in an empty mollusc shell, and drags that about with it on the floor
of the sea, withdraws with lightning-like rapidity into its house
as soon as any suspicious movement catches its eye, and it is very
difficult to grip one of its legs with the forceps in time to draw it
out of its shell. The same is the case with the so-called Serpulids,
worms of the genus _Serpula_, and its allies; it is not easy to seize
them, because, however quick one is with the forceps, their instinct
of fugitive self-preservation acts more quickly still, and they shoot
back into their protecting tubes before one has had time to grasp them.
But this impulse to flee from enemies, though it seems almost a matter
of course, is by no means common to all animals, for in quite a large
number the instinct of self-preservation finds expression in an exactly
contrary manner, namely, in the so-called 'death-feigning,' that is,
remaining absolutely motionless in a definite position precisely
prescribed to the animal by its instinct. In speaking of protective
colouring, I drew attention to the 'wood-moth' (_Xylina_), which
resembles a broken fragment of half-decayed wood so deceptively, and
I pointed out that the colour-resemblance to wood would be in itself
of but little use to the insect if it were not combined with the
instinct to remain motionless in danger, to 'feign death.' The antennæ
and legs are drawn close to the body, so that they rather heighten
the disguise, and, instead of running away, the insect does not move
a muscle until the danger is past. This instinct must have evolved
hand in hand with the resemblance to a piece of wood, and, just as
we sought to interpret the latter from the fact that the moths which
most resembled the wood had always the best chance of surviving, so we
maintain that those moths would profit most by their resemblance which
drew in their legs and antennæ closely and lay most perfectly still.
Thus the brain-mechanism, which effected the keeping still whenever the
senses announced danger, would be more and more firmly established and
perfected in the course of selection.
Even nearly related animals may have quite different instincts
which secure them against danger. Thus in the group of pocket crabs
(Notopoda) there are some species which run away when danger threatens,
but others which anticipate the risk of discovery by masking themselves
to a certain extent. With the last pair of legs they hold over
themselves a large piece of sponge, which then grows till it often
leaves only the limbs and frontal region uncovered. Of course there
can be no question of consciousness in what the crab does, as is
proved by the fact that these crabs will, in case of necessity, take a
transparent piece of glass instead of the sponge; but the impulse to
cover themselves with something is strong in them, and finds expression
not only when they see a really protective substance, but even when
they see one which is transparent and therefore wholly useless for the
purpose. Crabs from which the sponge has been taken away wander about
until they find another; the impulse is thus set up not only by the
sight of the sponge or of a stone, but also by the feeling that their
back is uncovered. The large spider-crab of the Mediterranean (_Maja
squinado_) effects its disguise in a somewhat different manner. It has
peculiar hooked bristles on the back, and on these it hooks little
bunches of seaweed, often many of them, so that it is entirely covered
and looks like a bunch of wrack rather than like an animal. Here again
a bodily variation has gone hand in hand with the development of the
instinct to cover itself: the bristles of the back have become hooked.
Many instincts are accompanied by structural modifications, and in the
crabs which cover themselves with sponge or stone this is the case, for
the last pair of thoracic legs is turned towards the back, instead
of being set at the side of the body, as is usual among crabs. They
are thus enabled to hold the sponge much better and more permanently,
and as this is advantageous we may well ascribe the change to natural
selection.
Let us now turn our attention to another category of instincts, the
most common and most indispensable of all, those which lead to the
seeking and devouring of food.
The chicken just emerged from the egg picks up the seeds thrown to it
with no experience of what eating is, or what can be made to serve it
as food; its instinct for food expresses itself in picking up, and it
is awakened or stimulated to action by sight of the seeds. As Lloyd
Morgan in his excellent book on _Habit and Instinct_ well says, 'It
does not pick at the seeds because instinct says to it that this is
something to be picked up and tested, but because it cannot do anything
else.'
In the same way the instinct to seek for food wakes in the kitten at
the sight of a mouse. I once set before a kitten which had never seen
a mouse a living one in a trap. The kitten became greatly excited, and
when I opened the trap and let the mouse run away she overtook and
caught it in a few bounds. The instinct in this case does not express
itself as in the chicken by the rapid lowering of the head and seizing
the food, but in quite a different combination of movements, in running
after and grasping the fleeing victim. But that is not all that is
included in the instinctive action in the case of the cat, for there
is also the whole wild and gruesome play, the familiar letting go and
catching again, the passionate growling of satisfaction which, in its
wildness, reminds us much more of a blood-thirsty tiger than of a tame
domestic animal.
As the egg-laying instinct of the female butterfly is excited only by
the sight and odour of a particular plant, so also is the food instinct
of the caterpillar. If we put a silkworm caterpillar (_Bombyx mori_)
just out of the egg upon a mulberry leaf it will soon begin to gnaw at
it; but put it on a beech leaf or on that of any other indigenous tree,
shrub, or herb and it will not touch it, but simply die of hunger.
And yet it could quite well eat many of these leaves, and thrive on
them too, but the smell and perhaps also the sight of them is not the
appropriate stimulus to liberate the instinct of eating. There are
many species of caterpillar which are 'monophagous,' that is to say,
restricted to a single species of plant in a country. One may ask how
such a restriction of the liberating stimulus to a single species
could have been brought about by natural selection, since it could not
possibly be advantageous to be so much restricted in food. The answer
to this will be found in the following facts. On the Belladonna plant
there lives a little beetle whose feeding instinct is aroused by this
plant alone. But as _Atropa belladonna_ is avoided entirely by other
animals on account of its poisonousness, this beetle is, so to speak,
sole proprietor of the Belladonna; no other species disputes its food,
and in this there must assuredly be a great advantage, as soon as the
other instincts, above all that of egg-laying, are so regulated as to
secure that the larva shall have access to its food-plant; and this
is the case. The monophagy of many caterpillars is to be understood
in the same way; it is an adaptation to a plant otherwise little
sought after, and it is combined with a more or less complete loss
of sensitiveness to the stimulus of other plants. The establishment
of such a specialized food-instinct depends on its utility, and has
resulted from the preference given, through natural selection, to those
individuals in which the food-instinct responded to the stimulus of
the smallest possible number of plants, and at the same time to those
which showed themselves best adapted to a plant especially favourable
to their kind, whose food-instinct was not only most strongly excited
by this one plant, but also whose stomach and general metabolism made
the best use of it. So we understand why so many caterpillars live
on poisonous plants, not only some of our indigenous Sphingidæ, like
_Deilephila euphorbiæ_, but whole groups of tropical Papilionidæ,
Danaides, Acræides, and Heliconiidæ. With this again is connected the
poisonousness or nauseousness of these butterflies.
How diversely the instinct to procure food may be developed in one and
the same group of animals is shown by the fact that not infrequently
plant-eating, saphrophytic, and flesh-eating animals occur in a single
group of organisms, as, for instance, in the order of water-fleas or
Daphnidae, or in the class of Infusorians. Many species find their
food by making an eddy in the water, which brings a stream towards the
mouth, and with it all sorts of vegetable or dead particles; others
live by preying upon other animals like themselves.
But even when the food-instinct in all the species of a group is
directed towards living prey, the procuring of it may be achieved by
means of quite different instincts. Such finer graduations of the
food-instinct are found not infrequently within quite small groups of
animals, as in the Ephemeridæ or Day-flies. All their larvæ live by
preying on other animals, but those of one family, represented by the
genus _Chloëon_, seek to secure their victims by agility and speed,
while the larvæ of the second family, with the typical genus _Baëtis_,
have the instinct to press their smooth broad bodies, with large-eyed
head, close to the brook pebbles on which they sit. They are exactly
like these in colour, and thus they lurk almost invisible, until a
victim comes within their reach, when they throw themselves upon it
with a bound. The third group, with the typical genus _Ephemera_,
follows its instinct to dig deep tubes in the mud at the bottom of
the water, and to lurk in these for their prey. We have thus within
this small group of Day-flies three distinct modifications of the
food-instinct, which differ essentially from one another, are made
up of quite different combinations of actions, and, consequently,
must have their foundation in essentially different directive
brain-mechanisms. All these cases have only one feature in common; the
animals all throw themselves upon their prey as soon as they are near
enough.
[Illustration: FIG. 31. Sea-cucumber (_Cucumaria_), with expanded
tentacles (_a_), and protruded tube-feet (_b_); after Ludwig.]
But even this common feature is not everywhere part of the
food-instinct. The sea-cucumber (_Cucumaria_) (Fig. 31), according to
the observations made on it by Eisig in the Aquarium of the Zoological
Station at Naples, gets its food in the following manner. The animal
sits half or entirely erect on a projecting piece of rock and unfolds
its ten tree-like tentacles which surround the mouth. These are
branched, and have quite the effect of little tufts of seaweed. They
are probably taken for such by many minute animals; for larvæ of all
kinds, Infusorians, Rotifers and worms settle down on them. But the
sea-cucumber bends inwards first one tentacle and then another, so
slowly as barely to be noticeable, brings the point to its mouth, lets
it glide gradually deeper into the gullet, until the whole tentacle is
within, and after a time draws it out again equally slowly and unfolds
it anew. Obviously it wipes the tentacle inside the gullet, and retains
everything living that was upon it. This performance is repeated
continually, day and night, and it is usually the only externally
visible sign of life which the animal displays.
This remarkable instinct is associated with a structural peculiarity,
for without the arborescent tentacles the fishing would not be nearly
so successful. Other sea-cucumbers or Holothurians have different
tentacles, and use them in quite a different manner, filling the mouth
with mud by means of them.
Very frequently, indeed, there are visible structural changes
associated with the modified food-instinct. Most predatory fishes chase
their prey, like the pike, the perch, and the shark, but there are also
lurkers, and these show in addition to the lurking instinct certain
definite bodily adaptations, without which this instinct could not have
such full play.
Thus in a marine fish known as the 'star-gazer' (_Uranoscopus_) the
eyes are situated not on the sides but on the top of the head, and the
mouth is also directed upwards. Its instinct leads it to bury itself in
the sand so that only the eyes are uncovered. It lies in wait in this
way until a suitable victim comes within reach, and then snaps at it
with a sudden movement. But it also possesses a decoying organ, a soft
worm-shaped flap, which it protrudes from the mouth as soon as little
fishes draw near. They make for this bait, and are thus caught.
Such ingenious fishing, which is quite suggestive of the human method
of catching trout with artificial bait, occurs in many predatory
fishes; but in every case the fish acts instinctively, without
reflection, on becoming aware of approaching prey. The suitability of
the action to the end does not depend upon consciousness of the end,
or upon reflection, but is a purely mechanical action, performed in
response to the stimulus of a sense-impression.
This is best shown by the fact that the instincts may lead their
possessors astray, which always happens when an animal is transferred
to an unnatural situation, to which its instincts are not adapted, so
to speak. The mole-cricket, which is in the habit of escaping pursuit
by burrowing in the earth, makes violent motions with the forelegs,
even if it be placed upon a plate of glass into which it could not
possibly burrow; an ant-lion (_Myrmeleo_), whose instinct impels
it to bore into loose sand by pushing backwards with the abdomen,
goes backwards on a plate of glass as soon as danger threatens, and
endeavours, with the utmost exertions, to bore into it. It knows no
other mode of flight, and its intellect is much too weak to suggest any
novel mode. Even the mode of escape most universal among animals, that
of simply running away, does not occur to it; it acts as it must in
accordance with its inborn instinct; it cannot do otherwise.
The change of instincts in the different stages of development of
one and the same animal have always seemed to me very remarkable; for
instance, the change of the food-instinct in the caterpillar and the
butterfly, where the food-instinct is liberated in the caterpillar
by the leaf of a particular plant, but in the butterfly by the sight
and fragrance of a flower, the nectar of which it sucks. In this case
everything is different in the two stages of development, the whole
apparatus for seeking and taking food, as well as the nerve-mechanism
which determines these modes of action. And how far apart often are the
stimuli which liberate the instinct! The larva of the flower-visiting,
honey-sucking _Eristalis tenax_ is the ugly, white, so-called
rat-tailed larva, well described by Réaumur, which lives swimming
in liquid manure, and feeds on that! What complete and far-reaching
changes, not only in the visible structure, but also in the finer
nervous mechanisms, which we cannot yet verify, must have taken place
in the vicissitudes of time and circumstance during the life-history of
this insect!
[Illustration: FIG. 32. Metamorphosis of _Sitaris humeralis_, an
oil-beetle, after Fabre. _a_, first larval form, much enlarged. _b_,
second larval form. _c_, resting stage of this larva (so-called
'pseudo-pupa'). _d_, third larval form. _e_, pupa.]
Not the food-instinct alone, but the instinct of self-preservation,
of mode of motion, in short, every kind of instinct, may vary in the
course of an individual life. Let us follow the somewhat complex
life-history of a beetle of the family of the Blister-beetles or
Cantharides, as we learnt it first from Fabre. The female of the
red-shouldered bee-beetle (_Sitaris humeralis_) lays its eggs
on the ground in the neighbourhood of the underground nest of a
honey-gathering burrowing-bee (_Anthophora_). The larvæ, when they
emerge, are agile, six-legged, and furnished with a horny head and
biting mouth-parts, as well as with a tail-fork for springing (Fig. 32,
_a_). The little animals have at first no food-instinct, or at least
none manifests itself, but they run about, and as soon as they see a
bee of the genus _Anthophora_ they spring upon it and hide themselves
in its thick, hairy coat. If they have been fortunate the bee is a
female, who founds a new colony and builds cells, in each of which she
deposits some honey and lays an egg upon it. As soon as this has been
done the _Sitaris_ larva leaves its hiding-place, bites the egg of the
bee open, and gradually eats up the contents. Then it moults, and takes
the form of a grub with minute feet and imperfect masticating organs;
the tail-fork, too, is lost, for all these parts are now useless, since
it can obtain liquid nourishment without further change of place, from
the honey in the cell, in exactly the quantity necessary to its growth.
Then it spends the winter in a hardened, pupa-like skin, and it is
not till the next year (the third), after another short larval stage
(_d_) and subsequent true pupahood (_e_), that the fully-formed beetle
emerges. This again possesses biting mouth-parts, and eats leaves, and
has legs to run with and wings to fly with.
In this beetle, then, the food-instinct changes three times in the
course of its life; first the egg of the bee is the liberating
stimulus, then the honey, and finally leaves. The instinct of moving
about varies likewise, expressing itself first in running and jumping
and in catching on, then in lying still within the cell, and, lastly,
in flying and running about on bushes and trees.
We can well understand that, in the course of innumerable generations
and species of insects, the various stages of development would,
by means of selection, become more and more different from each
other, both structurally and in their instincts, as they adapted
themselves better to different conditions of life; and thus ultimately
instincts frequently and markedly divergent have been developed in
the successive stages of life. No other interpretation is possible;
through natural selection alone can we understand even the principle
of such adaptations. An animal can thus very well be compared to a
machine which is so arranged that it works correctly under all ordinary
circumstances, that is to say, it performs all the actions necessary to
the preservation of the individual and of its kind. The parts of the
machine are fitted together in the best possible way, and work on each
other so ingeniously that, under normal circumstances, a result suited
to the end is achieved. We have seen how precisely the liberating
stimulus for an action may be defined, and this secures a far-reaching
specialization of instincts. But as every machine can work only with
the material for which it was constructed, so the instinct can only
call forth an action effectively adjusted to its end when the animal
is under natural conditions. Its specialization has its limits, and
in this lies the reason of its limited purposiveness. For instance,
if the larva of _Sitaris_ were not impelled by the sight of every
bee to spring on it and cling to it, but only by the _females_, then
many of them would be saved from the fate that awaits them if they
attach themselves to male bees, which make no nest, or even to other
flying insects, in which case also there is no possibility of further
development. But both these things happen, although the latter has
not yet, to my knowledge, been recorded of _Sitaris_, but only of its
relative, the larva of _Melöe_.
'Instinct goes astray,' it is often said; but in truth it does not
go astray, but is only not so highly specialized in relation to the
liberating stimulus of the action as seems to us necessary for perfect
purposiveness. But in this very imperfection there lies, as it seems to
me, another proof that we have to do with the results of a process of
selection, for it is of the very nature of these never to be perfect,
but only relatively perfect, that is to say, just as perfect as is
necessary to the maintenance of the species. At the moment at which
this grade of perfection is reached every possibility of a further
increase in the effectiveness of adjustment to the end ceases, because
it would then no longer directly further the end. Why, for instance,
should the liberating stimulus in this case be more highly specialized,
since enough of the _Sitaris_ larvæ already succeed in attaching
themselves to female bees? It is not for nothing that the beetles of
this family are so prolific; what is lacking in the perfection of the
instinct is made up for by the multitude of young larvæ. A single
female of the oil-beetle (_Melöe_) lays several hundred eggs.
In speaking of the animal as a machine, it must be added that it
is a machine which can be altered in varying degrees, which can be
regulated to work at high or low pressure, slowly or quickly, finely or
roughly. This regulating is the work of the intelligence, the limited
'thinking-power,' which must be ascribed to the higher animals in a
very considerable degree, but which in the lower animals becomes less
and less apparent, until finally it is unrecognizable. That instinctive
actions can be modified or inhibited by intelligence and will is proved
by any trained beast of prey which masters its hunger and the impulse
to snap at the piece of flesh held before it, because it knows that
if it does not control itself painful blows will be the consequence.
In a later lecture I shall return to the connexion between will and
instinct; all that concerns us here is to regard instincts as the
outcome of the processes of selection, and as an indirect proof of the
reality of these.
From what I have already said at least so much must be clear, that
nothing, in principle, stands in the way of referring instincts to
selection, since their very essence is their adaptation to an end, and
such purposive changes are precisely those that are preserved in the
struggle for existence. It might, however, be supposed that in all this
the principle of use and disuse also had a share, and that without it
no changes in instincts could have come about.
There are, however, numerous instincts in considering which this can be
entirely excluded.
At an earlier stage we discussed in detail the protective colourings
which secure insects, and especially butterflies, from extermination
by their numerous enemies, and it was mentioned that this was always
accompanied by corresponding instincts, without which the protective
colouring and the deceptive form would have profited nothing, or at any
rate not nearly so much. If the caterpillar of the _Catocala sponsa_,
which resembles the bark of an oak so deceptively, did not possess at
the same time the instinct to creep away from the leaves and hide in
the clefts of the bark on the trunk of the oak-tree, its disguise would
be of very little use to it; and if the predatory and grass-coloured
praying mantis was not impelled by instinct to lie in wait among the
grass for its prey, instead of pursuing it, it would rarely succeed in
seizing any of its victims, because of its somewhat leisurely mode of
movement. This adaptation of the instincts to the protective colouring
is carried into the most minute and apparently trifling details. Thus
different observers have established the fact that the nauseous,
sometimes even poisonous, butterflies, which are distinguished by their
glaring or sharply contrasted colour-pattern, are all slow fliers.
This is the case with the Danaides and Euplœides of the Old World and
the Heliconiides of the New; many of their mimetic imitators also fly
slowly.