C. Lloyd Morgan

I.—The Nature of Intelligent Behaviour

Such an animal as a newly hatched bird or an insect just set free from the chrysalis is a going concern, a living creature. It is the bearer of wonderfully complex automatic machinery, capable, under the initiating influence of stimuli, of performing instinctive acts. But if this were all we should have no more than a cunningly wrought and self-developing automatic machine. What the creature does instinctively at first it would do always, perhaps a little more smoothly as the organic mechanism settled down to its work—just as a steam-engine goes more smoothly when it has been running for a while; but otherwise the action would continue unchanged. Instinctive behaviour would remain unmodified throughout life. The chick, however, or the imago insect is something more than this. It affords evidence of the accommodation of behaviour to varying circumstances. It so acts as to lead us to infer that there are centres of intelligent control through the action of which the automatic behaviour can be modified in accordance with the results of experience. When, for example, a young chick walks towards and pecks at a ladybird, the like of which he has never before seen, the behaviour may be purely instinctive; and so, too, when he similarly seizes a wasp-larva. Even when he rejects the ladybird or swallows the larva, this may be directly due to unpleasant stimulation in the one case, and pleasant in the other. But when, after a few trials, the chick leaves ladybirds unmolested while he seizes wasp-larvÆ with increased energy, he affords evidence of selection based on individual experience. And such selection implies intelligence in almost its simplest expression. We may say, therefore, that, whereas instinctive behaviour is prior to individual experience, intelligent behaviour is the outcome and product of such experience. This distinction is presumably clear enough; and it is one that is based on the facts of observation. But we must not fail to notice that, though the logical distinction is quite clear, the acquired modifications of behaviour, which we speak of as intelligent, presuppose congenital modes of response which are guided to finer issues. We may say, then, that where these congenital modes of response take the form of instinctive behaviour, there is supplied a general plan of action which intelligence particularizes in such a manner as to produce accommodation to the conditions of existence.

We have already frankly admitted that, in the present state of our knowledge, we do not know with any definiteness how intelligent modification of behaviour is effected. But it seems probable that from all parts of the automatically working organic machine messages come in to the centres of conscious control, and that in accordance with the net result of all these messages, and the past experience which they recall, other messages go out to the automatic centres, and, by checking their action here and enforcing it there, give new direction to the resulting behaviour. If this be so, the consciousness associated with the control-centres is like the person who sits in a central office and guides the working of some organized system in accordance with the information he is constantly receiving; who sends messages to check activity in certain directions and to render it more efficient and vigorous in others.

It may be said, however, that intelligent guidance is, at any rate in such simple cases as the selection of a palatable grub and the rejection of a nauseous ladybird, itself determined by instinctive likes and dislikes. All young chicks apparently find wasp-larvÆ palatable and ladybirds the reverse; and this is just as much the outcome of heredity as the instinctive act of pecking. Since, therefore, heredity determines what shall be selected and what rejected—since the likes and dislikes are themselves instinctive—any essential difference between congenital and acquired behaviour seems to be evanescent.

Now, if we apply to the affective qualities of mental states—the pleasurable tone or its opposite which characterize such states—the term “instinctive,” we do so in reference to the broader psychological conception of instinct, rather than in accordance with the narrower biological acceptation of the term. For the likes and dislikes constitute part of the conditions under which the behaviour occurs, and not elements in the co-ordinated response as such. Hence it is preferable to apply to these hereditary qualities the term innate, rather than the term instinctive. But, waiving this distinction, it is true that such pleasant or unpleasant qualities of the sensory results of stimuli are part of the animal’s hereditary outfit, and are not acquired in the course of individual experience. What, then, is acquired? What part does experience play in the development of intelligent behaviour? Let us consider the case of the chick and the ladybird, and see whether it helps us to answer these questions. The chick is stimulated to the instinctive pecking response by a small moving object. That is the first scene of the little drama. In the second scene the ladybird is seized, sensory centres are unpleasantly stimulated, and the insect is dropped or thrown on one side with signs of disgust. Let us grant that this aversion with its characteristic response is also instinctive. There is no hereditary connection between scene 1 and scene 2. After an interval the curtain rises on act ii. The characters are the same as in the first scene of the previous act. But the action of the drama is different. The chick does not seize the ladybird. Why? Because there is an acquired connection between scenes 1 and 2 of the previous act. The chick has gained experience of the nauseous character of the insect, and this experience influences and modifies his behaviour. The essentially new feature, therefore, is the establishment of a connection which is not provided through inheritance. To put the distinction in a brief form, we may say that instinct depends on how the nervous system is built through heredity; while intelligence depends upon how the nervous system is developed through use.

Assuming that an animal is capable of gaining experience and of acquiring new nervous connections in the course of individual experience, it follows that, as has already been indicated, instinctive behaviour in its logical purity is only presented in the first performance of any given co-ordinated act. For after this the animal has gained experience of its performance; and this can no longer conform to a definition of instinct, according to which it is characterized as “prior to experience.” On the other hand, intelligent behaviour cannot be presented on the first occurrence of any action, since there is no prior experience thereof in the light of which it may be guided. This logical distinction may be expressed by saying that instinctive behaviour is always prior to experience, while intelligent behaviour is always subsequent to experience. When, however, instinctive procedure continues throughout life practically unmodified or but little modified, we may still class it under instinct, since the hereditary connections are still the predominant factors. And where the latter part of an instinctive sequence is modified by the experience gained in the former part, we may still term the modification intelligent, however small may be the time-interval implied in the word “subsequent.” Sharp as the logical distinction is, the behaviour of animals is in the main a joint-product, and whether we term it instinctive or intelligent depends upon whether the hereditary or the acquired factor predominates.

Passing on now to consider some further characteristics of intelligent behaviour, we may first notice what Dr. Charles Mercier, in his work on “The Nervous System and the Mind,” terms the four criteria of intelligence. Intelligence is manifested, he says, first in the novelty of the adjustments to external circumstances; secondly, in the complexity; thirdly in the precision; and fourthly, in dealing with the circumstances in such a way as to extract from them the maximum of benefit.

Fig. 18.—Leaf-case of Birch-weevil.

If, however, we are to regard these severally as criteria of intelligence, each should serve to differentiate intelligent from instinctive behaviour. But this is not the case. The precision of the adjustment cannot be regarded as a criterion of intelligence, for many instinctive acts are remarkably precise. No grocer’s assistant rolls a paper funnel with more precision than is displayed by the birch-weevil (Rhynchites betulÆ) in constructing the leaf-case in which her eggs are laid. Curved incisions of constant form are made on either side of the midrib, and are “of just the right shape to make the overlaps in the rolling, and to retain them rolled up with the least tendency to spring back,”[49] while the tip of the leaf is rolled into a second smaller funnel, which is tucked in to close the opening of the first, after the eggs have been deposited. “The eggs hatch in their dark place, each giving rise to an eyeless maggot, which ultimately leaves the funnel for the earth.... Hence the beetle cannot be considered to have ever seen a funnel, and certainly has never witnessed the construction of one, though, when disclosed, it almost immediately sets to work to make funnels on the complex and perfect system” characteristic of the species. This is but one example of instinctive precision out of the many which could be cited. We may say, then, that though, when an act is otherwise shown to be intelligent, the precision is a criterion of the level attained by the intelligence, still it cannot be said to be a criterion which serves to distinguish intelligent from instinctive behaviour.

Nor can we regard apparent prevision (which is sometimes advanced as a criterion of intelligence) as specially distinctive of intelligent acts regarded objectively in the study of animal behaviour. For, as we have had occasion to show, there are many instincts which display an astonishing amount of what may be termed “blind prevision”—instance the instinctive regard for the welfare of unborn offspring which the mother will never see, and the instinctive preparation for an unknown future existence in the case of insect larvÆ.

Nor, again, is the complexity of the adjustment distinctive of intelligence as contrasted with instinct. We have cited examples which afford evidence of much complexity in instinctive behaviour. The construction and storage of the nest among solitary wasps, and their methods of capturing and conveying the insects or spiders on which they prey, are sufficiently complex. So, too, is the behaviour of the Sitaris larva which attaches itself to the male bee, passes to the female, and then slips on to the eggs she lays; and so, again, is that of the Yucca moth, which collects pollen from the anthers, conveys it to the stigma, and then lays her eggs among the ovules. These cases show, too, that the circumstances may be dealt with in such a way as to extract from them the maximum of benefit. It would be difficult intelligently to improve upon the manner of dealing with the circumstances displayed in many familiar modes of instinctive procedure.

There remain the novelty of the adjustment and the individuality displayed. And here we seem to have valid criteria of intelligent behaviour. The ability to perform acts in special adaptation to new circumstances, and the individuality manifested in dealing with the complex conditions of a variable environment,—these seem to be distinctive features of intelligence. On the other hand, in instinctive behaviour there seems to be no choice; the animal is impelled to their stereotyped performance through impulse, as by a stern necessity; they are so far from novel that they are performed by every like individual of the species, and have been so performed by their ancestors for generations; and in performing the instinctive act, the animal seems to have no more individuality or originality than a piece of adequately wound clockwork.

Granting, then, that behaviour is shown to be intelligent by the fact that there is evidence of profiting by experience, we may say that the level attained by the intelligence is indicated by the complexity of the adjustment, its precision, the individuality shown, the amount of prevision disclosed, and in its being such as to extract from the circumstances the maximum of benefit. Many of these points, however, serve equally well to mark the level of instinctive procedure.

II.—Intelligent Behaviour in Insects

It is, as we have seen, among the higher invertebrates, especially in insects, that some of the most remarkable and complex instincts may be found. There is,[50] however, a tendency to ascribe the behaviour of insects entirely to instinct, without sufficient evidence that neither imitation, instruction, nor intelligent learning play any part. This is, perhaps, a survival of the old-fashioned view that all the acts of the lower animals are performed from instinct, whereas those of human beings are to be regarded as rational or intelligent. In popular writings and lectures, for example, we frequently find some or all of the following activities of ant-life ascribed to instinct: recognition of members of the same nest; powers of communication; keeping aphides for the sake of their sweet secretion; collection of aphid eggs in October, hatching them out in the nest, and taking them in the spring to the daisies, on which they feed, for pasture; slave-making and slave-keeping, which, in some cases, is so ancient a habit that the enslavers are unable even to feed themselves; keeping insects as beasts of burden, e.g. a kind of plant-bug to carry leaves; keeping beetles, etc., as domestic pets; habits of personal cleanliness, one ant giving another a brush-up, and being brushed-up in return; habits of play and recreation; habits of burying the dead; the storage of grain and nipping the budding rootlet to prevent further germination; the habits described by Dr. Lincecum, and to a large extent confirmed by Dr. McCook, that Texan ants prepare a clearing around their nest, and six months later harvest the ant-rice, a kind of grass of which they are particularly fond, even, according to Lincecum, seeking and sowing the grain which shall yield this harvest; the collection by other ants of grass to manure the soil on which there subsequently grows a species of fungus upon which they feed; the military organization of the ecitons of Central America; and so forth. Now, the description of the habits of ants forms one of the most interesting chapters in natural history. But to class them all as illustrations of instinct is a survival of an old-fashioned method of treatment.

To put the matter in another way. Suppose that an intelligent ant were to make observations on human behaviour as displayed in one of our great cities or in an agricultural district. Seeing so great an amount of routine work going on around him, might he not be in danger of regarding all this as evidence of hereditary instinct? Might he not find it difficult to obtain satisfactory evidence of the establishment of our habits, of the fact that this routine work has to some extent to be learnt? Might he not say (perhaps not wholly without truth), “I can see nothing whatever in the training of the children of these men to fit them for their life-work. The training of their children has no more apparent bearing upon the activities of their after-life than the feeding of our grubs has on the duties of ant-life. And although we must remember,” he might continue, “that these large animals do not have the advantage which we possess of awaking suddenly, as by a new birth, to their full faculties, still, as they grow older, now one and now another of their deferred instincts is unfolded and manifested. They fall into the routine of life with little or no training as the period proper to the various instincts arrives. If learning thereof there be, it has at present escaped our observation. And such intelligence as their activities evince (and many of them do show remarkable adaptation to uniform conditions of life) would seem to be rather ancestral than of the present time; as is shown by the fact that many of the adaptations are directed rather to past conditions of life than to those which now hold good. In the presence of new emergencies to which their instincts have not fitted them, these poor men are often completely at a loss. We cannot but conclude, therefore, that, although shown under somewhat different and less favourable conditions, instinct occupies fully as large a space in the psychology of man as it does in that of the ant, while their intelligence is far less unerring and, therefore, markedly inferior to our own.”

Of course, the views here attributed to the ant are very absurd. But are they much more absurd than the views of those who, on the evidence which we at present possess, attribute all the varied activities of ant-life to instinct? Take the case of the ecitons, or military ants, or the harvesting ants, or the ants that are said to keep draught-bugs as beasts of burden: have we sufficient evidence to enable us to affirm that these modes of behaviour are purely instinctive and not intelligent; that all the varied manoeuvres of the military ants, for example, are displayed to the full without any learning or imitation, without teaching and without intelligence on the part of every individual in the army.

That in some cases there is something very like a training or education of the ant when it emerges from the pupa condition is rendered probable by the observations of M. Forel. As Romanes says,[51] “The young ant does not appear to come into the world with a full instinctive knowledge of all its duties as a member of a social community. It is led about the nest and ‘trained to a knowledge of domestic duties, especially in the case of larvÆ.’ Later on, the young ants are taught to distinguish between friends and foes.”

We have only to weigh the evidence brought forward by such observers as Fabre and Dr. Peckham to see that among the solitary wasps and mason bees the behaviour, though founded on instinct, is in large degree modified by intelligence. The care with which a site for the tunnelled nest in the ground is selected, betokens something more than instinct. The following is a slightly condensed statement of Dr. and Mrs. Peckham’s observations on one of the solitary wasps (Aporus fasciatus).[52] “We were working one day in the melon-field when we saw one of these little wasps going backwards and dragging a spider. She twice left it on the ground while she circled about for a moment, but soon carried it up on to one of the large melon leaves, and left it there while she made a long and careful study of the locality, skimming close to the ground in and out among the vines; at length she went under a leaf close to the ground, and began to dig. After her head was well down in the ground we broke off the leaf that we might see her method of work. She went on for ten minutes without noticing the change, and then, without any circling, flew off to visit her spider. When she tried to return to her hole it was evident that some landmark was missing. Again and again she zig-zagged from the spider to the nesting-place, going by a sort of path among the vines from leaf to leaf, and from blossom to blossom, but when she reached the spot she did not recognize it. At last we laid the leaf back in its place over the opening, when she at once went in and resumed her work, keeping at it steadily for ten minutes longer. At this point she suddenly reversed her operations, and began to fill in the hole that she had made. She then glanced at the spider, selected a new place, and began to dig again. This hole was also filled in; she looked once more at the spider, and started a nest in a new place. This, in turn, was soon abandoned, as was a fourth. The fifth beginning was made under a leaf that lay close to the ground, but after twenty minutes’ work this place also was abandoned and a sixth started. This, however, was the final choice, and after forty-five minutes spent in digging it was completed.”

Fig. 19.—Solitary Wasp using a stone to beat down the earth over its nest (after Peckham).

This description shows an amount of apparent fastidiousness which is quite irreconcilable with the hypothesis that the behaviour is merely instinctive. Not less fastidious are some wasps in the temporary closure of the hole with a stone or pellet of earth, the operation being repeated several times with different covers before the insect seems to be satisfied; while in other cases the hole is hidden by bringing earth in such quantity as to render the place indistinguishable from the rest of the field. But in one case observed by Dr. Peckham, intelligent procedure was carried so far as apparently to involve the use of a tool, the same behaviour having been independently observed in the same genus (Ammophila) by Dr. S. W. Williston of Kansas University. “Just here,” writes Dr. Peckham,[53] “must be told the story of one little wasp whose individuality stands out in our minds more distinctly than that of any of the others. In filling up her nest she put her head down into it and bit away the loose earth from the sides, letting it fall to the bottom of the burrow, and then, after a quantity had accumulated, jammed it down with her head. Earth was then brought from the outside and pressed in, and then more was bitten from the sides. When, at last, the filling was level with the ground, she brought a quantity of fine grains of dirt to the spot, and, picking up a small pebble in her mandibles, used it as a hammer in pounding them down with rapid strokes, thus making this spot as hard and firm as the surrounding surface. Before we could recover from our astonishment at this performance she had dropped her stone and was bringing more earth, and in a moment we saw her pick up the pebble and again pound the earth into place with it. Once more the whole process was repeated, and then the little creature flew away.”

Here we have intelligent behaviour rising to a level to which some would apply the term rational. For the act may be held to afford evidence of the perception of the relation of the means employed to an end to be attained, and some general conception of purpose. In this section, which deals with description of behaviour based on observation, the psychological explanation cannot be discussed. Similar indications of deliberate action may be held to be afforded by the sometimes elaborate “locality studies” which these insects seem to make,—by the “care that is taken by wasps to acquaint themselves with the surroundings of their nests.” A Sphex, for example, which had partially made and then abandoned several nests, left them without any locality study; but when she had completed a nest in a suitable spot she made “a most thorough and systematic study of the surroundings. She flew in and out among the plants, first in narrow circles near the surface of the ground, and then in wider and wider ones as she rose higher in the air, until at last she took a straight line and disappeared in the distance.” Another species (Cerceris deserta) “has the habit of making a number of half circles in front of the nest, and then, after rising a little higher, of flying several times completely round it.” The method of procedure is, it seems, so normal to the species that it is probably founded on an instinctive basis. Dr. and Mrs. Peckham, in commenting on their observations, say:[54] “If the examination of the objects about the nest makes no impression upon the wasp, or if it is not remembered, she ought not to be inconvenienced nor thrown off her track when weeds and stones are removed and the surface of the ground is smoothed over; but this is just what happens.” For convenience of observation they “sometimes gently moved intercepting objects to one side, but even such a slight change threw the wasp out of her bearings, and made it difficult for her to recover her treasure.” Where wasps form a number of nests in a small plot of ground, as in the case of Bembex, each knows and returns to its own hole, as was proved by Dr. Peckham, who marked the insects and their nests with paint.

So, too, with regard to prey. In the course of his observations on Pompilus, Fabre removed the spider which the wasp had deposited on a tuft of vegetation before she made her nest. As she was at work beneath the surface she could not see what went on above ground or where the spider had been redeposited some twenty inches from its former position. On emerging from the nest the wasp went straight to the original spot, searched round it for some time, then made further excursions, and discovered the spider. After slightly altering its position, and placing it on another tuft of vegetation, she returned to her subterranean labours, giving the observer an opportunity of again moving the spider. Five times did Fabre repeat the operation, and every time the wasp returned to the spot where she had last deposited her prey.

The same observer records some interesting experiments with the mason bee, Chalicodoma. The mud nests of the species investigated were affixed to stones on the banks of the Rhone. When a nest was partially constructed, the bee having flown off for more material, Fabre moved the stone to a new position, near at hand and easily visible from the original site. The bee went straight to the place where the nest had been, searched the immediate neighbourhood, flew off, and returned to the same spot to continue the search. If she came across her own nest in its new position she did not recognize it as hers, but left it after examination. But if a stone with the nest of another bee in about the same stage of construction was placed in the position occupied by her own, she adopted it. And when two nests near together, both half built, were transposed, each bee unhesitatingly adopted the nest which occupied the position where its own nest had been. It may well seem strange that, the general locality-memory being so well marked, the recognition of the particular stone and nest should be deficient. This may be due to the fact that the so-called compound eyes are the organs concerned in locality vision, while the ocelli deal with details at very close range, and that the former alone afford the requisite data for recognition; by their instrumentality alone arises the conscious situation which affords guidance in behaviour. And in that situation slight changes which for us make it “still the same but with a difference” render it no longer the same for a being of more limited intelligence—one probably incapable of analyzing the situation and seeing that the sameness preponderates over the difference. Be this as it may, the failure of a bee to recognize its own nest under circumstances so foreign to its experience as removal to a new spot may be paralleled with what I have observed in the case of sticklebacks. A nest had been built in a round glass bell jar which stood near a window. Some aquatic vegetation grew in the tank, and the nest was built on the window side. An experiment was made by turning the large bell jar through a right angle. The male stickleback searched for its nest in the old direction on the window side—that is to say, the same position in reference to the incidence of the light. The search was, of course, fruitless, and a new nest was begun in this position. Presently the old nest was discovered, and was then vigorously destroyed in just the same way as the nest of a rival is pulled to pieces and scattered. Here a new incidence of light and new direction of shadows seemed to have completely transformed the visual situation.

To return to insects, it is probable that the homing faculty is not the result of an inborn mysterious instinct dependent on some sense of direction of which we have no knowledge, but is based upon experience gained during their flight hither and thither—that, in a word, it is intelligent and not instinctive. Experiments of Fabre at first seemed to suggest some magnetic influence to which bees were sensitive; for when a minute magnet was fixed to a bee as it started on its return journey, the insect was at fault; but as a check experiment he affixed a piece of straw instead of a magnet, with similar results. Some of Fabre’s observations and those of Dr. Bethe[55] are difficult to reconcile with the hypothesis that, in the homing, guidance is due to acquired acquaintance with the locality. But, on the other hand, the experiments of Lord Avebury and of Romanes seem to favour this view. Romanes found that when bees were taken inland from their hive near the seaboard, and then liberated, they returned from considerable distances, the whole locality being familiar; but taken to the seashore, where the objects around them were unfamiliar (since the seashore is not the place where flowers and nectar are to be found), the bees, though not far distant from the hive, were nonplussed and lost their way. Dr. and Mrs. Peckham, as the result of their extremely careful observations, seem to have no doubt that the homing of solitary wasps is due to locality-experience; and of the social wasp, Polistes fusca, they say:[56] “We have seen the young workers make repeated locality studies when they first began to venture away from home, but as they occupy the same nest all summer they, of course, grow more and more familiar with their surroundings, until they become so thoroughly acquainted with them that they can find their way without the least difficulty. We have no doubt that with them, as with the solitary wasps, the faculty is not instinctive, but is the direct outcome of individual experience.”

In the interesting pages of the works in which Dr. and Mrs. Peckham describe their investigations, there are many observations which show that wasps are capable of intelligently profiting by the experience which their instinctive behaviour places them in a position to acquire. The inherited tendencies and aptitudes pave the way for acquired modification and accommodation of behaviour. To catch and paralyze spiders, to dig and prepare a tunnelled nest, and to carry the prey to the nest, all this affords the instinctive basis; but when the observers tell us that they “have several times seen wasps enlarge their holes when a trial had demonstrated that a spider would not go in,” and even on one occasion without trial when an unusually bulky spider was brought, there is something beyond instinct; there is intelligent adjustment to special circumstances given in experience. Presumably intelligent is the habit frequently observed in one species of Pompilus, and occasionally in another, of hanging the paralyzed spider in a crotch of a branching stem, usually of bean or sorrel, where it will be safe from the depredation of ants. On one occasion Dr. Peckham, desirous of seeing the exact manner in which the victim was stung, substituted an unhurt spider for that which the wasp had paralyzed.[57] “According to the habit of its species when danger threatens, it kept perfectly quiet, and when the wasp returned it was hanging there as motionless as a piece of dead matter; but she would not touch it; she hunted all over that plant and then over several others near to it, returning continually to look again at the right spot. After five minutes she flew off in the direction of the woods to catch another spider. Why did she go to the woods? Why did she not take the one that hung there in plain view? It could not have been due to the fact that we had handled the spider, since when, on other occasions, we took one that had been paralyzed, examined it, and then returned it to the wasp, she accepted it without hesitation.... In forty minutes she came back with another spider, but, instead of taking it into the nest, she hung it upon a bean plant near by, and then proceeded to dig a new hole a few inches distant from the first. Foolish little wasp, what a waste of labour! Truly, if you are endowed with energy beyond your fellows, you are but meagrely furnished with reason.”

Fig. 20.—Spiders placed by Solitary Wasps in the crotches of branching stems (after Peckham).

Here we have the routine of instinct—the normal mode of hunting and capturing prey, the normal procedure of bringing the spider, and then making the nest, predominating over any tendency to initiate intelligent improvements. This, however, should not surprise us, in whom the force of habit is often so strong. Nor should we feel surprise at the apparently stupid tolerance some of these wasps display in presence of parasites. Bembex, which does not store and close its cell, but brings continual supplies of food to its larvÆ, is not disturbed by the presence in the cell of the grubs of the parasitic fly Miltogramma. She could, we think, easily free her nest of these intruders, but she continues to bring supplies, though the parasites may absorb it all and leave her own larvÆ to perish. She adapts her procedure to the new conditions, being incapable of knowing that she is feeding the enemies of her race.

Enough has now been said to show the extent and the limitations of the intelligence of such insects as the solitary wasps. It will be noticed that the acquired modifications of behaviour occur in close connection with the inherited ground-plan of instinctive procedure. We shall have occasion to note the same connection in our discussion of social behaviour in the next chapter. And we shall consider the influence of intelligence on instinct before we bring this chapter to a close.

III.—Some Results of Experiment

It is unnecessary to give a resumÉ of entertaining anecdotes illustrative of intelligent behaviour in the higher animals. Such anecdotes are too often the outcome of casual observations by untrained observers; and the interpretation put upon the facts frequently shows a want of psychological discrimination. Such is not the material of which science is constituted. What is needed is systematic observation conducted, so far as possible, under controlled conditions. Two things are necessary: first, to distinguish instinctive behaviour, inherited as such, from the acquired modifications or new departures due to intelligence; and secondly, to determine the method and range of intelligent procedure. These problems can only be solved in their entirety by a complete knowledge of the life-history of the animal concerned. But they may be attacked in detail by a systematic study of particular modes of behaviour, and by an investigation into their manner of origin. That this may be done with some approach to accuracy, resort must be had to experiment, which permits of observation under controlled conditions.

To ascertain, for example, how far nest-building is instinctive in birds, Mr. John S. Budgett hatched a hen greenfinch under a canary. In the following autumn he bought a caged bird, a cock, probably of the same year, and in the succeeding spring turned the pair into a large aviary, supplying such material as twigs, rootlets, dried grass, moss, feathers, sheep’s wool and horsehair. The hen soon began to build, the cock bird taking no share in the work, and finished her nest in a few days. On careful comparison it was found to resemble that of a wild greenfinch in every particular, being made of wool, roots, and moss, lined with horsehair. A second nest the aviary greenfinch built was also quite normal.

In the case of a bullfinch which Mr. Budgett reared, having obtained it when a few weeks’ old, the first nest was composed of dried grass with a little wool and hair, but without either rootlets or twigs. A second which she built was, however, quite typical, made of fine twigs and roots, and lined with horsehair; as was also a third nest.

It is just possible, though most improbable, that the bullfinch utilized its three weeks’ experience gained in the nest from which it was taken. But Mr. Jenner Weir describes[58] observations on canaries in which this source of experience is excluded. “It is usual,” he says, “with canary fanciers to take out the nest constructed by the parent birds, and to place a felt nest in its place, and when her young are hatched, and old enough to be handled, to place a second clean nest, also of felt, in the box, removing the other. But I never knew that canaries so reared failed to make a nest when the breeding time arrived. I have, on the other hand, marvelled to see how like a wild bird’s the nests are constructed. It is customary to supply them with a small set of materials, such as moss and hair. They use the moss for the foundation, and line with the finer materials, just as a wild goldfinch would do.”

Experiment seems, therefore, to show in a way, and with a clearness impossible of attainment by observation under natural uncontrolled conditions, that nest-building in birds is instinctive. That the manner and method of procedure is often modified in accordance with special conditions—that the instinctive outline of nidification receives its final touches through individual experience—is sometimes seen under nature, and more often under the semi-experimental conditions of domestication. Thus three pairs of pigeons in the Wilson Tower of Clifton College made their nests in 1898, as I am informed by Mr. H. C. Playne, of galvanized iron wire, pieces of which were left in a corner at the top of the tower, thus affording a parallel to the behaviour of the unconventional crow of Calcutta, mentioned by Mr. F. W. Headley,[59] which made its nest of soda-water bottle wires, which it picked up in a back yard. But even in this matter experiment serves to bring out clearly the selective influence which is exercised by intelligence. Bolton,[60] in 1792, observed a pair of goldfinches beginning to build their nest in his garden. They formed the ground-work of moss, grass, etc., as usual; but on his scattering small parcels of wool in different parts of the garden they, in great measure, left off the use of their own stuff and used the wool. Afterwards he gave them cotton, and they then used this instead of the wool; then he supplied fine down, and they finished their work with this, leaving the wool and cotton.

In studying the behaviour of wild animals under natural conditions, it must always be difficult to distinguish the congenital basis from the acquired elements; for both tend to bring about a working adjustment to the conditions of life, and we can seldom have opportunities of tracing the interplay of the factors which produce the instinct-habits of adult life. But under domestication we seek to bring about a new working adjustment to conditions imposed by man. The skilful trainer utilizes the natural instinctive tendencies as a basis; and, by a system of rewards and punishments, leads the intelligent modifications of behaviour along lines directed by his deliberate purpose. The conditions are largely those of experiment, and they bring out the play and range of intelligence in a way which would otherwise elude our observation. The training of falcons for the chase affords a good illustration, since they cannot be bred in confinement, and the effects of training cannot therefore be hereditary. The falconer’s object is to modify the congenital instinctive behaviour of a bird of prey for the purposes of sport. She is trained to the lure at first at short distances, and step by step through longer flights; she is taught by snatching away the lure to stoop at it repeatedly as often as it is jerked aside; and then she is trained on living quarry, at first under easy conditions, till eventually she can be flown at a wild bird. And as a result a well-trained falcon will follow her master from field to field, regulating her flight by his movements, always ready for a stoop when the quarry is sprung. The fact that she can be thus educated for her work shows that her behaviour is plastic, and can be moulded by intelligence. Experimental conditions reveal the fact; but under nature the moulding influence of intelligence is presumably not less important, though it is more directly in line with the congenital instinctive tendencies.

That much of the behaviour of the higher animals is guided by experience similar to that which plays so large a part in their training under the experimental conditions of domestication is generally admitted. But what are the range and limits of animal intelligence, and whether it attains the level of rational conduct, in the restricted sense of the term “rational,” are questions open to discussion, to which answers are more likely to be obtained through experiment than by chance observations.

Before giving some of the results of such experiment it will be well to revert to the distinction, which was drawn in the second chapter, between the lower or intelligent stage of mental development and the higher or rational stage. It will be remembered that rational processes were characterized by the fact that the situations contain the products of reflective thought, presumably absent in the earlier stages of development; that they were further characterized by a new purpose or end of consciousness, namely, to explain the situations which at an earlier stage are merely accepted as they are given in presentation or re-presentation; they require deliberate attention to the relationships which hold good among the several elements of successive situations; and they involve, so far as behaviour is concerned, the intentional application of an ideal scheme with the object of rational guidance.

On the other hand, the animal at the stage of intelligent behaviour deals with the circumstances of his comparatively simple life by making use of the particular situations which have been presented to consciousness in the course of his practical experience. If such an animal be placed in the midst of new circumstances he has to find out by a process of trial and error how they are to be met. After a longer or shorter period of trial, guided only by particular experiences, he chances to hit upon a mode of procedure which is successful. The successful act is then incorporated in a new situation; at first, perhaps, only incompletely. The association is eventually established by repetition, through which is acquired the habit of doing the right thing in the appropriate manner. Why he does this and not something else, in so far as he is intelligent and not rational, he probably neither knows nor has the wit to consider. The satisfaction of success suffices for intelligence as such. If the circumstances be so modified as to render the particular mode of meeting them ineffectual, after trying again and again in the old way, he will sometimes stumble upon the proper mode of overcoming the difficulty, and after doing so two or three times a new conscious situation involving the requisite associations will be established, and the appropriate behaviour will become habitual. But why this new mode of procedure rather than any other is adopted, intelligence as such does not know, because it does not analyze the situation and disentangle the essential relationships. The satisfaction of success again suffices. In a word, such an animal in the perceptual stage of mental development seems wanting in the power of reflection. He does not appear to show evidence of framing anything like a general scheme of knowledge which he can apply to the solution of particular problems, of a practical nature, involving difficulties and obstacles.

The method of intelligence—in the sense in which we are using this term—the method of varied trial and error with the utilization of chance success, is a lengthy and somewhat clumsy process; but it suffices. Now contrast it with the procedure of a rational animal, such as man is or may be. When he is confronted by a difficulty he is not content to meet it by trying this way, and that way, and another way, anyhow, and trusting to chance to bring success, but he considers the problem in all its relations with a view to ascertaining the essential nature of the difficulty. For each attempted mode of meeting the case he has a definite reason. He knows why he does this and not that. He has a plan or scheme which he puts into execution. And if it fail, he is not content till he finds out wherein the failure lay. This enables him to plan a better scheme. He sees why it is better; and if at last he be successful by a happy hit, as in the chance procedure of intelligence, he looks for the reason of it. And seeing why this fortunate attempt, unlike his previous efforts, just meets the case, he repeats it because he perceives that herein lies the essential solution of the difficulty. Both in the case of intelligence and in that of reason, as here distinguished, present procedure is based upon past experience; but reason has built upon the foundations thus laid an orderly scheme, and knows its whys and wherefores, while intelligence is at the mercy of chance associations. The reason for success it has not the wit to assign.

The essential difference between the two cases may be put in another way by saying that the intelligent being forms sensory impressions and sensory images linked together by bonds of association, combining and coalescing to constitute a conscious situation effective in behaviour under the guiding influence of pleasure and pain; while the rational being not only does all this, but goes further. He fixes his attention on the way in which the elements in the situation are connected and related; he builds an ideal framework on which the sensory impressions are set or move in an orderly manner. And it is this scheme, fashioned by reason and transforming the situation, which he utilizes in dealing with difficulties.

Yet another way of putting the same essential distinction is to say that intelligence deals with pictures, either directly presented to the senses or called up in re-presentation. If we state the matter thus, however, we must remember that the “pictures” may be painted in colours supplied by any of the senses; and that smells, tastes, sounds, touches, pressures, limb-movements, and so forth, are elements in the pictured product. Bearing this in mind, we may say that intelligence deals with sensory impressions and their revived images in concrete and particular situations; while reason analyzes the pictures, and extracts from them general notions in terms of which the pictures may be explained. For example, we picture a stone falling to the earth; but we explain it by the general notion of gravitative attraction. The conception forms part of our ideal scheme of knowledge, which is not itself picturable, though this or that example of its action may be presented or re-presented in sensory imagery.

Once more we may say—and this way of looking at the question arises naturally out of what has gone before—that intelligence deals with concrete examples, and does not rise to the abstract and general rule. The ideal scheme of reason is the result of abstraction and generalization. It is a framework of conceptions which can be applied to the particular facts which fall under observation to see whether it fits and meets the case. Intelligence has to deal with the facts as they present themselves, without the aid of an organized system of knowledge built up into an ideal scheme.

Enough has now been said to indicate the distinction between the method of intelligence and that of reason. It may, no doubt, be said that the terminology used is open to criticism; for, on the one hand, the words “intelligence” and “intelligent” are frequently used as synonymous with “reason” and “rational;” and, on the other hand, acts requiring neither abstraction, generalization, nor the application of any scheme of knowledge are frequently spoken of as “rational.” Hence there is, it may be urged, some danger of misunderstanding. This may be granted. And unless some such restriction of meaning under suitable terms be accepted by psychologists, misunderstanding will continue. More essential, however, than the distinctive terms we are to use is the distinction of method which underlies them. That, I trust, is sound. Dr. Lindley, in an interesting paper on “A Study of Puzzles,”[61] has utilized the distinction in his investigation of the mental development of children, and has found that the procedure of young children is predominantly of the “sense-trial and error” order which has above been termed intelligent; and he expresses the opinion that “most of the adaptations of animals are on this sense-trial and error level.”

Such certainly seems to be the conclusion to be drawn from my own experimental observations on dogs. It has frequently been asserted that the behaviour of a dog with a stick in his mouth, when he comes to a narrow gap, shows that he at once perceives the nature of the difficulty, and meets it in a rational manner by adopting the appropriate plan of action. He pulls the stick through by one end. But experiments, which I have elsewhere described,[62] showed that a fox terrier, fourteen months old, seemed to be incapable of perceiving the nature of the difficulty which vertical iron railings presented to his passage with a stick in his mouth, and only imperfectly learnt to overcome it after many ineffectual trials and many failures. The results obtained on the first afternoon may be quoted to indicate the nature of the evidence. The dog was sent after a short stick into a field, and had to pass through vertical rails about six inches apart. On his return the stick caught at the ends. I whistled and turned as if to leave; and the dog pushed and struggled vigorously. He then retired into the field, lay down, and began gnawing the stick, but, when called, came slowly up to the railings and stuck again. After some efforts he put his head on one side, and brought the stick, a short one, through. After patting and encouraging him, I sent him after it again. On his return he came up to the railings with more confidence, but, holding the stick by the middle, found his passage barred. After some struggles he dropped it and came through without it. Sent after it again, he put his head through the railings, seized the stick by the middle, and then pulled with all his might, dancing up and down in his endeavours to effect a passage. Turning his head in his efforts, he at last brought the stick through. A third time he was again foiled; again dropped the stick; and again seizing it by the middle tried to pull it through. I then placed the stick so that he could easily seize it by one end and draw it through the opening between the rails. But when I sent him after it, he went through into the field, picked up the stick by the middle, and tried to push his way between the railings, succeeding, after many abortive attempts, by holding his head on one side.

Subsequent trials on many occasions yielded similar results. But the following summer, when I resumed the experiments, I was able with some guidance to teach him to bring a long stick to the railings, drop it, and then draw the stick through by one end; though even then, if he had dropped it so that one end just caught a rail, he often failed, shaking his head vigorously, dropping the stick and seizing it again, and repeating this behaviour until it chanced to fall in a more favourable position. He did not apparently perceive that by gently moving the stick a little one way or other the difficulty could be simply overcome with little effort. Nor when given a crooked stick, which caught in a rail, did he show any sign of perceiving that by pushing the stick and freeing the crook he could pull the stick through. Each time the crook caught he pulled with all his strength, seizing the stick now at the end, now in the middle, and now near the crook. At length he seized the crook itself, and with a wrench broke it off. A man who was passing, and who had paused for a couple of minutes to watch the proceedings, said, “Clever dog that, sir; he knows where the hitch do lie.” The remark was the characteristic outcome of two minutes’ chance observation. During the half hour or more during which I had watched the dog he had tried nearly every possible way of holding and tugging at the stick. Such is the mode of behaviour based on intelligence—continued trial and failure, until a happy effect is reached, not by methodically planning, but by chance.

Two of my friends criticized these results, and said that they only showed how stupid my dog was. Their dogs would have acted very differently. I suggested that the question could easily be put to the test of experiment. The behaviour of the dog was in each case—the one a very intelligent Yorkshire terrier, the other an English terrier—similar to that above described. The owner of the latter was somewhat annoyed, used forcible language, and told the dog that he could do it perfectly well if he tried.

In experimenting with my fox terrier on the method adopted in seizing and carrying differently balanced objects, I used (1) a straight stick, the centre of gravity of which was at the middle; (2) a Kaffir knob-kerrie, the centre of gravity of which was about six inches from the knob; (3) a light geological hammer; and (4) a heavier hammer. In the last, the centre of balance was close to the hammer head. The net result of the observations was that the best place for seizing and holding the object was hit upon in each case after indefinite trials; that after three or four days’ continuous experience with one (say the knob-kerrie), another (say the stick) was at first seized nearer one end, showing the influence of the more recent association; and that there was little indication of the dog’s seizing any one of the four at once in the right place, that is to say, the point of seizure was not clearly differentiated in accordance with the look of the object. I tied a piece of string, in later trials, round the centre of balance, but this, at the time of the dog’s death, had not served as a sure guide to his experience.

The way in which my dog learnt to lift the latch of the garden gate, and thus let himself out, affords a good example of intelligent behaviour. The iron gate outside my house is held to by a latch, but swings open by its own weight if the latch be lifted. Whenever he wanted to go out the fox terrier raised the latch with the back of his head, and thus released the gate, which swung open. Now the question in any such case is: How did he learn the trick? In this particular case the question can be answered, because he was carefully watched. When he was put outside the door, he naturally wanted to get out into the road, where there was much to tempt him—the chance of a run, other dogs to sniff at, possibly cats to be worried. He gazed eagerly out through the railings on the low parapet wall shown in the illustration; and in due time chanced to gaze out under the latch, lifting it with his head. He withdrew his head and looked out elsewhere; but the gate had swung open. Here was the fortunate occurrence arising out of natural tendencies in a dog. But the association between looking out just there and the open gate with a free passage into the road is somewhat indirect. The coalescence of the presentative and re-presentative elements into a conscious situation effective for the guidance of behaviour was not effected at once. After some ten or twelve experiences, in each of which the exit was more rapidly effected with less gazing out at wrong places, the fox terrier had learnt to go straight and without hesitation to the right spot. In this case the lifting of the latch was unquestionably hit on by accident, and the trick was only rendered habitual by repeated association in the same situation of the chance act and the happy escape. Once firmly established, however, the behaviour remained constant throughout the remainder of the dog’s life, some five or six years.

Fig. 21.—Fox-terrier lifting the latch of a gate.

Mr. E. J. Shellard observed[63] an act of similar import in a Scotch staghound, which “appeared at first to be the result of thought,” but which, on closer observation, was clearly seen to be the result of intelligence in the restricted sense of the term. The dog released the lever-latch of a yard door. “At first he raised his paws to the door and scratched violently, manifesting various signs of impatience. His scratches, which extended from the top of the door downwards, and over the whole area, would thus inevitably at some time or other reach the handle of the latch, which was thus struck forcibly downwards, the latch itself rising upwards. The door would then open from the weight of the dog pushing against it. The dog always opened the door in this manner from the time when the incident was first noticed until he left, a period of about three years. The door was opened with no greater ease at the expiration of that period than at the commencement. His paws would strike various parts of the door, and he never appeared to exercise any degree of judgment in the localization of his strokes, the fact of his paws striking the handle of the latch being a necessary result, providing the dog had sufficient patience and strength to continue.”

One or two more experiments with my fox terrier may be briefly described. I watched his behaviour when a solid indiarubber ball was thrown towards a wall standing at right angles to its course. At first he followed it right up to the wall and then back as it rebounded. So long as it travelled with such velocity as to be only just ahead of him he pursued the same course. But when it was thrown more violently, so as to meet him on the rebound as he ran towards the wall, he learnt that he was thus able to seize it as it came towards him. And, profiting by the incidental experience thus gained, he acquired the habit—though for long with some uncertainty of reaction—of slowing off when the object of his pursuit reached the wall so as to await its rebound. Again, when the ball was thrown so as to glance at a wide angle from a surface, at first—when the velocity was such as to keep it just ahead of him—he followed its course. But when the velocity was increased he learnt to take a short cut along the third side of a triangle, so as to catch the object at some distance from the wall. A third series of experiments were made where a right angle was formed by the meeting of two surfaces. One side of the angle, the left, was dealt with for a day or two. At first the ball was directly followed. Then a short cut was taken to meet its deflected course. On the fourth day this method was well established. On the fifth, the ball was thrown so as to strike the other or right side of the angle, and thus be deflected in the opposite direction. The dog followed the old course (the short cut to the left) and was completely non-plussed, searching that side, then more widely, and not finding the ball for eleven minutes. On repeating the experiment thrice, similar results were that day obtained. On the following day the ball was thrown just ahead of him, so as to strike to the right of the angle, and was followed and caught. This course was pursued for three days, and he then learnt to take a short cut to the right. On the next day the ball was sent, as at first, to the left, and the dog was again non-plussed. I did not succeed in getting him to associate a given difference of initial direction with a resultant difference of deflection.

I may here mention that, whenever searching for a ball of which he had lost sight in the road, he would run along the gutter first on one side and then on the other. A friend who was walking with me one day regarded this as a clear case of rational inference. “The dog knows,” he said, “the effects of the convex curvature of the road as well as we do.” I am convinced, however (having watched his ways from a puppy), that this method of search was gradually established on a basis of practical experience. No logical inference on his part is necessary for the interpretation of the facts; and we should not assume its presence unless the evidence compels us to do so.

Dr. E. L. Thorndike, in a monograph on “Animal Intelligence” published as a supplement to the Psychological Review (June, 1898), has fully described and carefully discussed a number of interesting experiments. The subjects (one might, alas! almost say victims) of some of these were thirteen kittens or cats from three to eighteen months old. His method of investigation shall be stated in his own words.

To Dr. Thorndike’s monograph we must refer those who desire detailed information as to apparatus and procedure. It must here suffice to state that the box-cages employed were rudely constructed of wooden laths, and formed cramped prisons about twenty inches long by fifteen broad and twelve high. Nine contained such simple mechanisms as Dr. Thorndike describes in the passage above quoted. When a loop or cord was pulled, a button turned, or a lever depressed, the door fell open. In another, pressure on the door as well as depression of a thumb-latch was required. In one cage two simple acts on the part of the kitten were necessary, pulling a cord and pushing aside a piece of board; and in yet others three acts were requisite. In those boxes from which escape was more difficult a few of the cats failed to get out. The times occupied in thoroughly learning the trick of the box by those who were successful are plotted in a series of curves, the essential feature of which is the graphic expression of a gradual diminution in the time interval between imprisonment and escape in successive trials. This is shown in Fig. 23, which is constructed from some of Dr. Thorndike’s data. In some cases the cats were set free from a box when they (1) licked themselves or (2) scratched themselves.

Such experiments carried out on a different method give results in line with my own. The conditions are, however, somewhat unnatural, which I regard as in some respects a disadvantage. But we need experiments on different methods—the more the better,—and if the results they furnish are in accord, their correctness will be rendered the more probable. It is to be hoped that Dr. Thorndike will devise further experiments in which (1) the conditions shall be somewhat less strained and straitened, while the subjects are in a more normal state of equanimity (cannot “utter hunger” be avoided?), and (2) there shall be more opportunity for the exercise of rational judgment, supposing the faculty to exist. To establish the absence of foresight in the procedure of the cats, it is surely necessary so to arrange matters that the connections are clearly open—nay, even obvious—to the eye of reason. It appears that this consideration has not weighed sufficiently with Dr. Thorndike.

A series of experiments were made to ascertain whether instruction (in the form of putting the animal through the procedure requisite for a given act) was in any degree helpful. The conclusion is that such instruction has no influence. Those who have had experience in teaching animals to perform tricks will probably agree here—though some trainers give expression to a different opinion. It is, however, essential to distinguish carefully between showing an animal how a trick is done, and either stimulating its attention or furnishing accessory guidance (such as the occasional taps of the trainer’s whip when he wants a performing horse to kneel), or affording suitable conditions the results of which temporarily enter into the association complex. If the latter be eliminated the practice of trainers, I believe, bears out the general result of the experiments. Dr. Thorndike never succeeded in getting an animal to change its way of doing a thing for his. Nor was I, after repeated trials, able to modify the way in which my dog lifted the latch of the gate. He did it with the back of his head. I could not get him to do it (more gracefully) with his muzzle.

It may be said that the remarkable feats of performing animals imply the existence of faculties of a higher order than Dr. Thorndike and I are prepared to admit on the basis of our experiments. Mr. P. G. Hamerton many years ago described[64] how, in his own house, a cleverly trained dog would fetch in their right order the letters which spelt the English or German equivalents of common French words, and do other wonderful things. But the owner of the dog (M. du Rouil) admitted that there was a means of rapport between them which he was not prepared to divulge. It is just because the trainer has to lead up to and utilize chance experiences that such prolonged patience and care are required. The animal is but the instrument on which his clever trainer plays; an instrument of wonderful intelligence, but lacking in the higher rational faculty. The organized scheme is the master’s, not that of his willing slave. A rational being might not do more wonderful things; but he would learn them more rapidly and by a less wearisome method. As it is, the clever performing dog originates little or nothing, and repeats again and again the same stereotyped behaviour, which—if one witnesses the performance often—touches one with a profound sense of its lack of rational spontaneity.

As at present advised, therefore, I see no reason for withdrawing from the position provisionally taken up. The utilization of chance experience, without the framing and application of an organized scheme of knowledge, appears to be the predominant method of animal intelligence.

On this view, then, we may see in instinctive behaviour, and the multifarious automatic acts of animals, a means of providing experience of the right kind and on profitable lines. We may see in the play-instincts of the young a training ground for the more serious business of animal life—a theme developed by Professor Groos. We may see in the imitative tendency—the innate proclivity to follow a lead blindly and at first unintelligently—a further means of providing those useful items of experience which intelligence finds so serviceable. And we may see in the intelligence which can profit by chance occurrences that arise in these several ways all that suffices for the simple needs of animal existence.

With some differences of opinion Dr. Thorndike and I have much in common in the conclusions to which we have been independently led as to the method and limits of animal intelligence. We seem to be in essential agreement in the belief that the method of animal intelligence is to profit by chance experience without rational foresight, and that unless such experience be individually acquired, the data essential for intelligent progress are absent. While in our attempts to realize the general nature of animal consciousness there is a close similarity of treatment. In my “Introduction to Comparative Psychology” a good deal of space is devoted to an analysis of the psychology of skill “in order that we may infer what takes place in the minds of animals;” and I said:—“When I am playing a hard game of tennis, or when I am sailing a yacht close to the wind in a choppy sea, self does not at all tend to become focal. Hence, though I am a self-conscious being I am not always self-conscious. And presumably when I am least self-conscious, I am nearest the condition of the animal at the stage of mere sense experience. I am exhilarated with the sense of pleasurable existence, my whole being tingles with sentient life. I sense, or am aware of, my own life and consciousness, in an unusually subtle manner. Experience is vivid and continuous. Such I take it to be the condition of the conscious but not yet self-conscious animal.”

I can therefore cordially endorse Dr. Thorndike’s conclusions as expressed in the following passages:—

“One who has watched the life of a cat or dog for a month or more under test conditions, gets, or fancies he gets, a fairly definite idea of what the intellectual [intelligent] life of a cat or dog feels like. It is most like what we feel when consciousness contains little thought about anything, when we feel the sense-impressions in their first intention, so to speak, when we feel our own body, and the impulses we give to it. Sometimes one gets this animal consciousness while in swimming, for example. One feels the water, the sky, the birds above, but with no thoughts about them or memories of how they looked at other times, or Æsthetic judgments about their beauty; one feels no ideas about what movements he will make, but feels himself make them, feels his body throughout. Self-consciousness dies away. Social consciousness dies away. The meanings, and values, and connections of things die away. One feels sense-impressions, has impulses, feels the movements he makes; that is all.”

And after an illustration from such a game as tennis, Dr. Thorndike adds: “Finally the elements of the associations are not isolated. No tennis-player’s stream of thought is filled with free-floating representations of any of the tens of thousands of sense-impressions or movements he has seen and made on the tennis-court. Yet there is consciousness enough at the time, keen consciousness of the sense-impressions, impulses, feelings of one’s bodily acts. So with the animals. There is consciousness enough, but of this kind.”

It may be said that between the method of intelligence and that of fully developed rational procedure there is a wide gap which must have been bridged in the course of mental evolution. Unquestionably. And in contending that the methods of the animal are predominantly intelligent, I am far from wishing to assert dogmatically that in no animals are there even the beginnings of a rational scheme. Indications thereof do not indeed at present appear to have been clearly disclosed by experiment. But the experimental development of the subject is still in its infancy. We shall probably have to await the further results which must be the outcome of patient and well-directed child-study. The human child does pass in the course of his individual development from intelligent to rational procedure. Here there is a bridge which is crossed by every child. When we know more about the stadia of this development we shall be in a position to apply the results obtained in child-study in the analogous field of animal-study. Till then we must possess our souls in patience, and base our provisional conclusions on the results of systematic investigation, rather than on those of casual observation and anecdote.

IV.—The Evolution of Intelligent Behaviour

No attempt can be made in this section to trace the successive stages of the evolutionary progress of intelligence from its lower to its higher developments. It is indeed questionable whether comparative psychology has, as yet, accumulated a sufficient body of data to render such a task profitable or even possible. And the lower the level of intelligence with which we have to deal, the less reliable are the scanty psychological data which we can obtain. To interpret the mental processes which accompany the acts of even the higher animals is a hard task, requiring careful psychological analysis. Still harder is the task to infer the psychological basis of the actions of the lower animals.

It is difficult to say where, in the hierarchy of animal progress, the beginnings of intelligence can first be traced. In the articulated animals, such as the insects, spiders, and crustacea, there is abundant evidence of intelligence of a relatively high grade. But even in their case, how hard it is to realize the nature of their experience—to get any adequate notion of their mental processes! We are inevitably forced to describe their psychology in the most general terms. So, too, with forms still lower in the scale of intelligence. Many molluscs unquestionably profit by experience. But can we clearly picture to ourselves the nature and manner of acquisition of this experience? The way in which limpets return to the scars on the rock which form their homes seems to show that they have acquired a practically adequate experience of their near surroundings. Romanes cites[65] some of the earlier observations which have been extended by Professor Ainsworth Davis.[66] I looked into the matter myself some years ago, at Mewps Bay near Lulworth in Dorsetshire. The method adopted[67] was to remove the limpets from the rock, and affix them at various distances from their scars. This can be done without difficulty or injury to the mollusc if one catches them as they are moving. But one must make sure that they are just leaving or returning to their proper homes, and are not taken in the midst of a more extended peregrination, as in that case their special scars cannot be noted. Failure to be careful in this matter vitiated my earlier observations, which are therefore excluded in the following table:—

From the nature of the rock surfaces the removal of a limpet to a distance of two feet almost invariably involved placing them on the further side of an angle. And though some returned over such an angle, the majority did not.

In most cases the individuals which failed to return to their respective scars took up new positions; and in several instances, when they were subsequently removed to a distance of a few inches from this new position, they returned to it. Their return to the scar was watched in many cases, and the course was fairly, but not quite direct. One limpet covered a distance of ten inches, over a somewhat curved course, in a little under twenty minutes. In another case the limpet on its return journey had to pass between two others, which necessitated the lifting of the shell to some height so as to clear one of them. On reaching the scar they twist and turn about so as to fit down in the normal position which is constant. When they come up the wrong way round they rotate pretty rapidly through the 180 degrees to get into position. One was observed to make a short excursion from and return to its scar under stillish water. But as a rule they seem to remain fixed when they are submerged, moving for the most part when the tide has just receded.

The greatest distance I have watched a limpet reach from its home was twenty-two inches. But I have found them at a distance of three feet from their scars—that is to say, from those to which they fitted perfectly. This was on a large flat surface.

When they move, the tentacles are projected out beyond the shell, and keep on touching and slightly adhering to the rock. On reaching the scar they carefully feel round it with the tentacles. By excision of these feelers Professor Davis was led to conclude that it is not through their instrumentality that the limpet finds its way back to its particular scar. But I am inclined to question these results. At any rate, further observations and experiments are needed to settle the point.

Snails will also return to special dark hollows or crannies in the wall after their foraging excursions. Such behaviour in molluscs affords evidence of something more than instinct. In popular speech, we should say that there is memory of the locality. And in any case it is difficult to interpret the facts without the assumption that the animals are conscious, and that re-presentative states are evoked through the mediation of presentative sense-impressions. But how difficult, if not impossible, it is to form anything like a satisfactory conception of the rudimentary mental processes of a limpet!

The most highly developed molluscs are the cephalopods. They have long sensitive mobile arms with which they feel for and capture their prey. “Now Schneider observed,” writes Dr. Stout,[68] “a very young octopus seize a hermit-crab. The hermit-crab covers the shell in which it takes up its abode with stinging zoophytes. Stung by these, the octopus immediately recoiled and let its prey escape. Subsequently it was observed to avoid hermit-crabs. Older animals of the same species managed cleverly to pull the crab out of its house without being stung.” Such cases afford evidence of profiting by experience through the exercise of intelligence.

Darwin’s careful observations on the manner in which earthworms drag leaves into their burrows seem to show that these annelids act intelligently, and deal with leaves of different shapes in different ways. The leaves of Pine trees, consisting of two needles arising from a common base, were almost invariably drawn down by seizing this basal point of junction; while the leaves of the Lime were, in 79 per cent. of the cases examined, drawn down by the apex; in only 4 per cent. by the base; and in the remaining 17 per cent. by seizing some intermediate portion. On the other hand, the leaves of the Rhododendron, in which the basal part of the blade is often narrower than the apical part, were in 66 per cent. of the observations drawn down by the narrower base. Triangles of paper were in the majority of cases seized by the apex. Commenting upon his observations, carried out with great care under experimental conditions, Darwin says,[69] “As worms are not guided by special instincts in each particular case, though possessing a general instinct to plug up their burrows, and as chance is excluded, the next most probable conclusion seems to be that they try in different ways to draw in objects, and at last succeed in some one way;” that is to say, they profit by experience based on the method of trial and failure. But Darwin adds that the evidence he obtained shows “that worms do not habitually try to draw objects into their burrows in many different ways.” And he seems to attribute to them an almost rational power of dealing with the circumstances in the light of general conceptions. “If worms,” he says, “are able to judge, either before drawing or after having drawn an object close to the mouths of their burrows, how best to drag it in, they must acquire some notion of its general shape. This they probably acquire by touching it in many places with the anterior extremity of their bodies, which acts as a tactual organ. It may be well to remember how perfect the sense of touch becomes in a man when born blind and deaf, as are worms. If worms have the power of acquiring some notion, however rude, of the shape of an object and of their burrows, as seems to be the case, they deserve to be called intelligent; for they then act in nearly the same manner as would a man under similar circumstances.”

Such power of perceiving the relation of the shape of a leaf or other object to the form of the burrow is presumably beyond the reach of an earthworm. It may be regarded as more probable that the earthworm inherits an instinctive tendency to draw down objects in special ways, and that this is subject to some modification under the play of experience, without the formation of anything so psychologically complex as a general notion, however rude. In any case the behaviour of earthworms in closing their burrows seems to afford indications of something more than instinct—of that profiting by the results of experience which characterizes intelligent procedure. More than this we cannot say.

Professor Whitman[70] has made some interesting observations on the leech Clepsine. “Place the animal,” he says, “in a shallow, flat-bottomed dish, and leave it for a few hours or a day, in order to give it time to get accustomed to the place, and come to rest on the bottom. Then, taking the utmost care not to jar the dish or breathe upon the surface of the water, look at the Clepsine through a low magnifying lens, and see what happens when the surface of the water is touched with the point of a needle held vertically above the animal’s back. If the experiment is properly carried out, it will be seen that the respiratory undulations (if such movements happen to be going on) suddenly cease, and that the animal slightly expands its body and hugs the glass. Wait a few moments until the animal, recovering its normal composure, again resumes its respiratory movements. Then let the needle descend through the water until the point rests on the bottom of the dish at a little distance from the edge of the body. Again the movements will cease, and the animal will hug the glass with its body somewhat expanded. Now push the needle slowly along towards the leech, and notice as the needle comes almost in contact with the thin margin of the body, that the part nearest the needle begins to retreat slowly before it. This behaviour shows a surprising keenness of tactile sensibility, the least touch of the water with a needle-point being felt at once.... If its back were rubbed with a brush or the handle of a dissecting needle, in order to test its sensitiveness to touch, the appearance would probably be that of insensibility and indifference to the treatment. Closer examination, however, would show that the flesh of the animal was more rigid than usual, and that the surface was covered with numerous stiff, conical elevations, the dermal papillÆ or warts, which are so low and blunt in the normal state of rest as to be scarcely visible. It would be seen that the animal, although motionless, was in a state of active resistance to attack.... Clepsine has another and entirely different method of keeping quiet. The animal rolls itself up (head first and ventral side innermost) into a hard ball, outwardly passive, free to roll or fall whithersoever gravity or currents of water may direct it.... If by chance the animal has eggs, it will not desert them to escape in this way.... This species, then, has two quite distinct and peculiar ways of keeping quiet, and thus avoiding its enemies. If the animal has no eggs, or if it has young, it may adopt either mode of escape, while if it has eggs it has no choice but to remain quiet over them.... The act of rolling up into a passive ball may be performed (a) under compulsion, as when it is her last resort in self-defence; (b) under a milder provocation, as one of three courses of behaviour, as when the resting-place is turned up to light, and the choice is offered between remaining quiet in place, creeping away at leisure, or rolling into a ball and dropping to the bottom; (c) or finally, under no special external stimulus, but rather from internal motive, the normal demand for rest and shady seclusion, presumably very strong in Clepsine after gorging itself with the blood of its turtle host.”

Professor Whitman rightly regards the act of rolling into a ball as instinctive, and due to natural selection. But he does not undertake to discuss the question as to how much intelligence, if any, Clepsine may have. Nor, indeed, is it an easy matter to determine. The differential reaction according as the animal has eggs or not suggests intelligence; but it may be instinct varying according to the conditions of stimulation external and internal. The different behaviour which may be seen in different cases when a stone is turned to the light again suggests intelligence, but again may be determined directly by the conditions of stimulation. Prompted by Dr. Whitman’s observations, I endeavoured to determine whether a leech would grow accustomed to frequent gentle stimulation with a camel’s-hair brush, and cease to react under circumstances which were followed by no ill effects. But though I incline to think that this is the case, the observations were not such as to be satisfying and convincing. If intelligence be present we seem to find it in an early and rudimentary state.

Observation, we must confess, seems to afford little indication of the conditions under which intelligence first makes its appearance in the animal kingdom. And if we turn to general considerations, which at the best afford uncertain guidance, little light is thrown on the subject. If we accept the view already indicated,[71] that the nerve-centres which are concerned in the conscious control begotten of experience are independent of those primarily concerned in normal reflex action, we may perhaps believe that the simplest nervous system, worthy of the name, contains both these elements, and that in the course of the evolution of nervous systems in higher and higher grades, there go on pari passu the further differentiation of these elements, and the progressive integration of reflex and control centres into a closely connected and effective whole. Not that any expression of the facts, if such they be, in terms of an evolution formula, adds anything to our knowledge of the organic modus operandi. We know but little of the intimate nerve physiology of even the highest invertebrates. We see ample evidence of the control of behaviour in the light of individual experience. Of any detailed knowledge concerning the manner in which this control is effected we do not seem to possess more than the rude initial phases.

When we compare, however, the several grades of intelligence which observation suggests, and when we watch the conscious development of the more intelligent animals, we seem to find evidence of the growth of a system of experience, at first in very close touch with inherited modes of procedure, but gradually acquiring more of independence and freedom. Increase of the range and complexity of behaviour brings with it, not only increase in the range and complexity of experience, but also—what is, perhaps, even more essential to effective progress—greater unity and closer connection into a well-knit whole. And with this greater unity and closer connection there goes what one may term a condensation of experience by an elimination of detail and the survival of essential features repeatedly emphasized. This is analogous in the development of intelligence to the generalization and abstraction which play so important a part in the development of reason. It affords, in fact, the data which reflection utilizes in the purposive and intentional condensation and concentration of knowledge at a higher stage of mental development.

The omission of detail and the survival of the salient features is well known to us in the familiar facts of memory. We have seen thousands of sheep and oxen, no two of which are probably alike in all their external details as presented to vision. But we remember what a sheep or an ox looks like, and many of us can form a visualized image of either of these animals. This, however, is not the re-presentative image of any particular sheep or ox. It is what psychologists term a generic image. It is like a composite photograph made by superimposing on the same plate a number of individual images so that the salient features which all possess in common stand out clearly by their coincidence on the plate, while the distinctive details are but dimly presented. Thus does memory preserve the essentials common to many impressions while the distinguishing details are lost and fade, eliminated by forgetfulness. And thus in the experience which intelligence practically utilizes are the net results of a thousand particular impressions condensed in one effective image.

Condensation of experience is also effected by the elimination, under the guidance of consciousness, of those modes of behaviour which are not efficacious—a process to which Professor Mark Baldwin applies the phrase Functional Selection. There is a tendency at first to the overproduction of relatively useless actions. The multifarious random movements of the human infant, though their inexactness renders the child terribly helpless, afford a wide store of plastic material which intelligence can guide to its appropriate use. And the prolonged period of pupilage in the child is correlated with an unsurpassed range of combination and recombination of the abundant plastic material. The hereditary legacy, though it contains fewer drafts for definite and specific purposes than are placed to the credit of an animal rich in instinctive endowment, affords a far larger general fund on which intelligence may draw for the varied purposes of the freer financial existence of a rational being.

The relatively helpless young of many of the higher mammalia exhibit also much overproduction of seemingly aimless movements. But from these intelligence selects those which are of value for the purposes of life—those which experience proves to be effective. These—the relatively few—afford the motor impressions which by repetition stand out in experience, while the rest lapse from memory and are eliminated from experience as they are eliminated from practical performance. This is a great gain. Motor experience is rendered generic; the composite image that is retained is the net result of effective behaviour; and all that is valuable in the acquisitions of early life is condensed within manageable limits.

This process of rendering generic the particular items of a widening experience has a marked effect in the development of the conscious situations in the light of which behaviour is intelligently guided. It is not the master holding this whip or that ball which suggests to the dog a hiding or a scamper; it is a generic situation with interchangeable details. It is not this, that, or the other previously unseen cat that at once determines the situation for the fox terrier; the particular animal has never entered into his past experience: it is the fulfilment of the essential conditions of the generic image that is operative in behaviour. The experience of animals must inevitably become in large degree generic by the elimination of the unessential and survival in re-presentative consciousness of the salient elements in many slightly diverse situations.

Stated in terms of this conception, the familiar phenomena of mimicry are due to the fact that the mimicking form accords sufficiently well with the generic image to carry the same suggested meaning. As is well known, the model has been proved in many cases to be unpalatable or hurtful, while the mimic is in itself neither the one nor the other. The drone-fly, Eristalis, mimics the drone. And it has been urged that this cannot be a true case of mimicry, since the drone is harmless, though the female and “neuter” bees are possessed of stings. But I have satisfied myself by experiments with young birds, that (1) after experience with bees drones are avoided, and (2) that after similar experience drone-flies are also left untouched. Hence it seems that all three fall within the same generic image, the points of resemblance outweighing the differences in detail—as they do, indeed, with many men and women.

Such examples of mimicry belong to what is known as the “Batesian type”—so called after H. W. Bates, who, in 1861, discussed its occurrence among Amazonian insects in the light of the theory of natural selection. There are, however, certain groups of insects which, although themselves “protected,” possess common warning colours, causing them to resemble each other. These are sometimes classed under the head of “MÜllerian mimicry”—so called after Fritz MÜller, who, in 1879, first offered an explanation of the facts based on the theory of natural selection. He suggested that such mutual resemblance is advantageous to both protected forms, since it lessens the number of those which are killed by young birds and other animals while they are learning by experience what to eat and what to leave. For, as the result of careful observation, Mr. Frank Finn concludes “that each bird has to separately acquire its experience, and well remembers what it has learnt,”—a conclusion with which, as already stated, my own observations are entirely in accord. There is therefore a certain amount of destruction of even well-protected forms by young and inexperienced birds. If, then, two such forms resemble each other, the acquisition of experience is thereby facilitated and the amount of destruction reduced, on the assumption that the two fall within the same generic image. Upholders of natural selection are not, indeed, at one in accepting this explanation, and further observation is unquestionably needed. It is not improbable, however, that common protective coloration, such as the banding of yellow and black, seen in such different forms as the caterpillar of the cinnabar moth and the imago of the wasp, is of mutual utility. The following experiment was made with young chicks. Strips of orange and black paper were pasted beneath glass slips, and on them meal moistened with quinine was placed. On other plain slips meal moistened with water was provided. The young birds soon learnt to avoid the bitter meal, and then would not touch plain meal if it were offered on the banded slip. And these birds, save in two instances, refused to touch cinnabar caterpillars, which were new to their experience. They did not, like other birds, have to learn by particular trials that these caterpillars are unpleasant. Their experience had already been gained through the banded glass slips; or so it seemed. I have also found that young birds who had learnt to avoid cinnabar caterpillars left wasps untouched. Such observations must be repeated and extended. But they seem to show that one aspect of the MÜllerian theory is not without some facts in support of it; and, so far as they go, they afford evidence that black and orange banding, irrespective of particular form, may constitute a guiding generic feature in the conscious situation.

It may be said that the generic condensation of experience here indicated implies the formation of general and abstract ideas, and that we cannot in face of the evidence accept Locke’s dictum that abstraction is “an excellency which the faculties of brutes do by no means attain to.” Romanes contended[72] that “all the higher animals have general ideas of ‘good-for-eating’ and ‘not-good-for-eating’ quite apart from any particular objects of which either of these qualities happens to be characteristic,” and he quoted with approval Leroy’s statement,[73] that a fox “will see snares when there are none; his imagination, distorted by fear, will produce deceptive shapes, to which he will attach an abstract notion of danger.” According to such views animals form concepts; and concepts belong to the sphere of rational thought. It is not my intention to enter at length into the refinements of psychological distinction. Many psychologists, however, seek to distinguish between, on the one hand, the predominance by natural emphasis, of certain qualities, such as that of being suitable for food, and, on the other hand, the intentional isolation of these qualities for the purposes of thought and rational explanation. Abstraction they regard as a deliberate process applied with rational intent to the material afforded by experience and reflection. Generalization, too, they regard as deliberate, and carried out with like intent. The result is not merely a composite or generic product, but something more subtle and less dependent on sense. “All trees hitherto seen by me,” said NoirÉ, “leave in my imagination a mixed image, a kind of ideal presentation of a tree. Quite different is my concept, which is never an image.” The concept “tree” is a deliberate synthesis of abstract qualities intentionally isolated, and recombined in accordance with the general relationships which subsist between them.

If we accept this distinction, if we regard abstraction and generalization as intentional mental processes carried out with the rational intent of discovering the relationships of phenomena with the object of explaining them and recombining their essential features in an ideal scheme of thought, we shall probably admit, with John Locke, that these are excellencies which the faculties of brutes do by no means attain to. But we shall none the less see that the predominance of certain salient features in experience by reiterated emphasis in association with natural needs, and the development of generic in place of merely particular re-presentations will afford the appropriate material for abstraction on the one hand, and generalization on the other. Intelligence supplies the embryonic mental structures from which, under the quickening influence of a rational purpose, abstract and general ideas may be evolved.

The essential features of the evolution of intelligence seem, then, to be, first, the development of controlling nerve-centres, by which the responsive action of reflex automatic or instinctive centres may be checked, augmented, or modified; secondly, the increased differentiation and integration of these control centres with extension of the range and complexity of experience in close touch with practical needs; thirdly, the condensation and concentration of experience by the formation of generic products through the reiterated emphasis begotten of recurrent situations having certain salient features in common, though differing in details; and fourthly, an increased plasticity of behaviour, especially in early life, enabling an animal to deal effectually with an environment far less simple than that to which the more stereotyped instinctive behaviour is fitted by inheritance to respond. And this evolution of intelligent behaviour is working its way up to, though as such it cannot reach, the succeeding phase of mental evolution in which the data, supplied by intelligence, are treated with a new purpose for higher ends in the rational thought which seeks to explain the phenomena, and frame an ideal scheme of their relations and interconnections.

Two further points may be noticed. First, that it is during the early and plastic days or months of life that intelligence is setting its seal on animal behaviour, and stamping it with its distinctive character. Adult life is very much what youth has made it; and old age is stereotyped through habit. In times of progress, the character of the race is determined by plastic possibilities of the young. Among them it is that the incidence of elimination makes itself felt, resulting in the survival of those whose intelligence can mould behaviour in accordance with the new circumstances of a wider life.

Secondly, this selection of the intelligent involves the survival of those in whose higher brain-centres there is room for a greater range and variety of interconnection by means of associating fibres. It involves a selective survival of the larger and more finely organized brains. It is probable, as Professor Ray Lankester has recently indicated, that the ridiculously small-brained mammals and reptiles of the past were creatures of instinct with little capacity for intelligent control. Their lives were simple, and their enemies and competitors no better provided with higher brain-centres than themselves. Stereotyped instinctive behaviour sufficed to enable them to hold their own, and meet the requirements of a life of dull and unprogressive monotony. Strength without cunning made these big-framed animals for a while masters of the situation. But among those existing animals whose skeletons indicate an analogous zoological position, there is none which exhibits a cerebral development so poor. And we may fairly conclude that the fact that these huge creatures have left no lineal descendants may be taken as evidence of the importance and value, in evolution, of that cerebral tissue which is the organic basis of intelligence. The higher brain contains the potentiality of that experience without which the evolution of intelligent behaviour in any race of vertebrate animals is impossible.

V.—The Influence of Intelligence on Instinct

We have seen that the relation of instinct to intelligence is essentially that of congenital to acquired behaviour. We have seen, too, that in the Lamarckian interpretation what is acquired in the course of life may be transmitted through inheritance, and thus the intelligent behaviour of one generation may become instinctive and congenital in the next. But serious biological difficulties stand in the way of the acceptance of this interpretation; there is, moreover, little or no evidence of the assumed transmission to offspring of any acquired modifications of structure or behaviour. We have, therefore, been led to infer that instinctive behaviour has been evolved through the selection of adaptive variations of germinal origin, the influence of intelligence being restricted to the fosterage of co-incident variations, that is to say, of those congenital variations which coincide in direction with the acquired modifications of behaviour due to intelligence. It is clear that on this interpretation the influence of intelligence on instinct is more indirect and less simple than that implied by the Lamarckian hypothesis. Intelligence and instinct are in large degree independent, though there is continual interaction between them. We have now to consider the nature of this interaction, and to this end we must indicate the relation of acquired modifications to the hereditary groundwork of the animal constitution.

The basal fact is, that the bodily tissues are subject to a certain amount of structural change during the course of individual life in accordance with the amount of functional strain put upon them. The labourer’s thickened skin, the enlarged and strengthened muscles of the athlete, the juggler’s acquired suppleness are familiar cases. Less familiar instances are afforded under abnormal conditions. Should one kidney from any cause be slowly destroyed, the other will slowly enlarge to carry on the increased work of elimination of waste products; when the larger shin bone of a dog has been removed after injury, the smaller bone becomes thickened to bear the added strain; new joint surfaces are sometimes formed where bones have been broken and the natural joints injured.

One may say that the normal development of any structure depends upon a due amount of use. But, since in the course of strenuous life any organ is from time to time subject to an abnormal amount of strain, it must be fitted to respond to a super-normal call on its strength and functional activity. Were the heart and the lungs, for example, unable to meet the greatly added drain on their energies, due to unwonted and severe exertion, collapse, perhaps death, would ensue if such exertion were imperatively demanded under special circumstances. And it is clear that many wild animals must be not infrequently placed in such circumstances as will subject their muscular structures and the functional activity of their organs of circulation and respiration to a strain nearly up to their extreme limits of endurance. The carnivorous hunter would often fail to secure his prey if his organization were unequal to a hard and prolonged chase; the hunted prey would not survive to procreate his kind if he fell a victim to the first pursuer through inability to stand the exertion necessary to enable him to make good his escape. It is thus, we may believe, through natural selection that a sufficiently high standard of strength and functional endurance is maintained. The failures in these respects are steadily eliminated. It is difficult to realize the great strain put upon a bird’s organization by the migration flight. Some ten times as many birds leave our shores in the autumn as return to them in the following spring. What proportion of these is weeded out in the act of migration we do not know; but we may be sure that only those fitted to stand a severe test of physical endurance return to rear broods which shall inherit in large degree similar vigour of constitution.

Two factors, then, determine the limits of efficiency in the bodily organs—heredity and use. And these two co-operate in such a way that we may say, either that due use is the essential condition of the effective development of the hereditary powers, or that heredity serves to condition their effective development through use. But though closely related, so that each may be regarded as conditional on the other, they are, if we accept the view that acquired characters are not transmitted as such, so far independent in that use adds nothing to, disuse subtracts nothing from, the hereditary store. It is, indeed, difficult to conceive how, on any view, the absence of the conditioning factor of normal use can be the efficient cause of a positive diminution of the balance at the bank of heredity. And Lamarckian thinkers have not succeeded in placing their conception of the matter in the clear light of a working hypothesis.

The amount of what we may term “modifiability” by use differs a good deal in the several organs and tissues. The teeth of carnivora and the antlers of deer may be cited as structures in which the conditioning effects of use form a relatively unimportant factor. On the other hand, the nervous system, with which we are here primarily concerned, is of all animal structures that in which what is acquired may attain the greatest importance in the successful conduct of life; the nature and the range of behaviour affording an index of the amount of modifiability in this respect.

We have already seen that instinctive behaviour is primarily a matter of the first occasion on which any given action is performed, and that many instinctive acts are subject to subsequent modification in the light of the experience gained during the early performances. The range of such modification varies both in different animals and also with respect to different modes of behaviour in the same animal. The more fixed and deeply rooted an instinct the less readily does intelligence obtain a hold on it, so as to direct the behaviour into new channels of better accommodation to the circumstances. M. Fabre describes how a Sphex, one of the solitary wasps, instinctively draws its prey, a grasshopper, into the burrow by its antennÆ. When these were cut off the wasp pulled the grasshopper in by the jaw appendages; but when these were removed she seemed incapable of further accommodation to the unusual circumstances. It would seem an easy and obvious application of intelligence to seize the prey by one of the forelegs. But this was not done; and the grasshopper was then left. Intelligence did not seem equal to meeting the altered conditions presented by the maimed grasshopper. Still, there was some modification of the normal instinctive behaviour; and, as Dr. Peckham has shown, there may be more than Fabre noted. Let us assume the existence of an animal whose every act is instinctive, whose whole behaviour is marked out in strictly hereditary lines, no new departures being acquired in the course of individual life. This extreme case would afford an example of what we may term completely stereotyped behaviour. On the other hand, let us assume the existence of an animal with no hereditary definiteness of reaction, whose every act is intelligent, whose whole behaviour is the result of individual acquisition. This antithetical extreme case would afford an example of what we may term completely plastic behaviour. It is questionable, however, whether either of these extreme types occur in nature. What we find in our study of animal behaviour is some intermediate condition in which both factors co-operate, with a predominance either of stereotyped instinctive response on the one hand, or of plastic intelligent acquisition on the other hand. And in the latter case, as such behaviour approaches its ideal limits, we have modifiability under the circumstances of individual life at its maximum.

The evolution of intelligence as such runs parallel with the evolution of plastic behaviour; and this plasticity is necessitated by the variety and the complexity of the conditions of life—a variety and a complexity requiring many subtle modifications of response to enable the behaviour to reach accommodation to the changeful exigencies of diverse circumstances. To meet constant and relatively fixed conditions stereotyped instinctive responses suffice; and the elimination under natural selection of those individuals which fail to respond in fixed ways by specially adaptive behaviour tends to render definite the hereditary channels of nervous intercommunication. An inherited system of no little complexity may thus be evolved; of which we have seen examples in our study of instinctive behaviour. But the essential condition of the successful working of such a system is constancy in the environment to which the instinctive behaviour is adapted. Completely stereotyped behaviour, in its theoretical perfection, is in exact adaptation to the circumstances. Where instincts are only relatively perfect, further adaptation is secured through congenital variation and the survival of the individuals in which the behaviour is better adapted to the comparatively invariable circumstances. This is one line of evolution. But the evolution of intelligence is along independent lines of progress. Both, however, result from the functional activity of the same nervous system, they jointly determine the behaviour, they interact not only in the course of individual life but in the process of evolution, and they are both subject to the incidence of natural selection, which can determine whether the one line or the other shall preponderate—whether instinct or intelligence shall dominate behaviour.

If an answer must be given to the question whether instinct or intelligence has priority in the course of the evolution of behaviour, it may be urged that, on theoretical grounds, the claims of instinct are the stronger. Taking animals as we actually find them, however, they afford numberless examples of behaviour at first instinctive but subsequently modified, in greater or less degree, in accordance with the teachings of experience. Let us, first, assume that the environment is slowly changing, or has changed, in some definite manner. Such change would, of course, be relative, and might be due, either to new conditions brought to bear on the animal, or to the animal being itself brought, in the expansion of its life, within their influence. The old instinct is no longer quite adapted to the changed circumstances. If the change were sufficient in amount, and occurred somewhat suddenly, variations of instinct might not occur soon enough to enable the animal to reach adaptation by the gradual process of natural selection. If dependent on instinct alone the animal would, under these circumstances, be eliminated. But if intelligence were able to modify the behaviour to meet the new conditions this elimination would be prevented. In successive generations intelligence would constantly modify behaviour in the same manner and in a definite direction. Meanwhile congenital variations in different directions would occur. Those which were in directions antagonistic to that dictated by intelligence would tend to thwart accommodation and render it less effectual; but those which were coincident in direction would conspire with accommodation and render it more effectual. The individuals in which variations of instinct tended to thwart intelligence would be eliminated; while those in which coincident variations assisted and aided intelligent modification would survive. Thus intelligence would lead the way along lines which congenital variations would follow. And in the course of a number of generations the new instinct would reach the fully adaptive level, and further modification by intelligence would become unnecessary unless the environment continued to change yet more. Individual accommodation of behaviour would in this way determine the direction of instinctive variation; and yet throughout the process there would be, strictly speaking, no transmission of the intelligently acquired characters of the behaviour.

But though under constant and uniform changes in the environment the net result would be only a guided variation of the original instinct, under more variable and indefinitely changing circumstances the result would be different. The higher animals exhibit an intelligent plasticity which enables them to meet the requirements of the more complex environment into which their wider life has risen; for evolution lifts the animal from narrower into progressively wider spheres of activity and behaviour, so that its environment becomes relatively more complex. Here stereotyped behaviour would be rather a hindrance than an advantage. The winning animal in life’s struggle would be the one in which behaviour was most rapidly and most surely modified to meet particular needs—the one in which the teachings of experience were most promptly utilized in effective action. The inevitable tendency of the evolution of intelligence must be disintegration of the stereotyped modes of behaviour and the dissolution of instinct. Natural selection, which under a uniform and constant environment leads to the survival of relatively fixed and definite modes of response, under an environment presenting a wide range of possibilities leads to the survival of plastic accommodation through intelligence. It is not that intelligence has any direct influence tending to undermine the hereditary foundations of instinct, for acquired plasticity is not inherited as such; it is rather that when the stereotyped and the plastic are pitted against each other in the struggle for existence in the wider, freer, and more varied life of the higher animals the plastic survives and the stereotyped succumbs.

Imperfect as is our present knowledge of the manner in which the nervous connections implied in psychological associations are established, there can be no question that they are acquired in the course of individual life; they are modifications of nervous structure due to a special mode of use under the conditions of experience. Here, then, in the case of the nervous system, as in that of the bodily organs before mentioned, two co-operating factors determine the limits of efficiency—heredity and use. Just as the heart and lungs must inherit the power of standing abnormal strain if the animal is to avoid elimination in times of unwonted exertion, so must the nervous system inherit some reserve power of dealing effectively with unwonted circumstances by intelligent accommodation, if the animal is not to fall a victim to such circumstances. In other words, at times of heightened competition those animals which can draw on a reserve fund of intelligent accommodation will survive, while the stupid blunderers will be eliminated. We may term this reserve fund of intelligent accommodation, this inherited ability to meet specially difficult circumstances as they arise, innate capacity. From the nature of the case it must be indefinite, for it must carry with it the ability to meet unforeseen combinations of the environing forces by new combinations of the results of experience. Its distinguishing mark is plasticity, in contradistinction to the stereotyped fixity of typical instinct. And accompanying its evolution there is probably, as we have seen, a dissolution of its antithesis, instinct. Thus may we account for the fact that man, with his great store of innate capacity, has so small a number of stereotyped instincts.

But the dissolution of instincts is not complete. Residua are left in the inherited mental constitution. And these we term congenital tendencies and propensities. They differ from the typical instincts in the fact that the definiteness of response has been lost. They dictate a general trend of action, but the particular application in behaviour is due to intelligent accommodation. They are commonly spoken of as instinctive; and their mode of origin justifies the use of the adjective in association with the term “propensities.” But it must be remembered that the behaviour to which they lead is not, as such, wholly instinctive; it is a joint product of instinct and intelligence, the general trend being due to the instinctive propensity, while the mode of application is guided by intelligence.

There is, however, another way in which analogous propensities may be ingrained in the mental constitution, not as residual vestiges of old instincts, but as congenital rudiments fostered by new habits. It is a well-known and familiar fact that the frequent repetition of intelligent accommodation in certain definite lines begets habits, which so far simulate instincts as to be commonly described in popular speech as instinctive. Professor Wundt indeed places them in the category of “acquired instincts”—a usage which we regard as unsatisfactory, seeing that it tends to mask the distinction between the congenital and acquired factors in behaviour, and seeing that we have the well-defined term “habits” for acts rendered to a large extent automatic through repetition. Lamarckian thinkers regard habit as the mother of instinct, assuming that the acquired automatism of one generation may be transmitted to become congenital in the succeeding generation. This conclusion we provisionally reject, regarding the basal assumption as at present unproven. But though we cannot accept the view that habit is the mother of instinct, we regard it as not improbable that habit may be the nurse of congenital propensities. Remembering that similar habits are acquired by animals of the same species throughout a series of succeeding generations, and assuming that congenital variations are constantly occurring in many directions, it seems probable that some of these variations will be coincident in direction with the acquired habits. Thus would arise a congenital propensity to perform the habitual acts; and should they be of sufficient importance in the conduct of life to be subject to the action of natural selection, those animals in which such propensities were congenital would survive, whereas those in which no such propensities existed would be eliminated. It is unnecessary, however, to elaborate this conception further, since it is in line with that already discussed in considering the influence of intelligence in fostering a diversion of instinct under changing circumstances. Then we were considering how habit may lead to a congenital change in an old instinct; here we are dealing with the development of a new propensity.

Sufficient has now been said to illustrate some of the ways in which instinct and intelligence interact in the evolution of behaviour. Such interaction is further exemplified in the social life of animals, which will be dealt with in the next chapter.