Mental evolution in animals

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Peefacb Inteoductiok Chaptee I.

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CHAPTER I. The Ceiterion of Mind

The subject of our enquiry being Mental Evolution, it is desirable to begin by understanding clearly what we mean by Mind,* and then defining the conditions under which known Mind is invariably found to occur. In this chapter, therefore, I shall deal with what I take to be the Criterion of Mind, and shall then pass on in the next chapter to a consideration of the objective conditions under which alone Mind is observed to exist.

It is obvious, then, to start with, that by Mind we may mean two very different things, according as we contemplate it in our own individual selves, or as manifested by other beings. Eor if I contemplate my own mind, I have an immediate cognizance of a certain flow of thoughts and feelings, which are the most ultimate things — and, indeed, the only things — of which I am cognizant. But if I contemplate Mind in other persons or organisms, I can have no such immediate cognizance of their thoughts and feelings ; I can only infer the existence of such thoughts and feelings from the activities of the persons or organisms which appear to manifest them. Thus it is that by Mind we may mean either that which is subjective or that wliich is objective. Now throughout the present work we shall have to consider Mind as an object ; and therefore it is well to remember that our only instrument of analysis is the observation of activities

* It was necessary in nay -work on Animal Intelligence briefly to touch on this question ; therefore the parts of the analysis which are common to the two works I shall render as much as possible in the same words.

which we infer to be prompted by, or associated with, mental antecedents or accompaniments analogous to those of which we are directly conscious in our own subjective experience. That is to say, starting from what I know subjectively of the operations of my own individual mind, and of the activities which in my own organism these operations seem to prompt, I proceed by analogy to infer from the observable activities displayed by other organisms, the fact that certain mental operations underlie or accompany these activities.

From this statement of the case it will be apparent that our knowledge of mental activities in any organism other than our own is neither subjective nor objective. That it is not subjective I need not wait to show. That it is not objective may be rendered obvious by a few moments' reflection. For it is evident that mental activities in other organisms can never be to us objects of direct knowledge ; as I have just said, we can only infer their existence from the objective sources supplied by observable activities of such organisms. Therefore all our knowledge of mental activities other than our own really consists of an inferential interpretation of bodily activities — this interpretation being founded on our subjective knowledge of our own mental activities. By inference we project, as it were, the known patterns of our own mental chromograph on what is to us the otherwise blank screen of another mind ; and our only knowledge of the processes there taking place is really due to such a projection of our own subjectively. This matter has been well and clearly presented by the late Professor Clifford, who has coined the exceedingly appropriate term eject (in contradistinction to subject and object), whereby to designate the distinctive character of a mind (or mental process) other than our own in its relation to our own. I shall therefore adopt this convenient term, and speak of all our possible knowledge of other minds as ejective.

Now in this necessarily ejective method of enquiry, what is the kind of activities that we are entitled to regard as indicative of Mind ? I certainly do not so regard the flowing of a river or the blowing of a wind. Why ? First, because the subjects are too remote in kind from my own organism to admit of my drawing any reasonable analogy between them and it ; and, secondly, because the activities which they present are invariably of the same kind under the same cir-

THE CEITERION OF MIND. 17

cmnstances : they therefore offer no evidence of that which I deem the distinctive character of my own mind as such — Consciousness. In other words, two conditions require to be satisfied before we even begin to imagine that observable activities are indicative of mind ; the activities must be displayed by a living organism, and they must be of a kind to suggest the presence of consciousness. What then is to be taken as the criterion of consciousness ? Subjectively., no criterion is either needful or possible ; for to me, individually, nothing can be more ultimate than my own consciousness, and, therefore, my consciousness cannot admit of any criterion having a claim to a higher certainty. But, ejectively, some such criterion is required, and as my consciousness cannot come within the territory of a foreign consciousness, I can only appreciate the latter through the agency of ambassadors — these ambassadors being, as I have now so frequently said, the observable activities of an organism. The next question, therefore, is. What activities of an organism are to be taken as indicative of consciousness ? The answer that comes most readily is, — All activities that are indicative of Choice ; wherever we see a living organism apparently exerting intentional choice, we may infer that it is conscious choice, and, therefore, that the organism has a mind. But physiology shows that this answer will not do ; for, while not disputing whether there is any mind without the power of conscious choice, physiology, as we shall see in the next chapter, is very firm in denying that all apparent choice is due to mind.

The host of reflex actions is arrayed against the proposition, and, in view of such non-mental, though apparently intentional adjustments, we find the necessity for some test of the choiceelement as real or fictitious. The only test we have is to ask whether the adjustments displayed are invariably the same under the same circumstances of stimulation. The only distinction between adjustive movements due to reflex action, and adjustive movements accompanied by mental perception, consists in the former depending on inherited mechanisms within the nervous system being so constructed as to effect particular adjustive movements in response to particular stimulations, while the latter are independent of any such inherited adjustment of special mechanisms to the exigencies of special circumstances. Eeflex actions, imder the influence of their appropriate stimuli, may be compared to the actions of a machine under the manipulations of an operator : when certain springs of action are touched by certain stimuli, the whole machine is thrown into appropriate action ; there is no room for choice, there is no room for uncertainty; but, as surely as any of these inherited mechanisms is affected by the stimulus with reference to which it has been constructed to act, so surely will it act in precisely the same way as it always has acted. But the case with conscious mental adjustment is quite different. For, without going into the question concerning the relation of Body and Mind, or waiting to ask whether cases of mental adjustment are not really quite as mechanical in the sense of being the necessary result or correlative of a chain of psychical sequences due to a physical stimulation, it is enough to point to the variable and incalculable character of mental adjustments as distinguished from the constant and foreseeable character of reflex adjustments. All, in fact, that in an objective sense we can mean by a mental adjustment, is an adjustment of a kind that has not been definitely fixed by heredity as the only adjustment possible in the given circumstances of stimulation. Tor, were there no alternative of adjustment, the case, in an animal at least, would be indistinguishable from one of reflex action.

It is, then, adaptive action by a living organism in cases where the inherited machinery of the nervous system does not furnish data for our prevision of what the adaptive action must necessarily be — it is only in such cases that we recognize the element of mind. In other words, ejectively considered, the distinctive element of mind is consciousness, the test of consciousness is the presence of choice, and the evidence of choice is the antecedent uncertainty of adjustive action between two or more alternatives. To this analysis it is, however, needful to add that, although our only criterion of mind is antecedent uncertainty of adjustive action, it does not follow that all adjustive action in which mind is concerned should be of an antecedently uncertain character ; or, which is the same thing, that because some such action may be of an antecedently certain character, we should on this account regard it as non-mental. Many adjustive actions which we recognize as mental are, nevertheless, seen beforehand to be, under the given circumstances, inevitable ; but analysis would show that such is only the case when we have in view agents whom we already, and from independent evidence;, regard as mental.

THE CRITERION OF MIND. 19

In positing the evidence of Choice as my objective (or ejective) criterion of Mind, I do not think it necessary to enter into any elaborate analysis of what constitutes this evidence. In a subsequent chapter I shall treat fully of what I call the physiology or objective aspect of choice; and then it will be seen that from the gradual manner in which choice, or the mind-element, arises, it is not practically possible to draw a definite line of demarcation between choosing and non-choosing agents. Therefore, at this stage of the enquiry I prefer to rest in the ordinary acceptation of the term, as implying a distinction which common sense has always drawn, and probably always will draw, between mental and non-mental agents. It cannot be correctly said that a river chooses the course of its flow, or that the earth chooses an ellipse wherein to revolve round the sun. And similarly, however ccmiplex the operations may be of an agent recognized as non-mental — such, for instance, as those of a calculating machine — or however impossible it may be to predict the result of its actions, we never say that such operations or actions are due to choice ; we reserve this term for operations or actions, however simple and however easily the result may be foreseen, which are performed, either by agents who in virtue of the non-mechanical nature of these actions prove themselves to be mental, or by agents already recognized as mental — i.e., by agents who have already proved themselves to be mental by performing other actions of such a non-mechanical or unforeseeable nature as we feel assured can only be attributed to choice.

And there can be no reasonable doubt that this common-sense distinction between choosing and non-choosing agents is a valid one. Although it may be difficult or impossible, in particular cases, to decide to which of the two categories this or that being should be assigned, this difficulty does not affect the validity of the classification — any more, for instance, than the difficulty of deciding whether Limulus should be classified with the crabs or with the scorpions affects the validity of the classification which marks off the group Crustacea from the group Arachnida. The point is that, notwithstanding special difficulties in assigning this or that being to one or the other class, the psychological classification which I advocate resembles the zoological classification which I have cited ; it is a valid classification, inasmuch as it recognizes a distinction where there is certainly something to distinguish. For even if we take the most mechanical view of mental processes that is possible, and suppose that conscious intelligence plays no part whatever in determining action, there still remains the fact that such conscious intelligence exists, and that prior to certain actions it is always affected in certain ways. Therefore, even if we suppose that the state of things is, so to speak, accidental, and that the actions in question would always be performed in precisely the same way whether or not they were thus connected with consciousness, it would still remain desirable, for scientific purposes, that a marked distinction should be drawn between cases of activity that proceed without, and those that proceed with this remarkable association with consciousness. As the phenomena of subjectivity are facts at any rate no less real than those of objectivity, if it is found that some of the latter are invariably and faithfully mirrored in those of the former, such phenomena, for this reason alone, deserve to be placed in a distinct scientific category, even though it were proved that the mirror of subjectivity might be removed without affecting any of the phenomena of objectivity.

Without, therefore, entertaining the question as to the connexion between Body and Mind, it is enough to say that under any view concerning the nature of this connexion, we are justified in drawing a distinction between activities which are accompanied by feelings, and activities which, so far as we can see, are not so accompanied. If this is allowed, there seems to be no term better fitted to convey the distinction than the term Choice ; agents that are able to choose their actions are agents that are able to feel the stimuli which determine the choice.

Such being our Criterion of Mind, it admits of being otherwise stated, and in a more practically applicable manner, in the following words which I quote from " Animal Intelligence :" — " It is, then, adaptive action by a living organism in cases where the inlierited machinery of the nervous system does not furnish data for our prevision of what the adaptive action must necessarily be — it is only here that we recognize the objective evidence of mind. The criterion of mind, therefore, which I propose, and to which I shall adhere throughout the present volume, is as follows : — Does the organism learn to make new adjustments, or to modify old

THE CRITERTON OF MIND. 21

ones, in accordance with the results of its own individual experience ? If it does so, the fact cannot be merely due to reflex action in the sense above described ; for it is impossible that heredity can have provided in advance for innovations upon or alterations of its machinery during the lifetime of a particular individual."

Two points have to be observed with regard to this criterion, in whichever verbal form we may choose to express it. The first is tliat it is not rigidly exclusive either, on the one hand, of a possibly mental character in apparently non-mental adjustments, or, conversely, of a possibly non-mental character in apparently mental adjustments. For it is certain that failure to learn by individual experience is not always conclusive evidence against the existence of mind; such failure may arise merely from an imperfection of memroy, or from there not being enough of the mind-element present to make the adjustments needful to meet the novel circumstances. Conversely, it is no less certain that some parts of our own nervous system, which are not concerned in the phenomena of consciousness, are nevertheless able in some measure to learn by individual experience. The nervous apparatus of the stomach, for instance, is able in so considerable a degree to adapt the movements of that organ to the requirements of its individual experience, that were the organ an organism we might be in danger of regarding it as dimly intelligent. Still there is no evidence to show that non-mental agents are ever able in any considerable measure thus to simulate the adjustments performed by mental ones ; and therefore our criterion, in its practical appKcation, has rather to be guarded against the opposite danger of denying the presence of mind to agents that are really mental. For, as I observed in " Animal Intelligence," " it is clear that long before mind has advanced sufficiently far in the scale of development to become amenable to the test in question, it has probably begun to dawn as nascent subjectivity. In other words, because a lowly organized animal does not learn by its own individual experience, we may not therefore conclude that in performing its natural or ancestral adaptations to appropiate stimuli, consciousness, or the mind-element, is wholly absent ; we can only say that this element, if present, reveals no evidence of the fact. But, on the other hand, if a lowly organized animal does learn by its own individual 2 experience, we are in possession of the best available evidence of conscious memory leading to intentional adaptation. Therefore, our criterion applies to the upper limit of non-mental action, not to the lower limit of mental."

Or, again adopting the convenient terminology of Clifford, we must always remember that we can never know the mental states of any mental beings other than ourselves as objects ; we can only know them as ejects, or as ideal projections of our own mental states. And it is from this broad fact of psychology that the difficulty arises in applying our criterion of mind to particular cases — especially among the lower animals. For if the evidence of mind, or of being capable of choice, must thus always be ejective as distinguished from objective, it is clear that the cogency of the evidence must diminish as we recede from minds inferred to be like our own, towards minds inferred to be not so like our own, passing in a gradual series into not-minds. Or, otherwise stated, although the evidence derived from ejects is practically regarded as good in the case of mental organizations inferred to be closely analogous to our own, this evidence clearly ceases to be trustworthy in the ratio in which the analogy fails : so that when we come to the case of very low animals — where the analogy is least — we feel uncertain whether or not to ascribe to them any ejective existence. But I must again insist that this fact — which springs immediately out of the fundamental isolation of the individual mind — is no argument against my criterion of mind as the best criterion available ; it tends, indeed, to show that no better criterion can be found, for it shows the hopelessness of seeking such.

The other point which has to be noted with regard to this criterion is as follows. I again quote from " Animal Intelligence :" —

" Of course to the sceptic this criterion may appear unsatisfactory, since it depends, not on direct knowledge, but on inference. Here, however, it seems enough to point out, as already observed, that it is the best criterion available ; and, further, that scepticism of this kind is logically bound to deny evidence of mind, not only in the case of the lower animals, but also in that of the higher, and even in that of men other than the sceptic himself. For all objections which could apply to the use of this criterion of mind in the animal kingdom, would apply with equal force to the evidence of any

mind other than that of the individual ohjector. This is obvious, because, as I have already observed, the only evidence we can have of objective mind is that which is furnished by objective activities ; and, as the subjective mind can never become assimilated with the objective so as to learn by direct feeling the mental processes which there accompany the objective activities, it is clearly impossible to satisfy any one who may choose to doubt the validity of inference, that in any case, other than his own, mental processes ever do accompany objective activities.

" Thus it is that philosophy can supply no demonstrative refutation of idealism, even of the most extravagant form. Common-sense, however, universally feels that analogy is here a safer guide to truth than the sceptical demand for impossible evidence; so that if the objective existence of other organisms and their activities is granted — ^without which postulate comparative psychology, like all the other sciences, would be an unsubstantial dream — common sense will always and without question conclude that the activities of organisms other than our own, when analogous to those activities of our own which we know to be accompanied by certain mental states, are in them accompanied by analogous mental states."

CHAPTER II. The Structure and Tunctions of Nerve-Tissue

Having thus arrived at the best available Criterion of Mind considered as an eject, we have now to pass on to the topic which has already been propounded, viz., to a consideration of the objective conditions under which known mind is invariably found to occur.

Mind, then, so far as human experience extends, is only certainly known to occur in association with living organisms, and, still more particularly, in association with a peculiar kind of tissue which does not occur in all organisms, and even in those in which it does occur never constitutes more than an exceedingly small percentage of their bulk. This peculiar tissue, so sparingly distributed through the animal kingdom, and presenting the unique characteristic of being associated with mind, is, of course, the nervous tissue. It therefore devolves upon us, first of all, to contemplate the structure and the functions of this tissue, as far as it is needful for the purposes of our subsequent discussion that these should be clearly understood.

Throughout the animal kingdom nerve-tissue is invariably present in all species whose zoological position is not below that of the Hydrozoa. The lowest animals in which it has hitherto been detected are the Medusae, or jelly-fishes, and from them upwards its occurrence is, as I have said, invariable. Wherever it does occur its fundamental structure is very much the same, so that whether we meet with nerve-tissue in a jelly-fish, an oyster, an insect, a bird, or a man, we have no difficulty in recognizing its structural units as everywhere more or less similar. These structural units are microscopical cells and microscopical fibres. (Figs. 1, 2.)

The fibres proceed to and from the cells, so serving to

THE STRUCTUEE AND FUNCTIONS OF NERVE-TISSUE. 25

Fig. 1. — Motor Nerve Cells connected by intercellular processes {b, b), and giving origin to outgoing fibres (c, c, c, and a). 4. Multipolar cell containing much pigment around nucleus. Diagrammatic. (Vogt.)

Fig. 2.— Multipolar Ganglion Cell from anterior grey matter of Spinal Cord of Ox, a, Axis cylinder process ; ft, branched processes, magnified 150 diameters. (Deiters.)

connect tlie cells with one another, and also with distant parts of the animal body. The function of the fibres is that of conducting stimuli or impressions (represented by molecular or invisible movements) to and from the nerve-cells, while the function of the cells is that of originating those of the impressions which are conducted by the fibres outwards. Those of the impressions which are conducted by the fibres inwards, or towards the cells, are originated by stimuli affecting the nerve-fibre in any part of its length ; such stimuli may be contact with other bodies or pressure arising therefrom (mechanical stimuli), sudden elevations of temperature (thermal stimuli), molecular changes in the nerve-substance produced by irritants (chemical stimuli), effects of electrical disturbance (electrical stimuli), or lastly, the passage of a molecular disturbance from any other nerve- fibre with which the one in question may be connected.

Nerve-cells are usually found collected together in aggregates, which are called ganglia, to and from which large bundles of nerve-fibres come and go. These rope-like clusters of nerve-fibres constitute the white threads and strings which we recognize as nerves when we dissect an animal, (See Fig. 3.) The relation of the clusters of fibres to the cluster of cells is now such as to supply the anatomical condition to the performance of a physiological

Fig. 3. — Small Sympathetic Ganglion (human) with Multipolar Ceils. Magnified about 400 diameters. (Leydig.)

process, which is termed Eeflex Action. If we suppose the left-hand bundle of fibres represented in the woodcut to be prolonged and to terminate in a sensory surface, while the other three bundles, when likewise prolonged, terminate in a group of muscles, then a stimulus falling upon the sensory surface would cause a molecular disturbance to travel along the left-hand or in-going nerve to the ganglion; on reaching the ganglion this disturbance would cause the ganglion to discharge its influence into the right-hand or outgoing nerves, which would then conduct this disturbance into the group of muscles and cause them to contract. This process is called reflex action, because the original stimulus falling upon the sensory surface does not pass in a direct line

THE STRUCTURE AND FUNCTIONS OF NERVE-TISSUE. 27

to its destination in the muscles, but passes first to tlie ganglion, and is thence reflected from the sensory surface to the muscles.* This, which at first sight appears a roundabout or cumbrous sort of process, is really the most economic that is available. For we must remember the enormous number and complexity of the stimuli to which all the higher animals are perpetually exposed, and the consequent necessity that arises for there being some system of co-ordination whereby these innumerable stimuli shall be suitably responded to. And such a system of co-ordination is rendered possible, and actually realized, through this principle of reflex action. For the animal body is so arranged that the innumerable nerve-centres, or ganglia, are all more or less in communication one with another, and so receive messages from all parts of the body, to which they respond by sending appropriate messages down the nerve-trunks supplying the particular groups of muscles which under the given circumstances it is desirable to throw into contraction. In other words, when a stimulus falls upon the external surface of an animal, it is not diffused in a general way throughout the whole body of the animal, so causing general and aimless contractions of all the muscles ; but it passes at once to a nerve-centre, and is there centralized; the stimulus is dealt with in a manner which leads to an appropriate response of the organism to that stimulus. For the nerve-centres which receive the stimulus only reflect it to those particular muscle-groups which it is desirable for the organism, under the circumstances, to throw into action. Thus, to take an example, when a small foreign body, such as a crumb of bread, lodges in the windpipe, the stimulus which it there causes is immediately conveyed to a nerve-centre in the spinal cord, and this nerve-centre then originates, by reflex action, a highly complicated series of muscular movements which we call coughing, and which clearly have for their very special object the expulsion of the foreign body from a position of danger to the organism. Now it is obvious that so complicated a series of muscular movements could not be performed in the absence of a centralizing mechanism ; and this is only one instance among hundreds of others that might be adduced of

* The term, however, is not a happy one, because the process is something more than the reflection of the original stimvilus or molecular disturbance ; the ganghon adds a new disturbance.

the co-ordinating power which is secured by this principle of reflex action.

Of course we may wonder how it is that the nerve-centres, which preside over reflex action, not being endowed with consciousness, know what to do with the stimuli which they receive. The explanation of this, however, is that the anatomical arrangement of ganglion and nerves in any particular case is such as to leave no choice or alternative of action, if the apparatus is called into action at all. Thus, to begin at the bottom of the series, in the Medusae the simple ganglia are distributed all round the margin of the animal, and respond by reflex action to the stimuli which are applied at any other part of the surface. This has the effect of increasing the rate and the strength of the swimming-movements, and so of enabling the animal to escape from the source of danger. Now, although this is a true reflex action, and has an obvious purpose to serve, it does not involve any co-ordi nation of muscular movements. For the anatomical plan of a jelly-fish is so simple, that all the muscular tissue in the body is spread out in the form of one continuous sheet ; so that the only function which the marginal ganglia have to perform when they are stimulated into reflex action, is that of throwing into contraction one continuous sheet of muscular tissue.

Hence we may infer that in its earliest stages reflex action is nothing more than a promiscuous discharge of nervous energy by nerve-cells, when they are excited by a stimulus passing into them from their attached nerve-fibres.* But as animals become more highly organized, and distinct muscles are by degrees set apart for the performance of distinct actions, we can readily understand how particular nerve-centres are likewise by degrees set apart to preside over these distinct actions ; the nervous centres then perform the part of triggers to the particular muscular mechanisms over which they preside — triggers which can only be loosened by the reception of stimuli along their own particular lines of communication, or nerves. Thus, for instance, in the star- fish — animals which are somewhat higher in the zoological scale than the jelly-fish, and which have a more highly de\'eloped neuro-muscular system — the ganglia are arranged in a ring round

* For a full account of reflex action in Medusaj, see Phil. Trans., Croonian Lecture, 1875 ; also Phil. Trans., 1877 and 1880