Part 18

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.

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!

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.

If we inquire how this instinct of fluttering, careless flight has come to be, we may leave habit as _primum movens_ out of the question altogether, for there are no external conditions which could have induced the butterfly to take to slower flight than its ancestors exhibited. That it is now advantageous for it--since it acts as a signal of its nauseousness--to be as clearly seen and recognized as possible can exercise no direct influence on its manner of flight, since it knows nothing about it. Even if we assume that individual variations cropped up which had an instinct for slower flight, there would still, without selection, be no reason why this variation in particular should multiply, still less why the originally slight slowing of the flight should become more marked in the course of generations. On the contrary, the butterflies fly a great deal, just as all other diurnal butterflies do; they exert their power of flight as long as the sun shines, and if the exercise of one generation influences the next, they ought to become gradually more capable of rapid flight. In this case exactly the opposite takes place to what is ascribed to the Lamarckian principle; more constant use must here have brought about a diminution of the activity of the relevant parts. It is quite otherwise when we look at it from the point of view of selection. The variants which cropped up by chance with slower flight survived because they were most easily recognized and avoided; they are the most frequent survivors; they leave descendants which inherit the slower flight-instinct, and this goes on increasing in them as long as the increase carries any advantage with it. As soon as this ceases to be the case the variation comes to a standstill, for it is adapted to the average of the conditions at a given time.

We may picture to ourselves the thousand kinds of regulations of animal movements through instinct as having come about in a similar way; in the majority of cases we _must_ picture it thus. For it is only in the case of those with high intelligence that we can ask whether the animal did not by deliberation help in establishing the purposive variation in its movements. Among insects in any case this could only be taken into account to a very limited extent, although I do not dispute that the more intelligent among them may learn, and may make experiments, and can modify their actions accordingly. But in fleeing from an enemy experience has nothing to do with it, for the first time it is caught it pays the penalty with its life. Without care, and with no idea of the dangers surrounding them on all sides, the butterflies float about, guided only by their instinct, which, however, is so exactly adapted to the conditions of their life that a sufficient number of them to preserve the species always happily escapes all the many dangers. I may remind you of Hahnel's case of the butterfly, already mentioned, which escaped the agile lizard by flying rapidly up from the sweet bait, but settled again upon it without fear immediately afterwards, to fly from the lizard as before, and did so several times in succession. We usually judge such actions far too much from the human standpoint; the butterfly does not wish to escape the death which threatens it; it knows nothing about death; it is not with it as it was with Dr. Hahnel himself, who when he was once in danger from a jaguar in a thicket was so affected by the thought of the death he had happily escaped that he never cared to pass the place again, but made a long circuit to his home. The butterfly does not act according to reflection and imagination; it flies up with lightning-like rapidity when the lizard rushes at it, because this rapid movement, which it _sees_, acts as the stimulus which liberates the flight-instinct, and this works so promptly that in most cases the insect is rescued from danger. Its disposition, however, is not otherwise affected by its narrow escape, and it obeys anew the food-instinct which impels it to settle again on the bait, until the flight-instinct is again set a-going by the visual impression of the re-advance of the lizard. It is the plaything of its instincts, a machine which works exactly as it must. That it is only sense-impressions and not conceptions which here liberate the actions can be well seen in the case of shy species of butterfly like our purple emperor (_Apatura iris_), which flies up like lightning from the moist wood-paths on which it loves to settle as soon as any rapidly moving visual image, even if it be only a shadow, strikes its eyes. For this reason the collector tries to approach it so as not to throw his shadow before him, for then the insect lets the advancing enemy get quite close, and only flies up when the net is quickly thrust towards it. In all probability the eye of this insect is particularly well adapted for perceiving movements, and certainly the flight-instinct reacts very promptly to such visual impressions, and we can understand that it must have been so regulated if, as we assume, the regulation came about through processes of selection, for the enemies of the butterflies, such as birds, dragon-flies, and lizards shoot quickly out on their prey, and therefore those butterflies must always have survived whose instinct impelled them to take to flight most quickly.

In this, then, as in a thousand other cases, the instinct of flight, or indeed any other mode of movement, cannot be interpreted as an 'inherited habit,' because there is no evidence of the possession of that degree of intelligence which could have induced the variation in the previous habit, that is, in manner of movement. The same is true of animals of low intelligence in regard to all the other instincts, which otherwise might seem to be explicable in terms of the Lamarckian principle.

In addition, there is a whole large group of instincts in regard to which the idea of the Lamarckian principle cannot be entertained, as I showed years ago, and it consists of all those instincts which are only exercised once in the course of a lifetime. These cannot possibly depend on practice in an individual lifetime, and transmission of the results of this exercise to the following generation; they can therefore only be interpreted in terms of selection, unless we are to give up all attempts at a scientific interpretation, and simply accept them as 'marvels.'

To this class belong all the diverse instincts by which insects protect themselves against attack during the pupa stage. Even the way in which the caterpillars of many diurnal butterflies hang themselves up in pupation is not by any means a very simple instinctive action. The caterpillar first spins, in a suitable place, a small round disk of silk threads, to which it then attaches the posterior end of its body, so securely that it cannot be easily torn away. More complicated still is the securing of the pupa when it does not hang freely, but is to remain pressed against a wall or a tree, as is the case in the Papilionidæ and the Pieridæ. In this case the caterpillar must, in addition to the usual cradle, spin a thread of silk, in an ingenious way, diagonally across the thorax, so that it may cross about the middle of the wing rudiments, and not be too loose, lest the pupa fall out, yet not too tight, lest the thread cut too deeply into the wing rudiments and hinder their development. When one remembers that it is the caterpillar that does all this, before it has taken the form of the pupa, and that it must all be adapted to the pupa's form, we are amazed at the extraordinary exactness with which instinct prescribes all the individual movements which make the whole of the complex performance effective. And yet, as each caterpillar only accomplishes this performance once in its life, it could at no time in the development of the species have become a habit in the case of any individual caterpillar, and it cannot therefore be an 'inherited habit.'