species present an interesting gradation in the perfection of their in-
stincts. The sexes of Molothrus badius are stated by an excellent observer,
Mr. Hudson, sometimes to live promiscuously together in flocks, and some-
times to pair. They either build a nest of their own or seize on one belong-
ing to some other bird, occasionally throwing out the nestlings of the
stranger. They either lay their eggs in the nest thus appropriated, or oddly
enough build one for themselves on the top of it. They usually sit on their
own eggs and rear their own young; but Mr. Hudson says it is probable
that they are occasionally parasitic, for he has seen the young of this species
following old birds of a distinct kind and clamoring to be fed by them. The
parasitic habits of another species of Molothrus, the M. bonariensis, are
much more highly developed than those of the last, but are still far from
perfect. This bird, as far as it is known, invariably lays its eggs in the nests
of strangers; but it is remarkable that several together sometimes com-
mence to build an irregular untidy nest of their own, placed in singular
ill-adapted situations, as on the leaves of a large thistle. They never, how-
ever, as far as Mr. Hudson has ascertained, complete a nest for themselves.
They often lay so many eggs — from fifteen to twenty — in the same foster-
nest, that few or none can possibly be hatched. They have, moreover, the
extraordinary habit of pecking holes in the eggs, whether of their own
species or of their foster-parents, which they find in the appropriated nests.
They drop also many eggs on the bare ground, which are thus wasted. A
third species, the M. pecoris of North America, has acquired instincts as
perfect as those of the cuckoo, for it never lays more than one egg in a
foster-nest, so that the young bird is securely reared. Mr. Hudson is a
strong disbeliever in evolution, but he appears to have been so much struck
by the imperfect instincts of the Molothrus bonariensis that he quotes my
words, and asks, "Must we consider these habits, not as especially endowed


or created instincts, but as small consequences of one general law, namely,

Various birds, as has already been remarked, occasionally lay their eggs
in the nests of other birds. This habit is not very uncommon with the

Online LibraryCharles DarwinThe origin of species → online text (page 24 of 50)