Sunday, October 27, 2013

On the Origin of Species

CHAPTER 7.
INSTINCT.
Instincts comparable with habits, but different in their origin. Instincts graduated. Aphides and ants. Instincts
variable. Domestic instincts, their origin. Natural instincts of the cuckoo, ostrich, and parasitic bees.
Slave-making ants. Hive-bee, its cell-making instinct. Difficulties on the theory of the Natural Selection of
instincts. Neuter or sterile insects. Summary.
The subject of instinct might have been worked into the previous chapters; but I have thought that it would be
more convenient to treat the subject separately, especially as so wonderful an instinct as that of the hive-bee
making its cells will probably have occurred to many readers, as a difficulty sufficient to overthrow my whole
theory. I must premise, that I have nothing to do with the origin of the primary mental powers, any more than
I have with that of life itself. We are concerned only with the diversities of instinct and of the other mental
qualities of animals within the same class.
I will not attempt any definition of instinct. It would be easy to show that several distinct mental actions are
commonly embraced by this term; but every one understands what is meant, when it is said that instinct
impels the cuckoo to migrate and to lay her eggs in other birds' nests. An action, which we ourselves should
require experience to enable us to perform, when performed by an animal, more especially by a very young
one, without any experience, and when performed by many individuals in the same way, without their
knowing for what purpose it is performed, is usually said to be instinctive. But I could show that none of these
characters of instinct are universal. A little dose, as Pierre Huber expresses it, of judgment or reason, often
comes into play, even in animals very low in the scale of nature.
Frederick Cuvier and several of the older metaphysicians have compared instinct with habit. This comparison
gives, I think, a remarkably accurate notion of the frame of mind under which an instinctive action is
performed, but not of its origin. How unconsciously many habitual actions are performed, indeed not rarely in
direct opposition to our conscious will! yet they may be modified by the will or reason. Habits easily become
associated with other habits, and with certain periods of time and states of the body. When once acquired, they
often remain constant throughout life. Several other points of resemblance between instincts and habits could
be pointed out. As in repeating a well-known song, so in instincts, one action follows another by a sort of
rhythm; if a person be interrupted in a song, or in repeating anything by rote, he is generally forced to go back
to recover the habitual train of thought: so P. Huber found it was with a caterpillar, which makes a very
complicated hammock; for if he took a caterpillar which had completed its hammock up to, say, the sixth
stage of construction, and put it into a hammock completed up only to the third stage, the caterpillar simply
re-performed the fourth, fifth, and sixth stages of construction. If, however, a caterpillar were taken out of a
hammock made up, for instance, to the third stage, and were put into one finished up to the sixth stage, so that
much of its work was already done for it, far from feeling the benefit of this, it was much embarrassed, and, in
order to complete its hammock, seemed forced to start from the third stage, where it had left off, and thus tried
to complete the already finished work. If we suppose any habitual action to become inherited--and I think it
can be shown that this does sometimes happen--then the resemblance between what originally was a habit and
an instinct becomes so close as not to be distinguished. If Mozart, instead of playing the pianoforte at three
years old with wonderfully little practice, had played a tune with no practice at all, he might truly be said to
have done so instinctively. But it would be the most serious error to suppose that the greater number of
instincts have been acquired by habit in one generation, and then transmitted by inheritance to succeeding
generations. It can be clearly shown that the most wonderful instincts with which we are acquainted, namely,
those of the hive-bee and of many ants, could not possibly have been thus acquired.
It will be universally admitted that instincts are as important as corporeal structure for the welfare of each
species, under its present conditions of life. Under changed conditions of life, it is at least possible that slight
modifications of instinct might be profitable to a species; and if it can be shown that instincts do vary ever so
little, then I can see no difficulty in natural selection preserving and continually accumulating variations of
instinct to any extent that may be profitable. It is thus, as I believe, that all the most complex and wonderful
instincts have originated. As modifications of corporeal structure arise from, and are increased by, use or
habit, and are diminished or lost by disuse, so I do not doubt it has been with instincts. But I believe that the
effects of habit are of quite subordinate importance to the effects of the natural selection of what may be
called accidental variations of instincts;--that is of variations produced by the same unknown causes which
produce slight deviations of bodily structure.
No complex instinct can possibly be produced through natural selection, except by the slow and gradual
accumulation of numerous, slight, yet profitable, variations. Hence, as in the case of corporeal structures, we
ought to find in nature, not the actual transitional gradations by which each complex instinct has been
acquired--for these could be found only in the lineal ancestors of each species--but we ought to find in the
collateral lines of descent some evidence of such gradations; or we ought at least to be able to show that
gradations of some kind are possible; and this we certainly can do. I have been surprised to find, making
allowance for the instincts of animals having been but little observed except in Europe and North America,
and for no instinct being known amongst extinct species, how very generally gradations, leading to the most
complex instincts, can be discovered. The canon of "Natura non facit saltum" applies with almost equal force
to instincts as to bodily organs. Changes of instinct may sometimes be facilitated by the same species having
different instincts at different periods of life, or at different seasons of the year, or when placed under different
circumstances, etc.; in which case either one or the other instinct might be preserved by natural selection. And
such instances of diversity of instinct in the same species can be shown to occur in nature.
Again as in the case of corporeal structure, and conformably with my theory, the instinct of each species is
good for itself, but has never, as far as we can judge, been produced for the exclusive good of others. One of
the strongest instances of an animal apparently performing an action for the sole good of another, with which I
am acquainted, is that of aphides voluntarily yielding their sweet excretion to ants: that they do so voluntarily,
the following facts show. I removed all the ants from a group of about a dozen aphides on a dock-plant, and
prevented their attendance during several hours. After this interval, I felt sure that the aphides would want to
excrete. I watched them for some time through a lens, but not one excreted; I then tickled and stroked them
with a hair in the same manner, as well as I could, as the ants do with their 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.
The occasional habit of birds laying their eggs in other birds' nests, either of the same or of a distinct species,
is not very uncommon with the Gallinaceae; and this perhaps explains the origin of a singular instinct in the
allied group of ostriches. For several hen ostriches, at least in the case of the American species, unite and lay
first a few eggs in one nest and then in another; and these are hatched by the males. This instinct may
probably be accounted for by the fact of the hens laying a large number of eggs; but, as in the case of the
cuckoo, at intervals of two or three days. This instinct, however, of the American ostrich has not as yet been
perfected; for a surprising number of eggs lie strewed over the plains, so that in one day's hunting I picked up
no less than twenty lost and wasted eggs.
Many bees are parasitic, and always lay their eggs in the nests of bees of other kinds. This case is more
remarkable than that of the cuckoo; for these bees have not only their instincts but their structure modified in
accordance with their parasitic habits; for they do not possess the pollen-collecting apparatus which would be
necessary if they had to store food for their own young. Some species, likewise, of Sphegidae (wasp-like
insects) are parasitic on other species; and M. Fabre has lately shown good reason for believing that although
the Tachytes nigra generally makes its own burrow and stores it with paralysed prey for its own larvae to feed
on, yet that when this insect finds a burrow already made and stored by another sphex, it takes advantage of
the prize, and becomes for the occasion parasitic. In this case, as with the supposed case of the cuckoo, I can
see no difficulty in natural selection making an occasional habit permanent, if of advantage to the species, and
if the insect whose nest and stored food are thus feloniously appropriated, be not thus exterminated.

No comments:

Post a Comment