Chapter IV: Introduction: §1 (2)

Such scepticism has much to commend it; but scepticism no less than piety can be employed as an excuse for mere intellectual laziness. Between those who advocate the mechanistic conception of life and those who reject it, there is a divergence of outlook more fundamental than usually appears in the course of controversy. Whether the same set of hypotheses will ultimately serve to interpret the properties of living and non-living matter may be left to the arbitrament of time. For practical purposes a decision one way or the other makes very little difference to the course of biological enquiry. The fundamental unity of scientific method in chemistry and physics is not invalidated by the fact that some phenomena can only be dealt with successfully in thermodynamical terms, while yet others yield only to treatment with the aid of kinetic and molecular hypotheses. It is less important to know how far the properties of living matter can be reduced to physical chemistry than to decide whether the logical structure of biological enquiry is essentially similar to or different from that of physical science. This is an issue of the most far-reaching consequences, not merely for philosophy but for biology as well. Though rarely stated explicitly, it represents the basic divergence of standpoint between the mechanist and the vitalist or holist. It is not merely a matter of taste or temperament: it is profoundly relevant to the way in which biological enquiry continues to develop. In this matter scepticism can only be justified by disinclination to face uncomfortable conclusions.

If the logical structure of biological enquiry is essentially similar to that of physical science, we must entertain the possibility of interpreting the whole domain of living matter without departing from the principle of ethical neutrality. This is not a pleasant possibility to admit; and it is hardly surprising that few biologists are enthusiastic in committing themselves with regard to it. If we find that there is no fundamental difference between the logical structure of biological and physical science, we cannot follow Dr. Whitehead in reviving the hope that scientific enquiry will eventually yield conclusions about the universe in conformity with our ethical predilections. If, without modifying the structure of its logic, biological science is capable of annexing as its parish the entire survey of living matter, there remains no nicely defined boundary at which science ends and philosophy begins. Philosophical enquiry must then abandon its pretensions to arrive at conclusions about the universe unaided by scientific discovery. It must restrict its operations to an examination of the logical structure of beliefs. It is therefore remarkable that the biological standpoint has been so little explored in contemporary criticism of traditional philosophy.

During the past two decades there have been three outstanding developments in biological research, the work of A. V. Hill and Meyerhof on the chemical mechanics of muscle, the extension of Mendel’s hypothesis by Morgan and his colleagues at Columbia, and the study of the conditioned reflex by Pavlov’s school. Of these the first alone represents an advance in the actual reduction of vital processes to physical chemistry. Yet no aspect of biology could be selected more appropriately than Morgan’s hypothesis to illustrate its logical unity with the study of chemistry. The study of the conditioned reflex has not as yet enlisted the resources of physical chemistry to any noticeable extent. Nor does it employ a logical technique as elaborate as that of the modern chromosome hypothesis. Its importance lies in the fact that it has emancipated biological study from the Cartesian dualism with its implicit assumption that method of enquiry applicable to one aspect of the properties of living matter is of a totally different kind from that employed in dealing with the remainder.

To estimate the significance of this advance it is necessary to start with a clear statement about the meaning of a word. The term _reflex_ is used by dentists, politicians and faith healers with a variety of implications irrelevant to the biologist. To exclude these irrelevant associations it is best to be concrete. Suppose that we decapitate or destroy the brain of a frog, and suspend it, legs downwards, in a vertical position. On raising a vessel of warm--about 40° C.--water, until the tips of the toes touch the surface, the legs of the animal are withdrawn after a short interval. This event takes place regularly and similarly under the same conditions. It is as definite and predictable a property of secular objects as is the precipitation of barium sulphate on mixing a solution of barium chloride with a solution of sodium sulphate. It is, if you care to express it in that way, a physical reaction between warm water and frog toes. In biological nomenclature it is a reflex.

The word reflex is not used in biology to denote every change that occurs in living matter. To clarify its meaning further we must consider how such a phenomenon can be studied more intimately. To the biologist it presents two types of problem. One is that of analysing the constituent parts of the reaction, and is analogous to what the chemist does, when he determines the solubility and dissociation constants of barium sulphate, barium chloride, sodium sulphate and sodium chloride to define more precisely what occurs during the reaction with a view to elucidating conditions under which it may be expected to occur. In the biological example that we have taken the first stage involves the purely spatial (or anatomical) examination of the reaction. It may be noted in this connexion that anatomy in its initial phase was an experimental science, and only became a catalogue in its dotage. We observe that we are dealing with a localized response to a localized agent involving a spatially localized structure the nervous system. We can in fact obtain the reaction from a preparation from which every structure but the skin of the toe, the nervous system and the muscles of the leg have been removed. From this point we proceed by a study of the temporal relations of the phenomenon, first undertaken by Helmholtz, to show that a disturbance is propagated at a measurable, predictable and modifiable rate from the seat of application of the agent to the seat of the visible reaction. The further analysis of the problem from the physico-chemical standpoint, an essentially modern development, will be referred to in a subsequent essay. We have now obtained the current definition of a reflex as a localized response to a localized stimulus, involving the intervention of the propagated disturbance known as the _nervous impulse_. Erroneous ideas implied in the common use of the term reflex arise chiefly in connexion with the second aspect of the study of reflex phenomena. This is not readily comparable with the investigation of a simple reaction like the precipitation of barium chloride. It might be compared with the interpretation of a more complex system such as the oxidation of oxalic acid in the presence of potassium permanganate and sulphuric acid, when the behaviour of any two reactants towards one another is already known. Frogs lift their legs from time to time in civil life, when they enjoy the use of a head. We may therefore ask what part do such reflexes, as we can study in the headless frog, play in the behaviour of the intact animal.

In any reflex displayed by the pithed frog the nervous impulse traverses a characteristic path. From the skin, the receptive area affected, it passes by one of numerous fibres of microscopic thickness to the spinal cord. Such fibres together with others carrying impulses from the cord to the muscles or glands collectively constitute the visible nerves. Fibres carrying impulses into the cord divide into very fine branches in the inner core or grey matter. These fine branches are intertwined with the ramifications of other fibres passing up and down the length of the cord. The latter branch at their other extremities around the fine endings of fibres which pass from the cord to the glands and muscles. An impulse entering the spinal cord first therefore passes across the junction or _synapse_ between the fibre along which it enters the cord and some other fibre running up or down the cord. Having traversed the latter, it passes across the junction or synapse between its branched ending and that of some fibre connecting the spinal cord with a muscle or gland. Reflex action depends upon the fact that an impulse travelling along a particular fibre can traverse some synapses more readily than others. This is a physical process, occupying a measurable time. By the use of certain physical reagents it is possible to increase the conductivity of the synapses, so that an impulse entering the cord irradiates to all the muscles of the body. Strychnine is such a reagent.

The familiar fact that the moth flies towards the candle will serve to illustrate how the study of a simple reflex, like the withdrawal of the toes of the pithed frog from warm water, makes it possible to make predictable conclusions about the normal behaviour of animals. If the nerves of the frog’s leg are severed, the leg hangs limply. Normally the muscles of the leg are never completely relaxed. They are maintained in a state of partial contraction or _tone_, reflexly determined by a number of agencies which for our present purpose it is unnecessary to specify. The nerve fibres which run up and down the length of the spinal cord in the frog cross from one side to the other at some level, and on this account most reflexes obtained in the pithed frog, when only one side is stimulated, involve muscular response of both sides of the body. Insects which move towards the light become noticeably more limp in darkness. Light reflexly increases the tone of their muscles. In insects there is little crossing of fibres from one side of the central nervous system to the other. It follows that, if light reflexly increases tone, the muscles of that side will be more contracted, when one eye is illuminated more strongly than its fellow. This will have the effect of bending the body round in the direction of the incident beam, until the head is brought into such a position that both sides are equally illuminated. Having attained this position the body will continue to move along the direction of the incident beam. If it swerves to the right or left, it is automatically readjusted.

This interpretation of the proverbial flight of the moth towards the candle permits us to make a very large number of easily verifiable predictions. One simple consequence repeatedly confirmed by experiment on a variety of insects which fly towards the light is the fact that, when one eye is blinded, they fly in circles. There is no need to mention the variety of predictable positions which such insects occupy, when allowed to crawl up rotating cylinders illuminated in various ways. One other rather interesting result of the experimental analysis of this phenomenon is worth mentioning. According to the common sense view the insect moves towards the candle, because it likes the light. There is one and only one fairly evident inference from the teleological way of looking at the matter. It implies that the moth should always fly from the darker to the brighter situation. Now the interpretation of its movement in terms of reflex action signifies that it is the direction of the light rays and not primarily the intensity of illumination which determines the direction of its movement. In Nature moving along the direction of the rays towards the source of light usually involves progression from a darker to a brighter region. In the laboratory it is easy to arrange conditions so that an insect crawling along the direction of an obliquely incident beam, moves from a brighter to a darker area, as it approaches the source. In doing so it behaves, as it would be predicted to behave in such a situation on the assumption that its behaviour is determined by reflex action. According to the teleological view it should do the opposite.

Even in the behaviour of so capricious an animal as man himself, it is possible to isolate units of behaviour to which the term _reflex_ is appropriate. The entire behaviour of a pithed frog or of a dog deprived of its brain can be regarded as the summation of a number of discrete reflexes compounded according to ascertainable laws. The problem is not a simple one; but the way in which the operation of one reflex affects the exercise of another has been elucidated with considerable success by Sherrington and his co-workers. Sherrington has paid special attention to what occurs in the simultaneous application of two stimuli whose appropriate responses involve the propagation of impulses along common fibres within the central nervous system. A further complication is introduced by the existence of inhibitory reflexes, responses which involve the cessation or the diminution of activity already in progress before the application of the stimulus. The work of Magnus and his colleagues, who have solved the riddle of how a cat falls on all fours, demonstrates to a very large extent the possibility of interpreting balancing movements of the body as the summation of such reflexes as are readily exhibited in the brainless or “spinal” animal. Yet few physiologists have ventured to entertain the likelihood that the entire behaviour of even such an animal as a cat, still less man himself, could be treated successfully in this way. Hence has arisen the traditional distinction between reflex and voluntary activity. So long as that distinction was a valid one, biology admitted a fundamental dualism in its subject matter and in its method. The vitalist was in a position to claim that there is a group of properties of living matter in dealing with which we must adopt introspective rather than physical methods of enquiry. The mechanist might reply epigrammatically that physiology deals with what we know about the central nervous system, psychology with what we do not know. The distinction still remained.

There are certain fairly evident reasons why the behaviour of a frog deprived of its brain should be simpler than that of the intact animal. One is that the number of possible paths along which nervous impulses can pass is much smaller. Another is the fact that the brain receives the nerves which bring in impulses from the three great receptor organs, or, in the older terminology, sense organs of the head. The eye and the ear bring the organism within the range of physical influence of innumerable events remotely situated in space. When we have allowed for all such differences there remains a perfectly tangible distinction between the behaviour of the spinal and that of the intact animal. The response that we have hitherto called a reflex is such that for a given agency under the same external conditions we may expect the same result. There are the best of reasons, based not on any introspective ideas but upon the study of behaviour to make us think that however much we standardize the external conditions at the moment, when the stimulus is applied, we can never predict from that alone exactly what will happen as the result of the application of certain types of stimuli. The performance of “learning” justifies this conclusion, and it has been customary in the past to refer this property of living matter to essentially non-physical concepts such as _memory_. By defining in this way the distinction between reflex behaviour in the traditional sense and voluntary or conscious behaviour, a new problem has emerged. This may be stated in the following way. If instead of concentrating exclusively on what is happening at the moment, we take into consideration the way in which a given stimulus has been presented to an organism on previous occasions, is it possible to establish any relation between the effect it now produces and the events associated with its application antecedently? In so stating the issue we have introduced no new and introspective concepts foreign to the traditional physiology of the reflex. We have simply envisaged the possibility of studying conditions under which new reflex systems may be brought into being.

§2

It is this problem which the Russian physiologist Pavlov and his co-workers have attacked with such conspicuous success during the past two decades. For some time their researches remained little known in this country, but two translations of Pavlov’s lectures are now accessible to the English-speaking reader. There is therefore no need to go into details concerning the experimental technique which is formidable. The more significant developments of the subject may be dealt with by considering how aspects of behaviour which were formerly referred to the introspective concepts of memory, attention and sensation can now be investigated without departing from the language adopted by physiologists, when describing the properties of simple reflex action.

Pavlov’s investigations commenced with the study of salivary secretion in dogs. A dog which has been deprived of the forebrain secretes saliva, when food is introduced into the mouth. The intact animal also secretes saliva, when food is brought within the range of its eyes or nostrils. In the adult the sight or smell of food is an appropriate stimulus for reflex salivary secretion. The ringing of a bell is ordinarily without effect on the secretion of saliva; but the ringing of a bell if repeated a certain number of times, when food is also presented, eventually comes to evoke salivary secretion, when food does not accompany it. In general it is found that, in the intact animal, a previously indifferent stimulus applied at suitable intervals simultaneously with the application of a stimulus which unconditionally evokes a reflex response is found to acquire the property of evoking the same reflex response, when unaccompanied by the original or “unconditioned” stimulus. A new reflex has been built up. Such reflexes are called by Pavlov _conditioned_ reflexes, and the previously indifferent stimulus is called the conditioned stimulus. Any event in the external world which affects a receptor organ may in the intact animal become a conditioned stimulus, provided external conditions are rigidly standardized in other respects, provided also that it accompanies the unconditioned stimulus a sufficient number of times depending on whether the application is precisely simultaneous, whether the conditioned stimulus begins to operate before the unconditioned, overlapping it in duration or separated from it by a short interval. The task of defining the facility with which a conditioned reflex is built up involves a study of the significance of the interval between successive applications of both stimuli and of the juxtaposition of conditioned and unconditioned stimulus. In defining the conditions which determine the bringing into being of a new reflex system by this method, we are investigating a class of phenomena which would formerly have been attributed to “memory.” At no point is it necessary to depart from the conventions of scientific nomenclature; and in place of a descriptive epithet, we arrive at a definite specification regarding when and whether an event will occur.

What it has been the custom to denote by the term memory is only one aspect of the problem of “conscious” or “voluntary” behaviour, that is to say those aspects of behaviour which are spatially referable to reflex paths in the fore brain. An animal is constantly subject to the simultaneous application of many indifferent and unconditioned stimuli, but its behaviour is selective. This introduces the problem of _attention_. To ascertain the conditions which prevent new reflex systems from coming into being, or extinguish them when they have become established, was perhaps the most important aspect of Pavlov’s work, because an understanding of this part of the problem underlies the successful control of experimental procedure. The possibility of isolating a conditioned reflex for study implies the existence of some inhibitory agencies which prevent the normal surroundings of the laboratory from exerting a significant influence on the course of the experiment. The inhibition of conditioned reflexes is a complex question; and its complexity emphasizes how broad a basis they offer for the interpretation of “conscious” behaviour in general and the interpretation of _attention_ in particular.

From this standpoint two important types of inhibition are called by Pavlov inhibition by extinction and conditional inhibition. The first term refers to the fact that, when an indifferent stimulus has been converted into a conditioned stimulus, and is then allowed to act repeatedly without the unconditioned stimulus, it gradually loses its potency, regaining it after an interval of rest. Conditional inhibition is the extinction which occurs, when a new indifferent stimulus is superimposed upon the effective phase of a conditioned stimulus. A third and especially important form of inhibition is the extinction of a state of inhibition by conditional inhibition, or as Pavlov calls it, inhibition of inhibition. Let us suppose that an organ note of one thousand vibrations per second has been made the signal for salivary secretion by repeated application of the stimulus, when food is administered to the animal. If it is now administered repeatedly without the accompaniment of food, it suffers inhibition by extinction, but recovers its efficacy after a period of rest. If, during the indifferent period, the experimenter superimposes on the now ineffective sound stimulus another indifferent agent such as the flash of a lamp before the dog’s eyes, secretion of saliva ensues. The sound regains its efficacy as a conditioned stimulus. One other type of inhibition which can be studied experimentally is “generalized inhibition” or elimination of the activity of the fore brain, which can be brought about in the dog by local warming or cooling of an area of the skin. This has an intimate bearing on the phenomena of sleep and hypnotic trance, as also on the advantages of summer time.

Perhaps the most radical consequence of the line of work which we are now considering lies in the possibilities which it presents for inverting our traditional attitude to the discussion of “sensation.” When we can isolate some simple unconditioned response to a particular stimulus, we can investigate the extent to which the efficacy of the stimulus is localized with reference to some receptive area, and discuss the sense organ in the same way as a piece of physical apparatus. We know for instance that a frog does not respond to white or black background by the appropriate change in colour of the skin, if its eyes are removed. The influence of the earth’s gravitational field on the way in which a frog maintains its normal balance in swimming provides another illustration of the way in which the experimental biologist deals with the phenomenon of receptivity, when it is possible to isolate a type of response which invariably accompanies a particular type of stimulation. In this instance the receptor is that part of the internal ear known as the labyrinthine organ. After destruction of the labyrinthine organ on one side only, a frog swims in a spiral path. If the internal ear of both sides is removed, it swims hither and thither, as likely as not upside down or sideways without any sign of its normal maintenance of balance. The inner ear of the frog or man with its three semicircular canals in the three Cartesian planes is a rather elaborate example of a type of receptor organ represented in shrimps by two little sacs called statocysts at the base of the feelers. These sacs contain concretions of sand known as the statoliths. Experimentally the sand can be replaced by iron filings. If this is done, the shrimp swims upside down, when a strong electromagnet is placed above it. The position occupied by the statolith in its sac is determined by the pull of gravity in ordinary circumstances. When the body is bent, the statocyst comes into contact with a new portion of the wall of the sac, thus stimulating a different set of nerve fibres, and initiating appropriate muscular reflexes. The balancing movements of a shrimp in swimming also depend on the eyes. With both feelers removed a shrimp swims normally in daylight. It loses its balance completely in a dark room; and swims on its back if illuminated from below. Removal of one eye or one statocyst does not affect its balance in daylight, unless the two operations are performed on the same animal. It then swims in spirals.

A modern biologist adopts to the statocyst and the eye the same attitude which he would adopt to the self starter of a motor car, if he were quite ignorant of its mechanism. Sometimes his problem is further complicated by the necessity of turning on the switch before the engine will start, adjusting the spark or cutting down the air. In an animal whose behaviour is largely conditioned behaviour, it is not so easy to isolate simple invariable responses to particular types of external agency. We lapse into the language of introspective psychology. Pavlov has shown that this is unnecessary. By employing the method of building up conditioned reflexes to define the limits of discrimination, the analysis of sensation can be carried out without departing from the attitude which we adopt to a motor car. Let us suppose that the sound of a tuning fork of 256 vibrations per second, i.e. middle C, is accompanied by electrical stimulation of the paw of the dog, until the note itself becomes an effective stimulus for withdrawal of the paw. A tuning fork of 264 vibrations will also evoke the withdrawal of the paw; but the application of the second stimulus suffers inhibition by extinction before the original (middle C), as can be shown by applying the latter after response to the tuning fork of 264 vibrations has been extinguished. Applying series of tuning forks in such experiments it is found that the limits of discrimination in dogs is a fraction of a tone. The delicacy of this method of testing discrimination or selective receptivity to a given range of stimuli depends on the fact that it is possible not merely to show whether one stimulus can be substituted for another in a conditioned reflex but to measure the extent to which a given stimulus can replace another. Judged from this standpoint dogs and cats are colour blind, as far as such a statement can have any tangible meaning. That is to say, differences of light intensity but not of wave length in the effective range determine the reactions of these animals to photic stimuli.

§3

In the light of Pavlov’s work the problem of conscious behaviour, or as we should now say conditioned behaviour, no longer presents itself to biological enquiry as a domain in which the methods of traditional physiology must be abandoned in favour of introspective speculation. It becomes the problem of defining how new reflex systems can be built up. The possibility of a further analysis of the process on mechanistic lines will be discussed elsewhere. Whatever success attends such an attempt, the fact remains that the controversy between the mechanistic and vitalistic schools must now be conducted on a new basis. Mechanistic biology could not claim to take a comprehensive view of the properties of living matter, so long as it failed to indicate how “voluntary” activity, as it was almost universally denoted by physiologists, differs from reflex activity. It is true that some of the more radical mechanists like Loeb preferred to speak of associative behaviour as having a more objective flavour. But Loeb’s own use of the concept of “brain images” emphasizes how fundamental is the innovation which the work of Pavlov’s school has introduced into philosophical discussion. The mechanist never legitimately claimed more than the right to investigate the properties of living matter in its simpler manifestations by those methods whose success had been justified in the domain of physics and chemistry. If the mechanist ventured to speculate beyond those limits he transgressed his terms of reference. Until the publication of the work of Pavlov’s school physiology was tied hand and foot to the traditional distinction between reflex and voluntary behaviour. Thus the author of a standard work on human physiology with a distinctly mechanistic tendency writes on the functions of the cerebellum: “... the degree of consciousness, if any, exhibited by the cerebellum is of a much lower order than that shown by the cerebrum. All observers agree that there is no apparent loss of sensation after removal of the cerebellum, but Luciani, Russell and others state their belief that in some indefinable way it is affected by such operations. Whatever functions of this kind are present we can define only by the unsatisfactory terms of subconscious rather than unconscious...” What Howell wrote in 1905 might have been written by any mechanist of that period. The physiologist inevitably lapsed into introspective terminology, when dealing with brain physiology; and it is this restricted mechanistic outlook which Dr. Haldane has attacked in his recent Gifford Lectures. It is not difficult to show that the mechanist, as that term is used by Dr. Haldane, accepted implicitly the Cartesian compromise. It is surprising that, although Pavlov’s work has been generally accepted by contemporary biologists, Dr. Haldane completely refrains from considering its bearing on the present status of the mechanistic conception of life.

Dr. Haldane’s statement that the method of traditional, i.e. mechanistic, physiology “tells us nothing, however far we may extend it, regarding the distinctive characters of conscious behaviour” is especially remarkable. Although few writers have hitherto ventured to formulate the far-reaching philosophical consequences of Pavlov’s work, more than fifteen years have passed since the veteran physiologist Sir William Bayliss made the following pronouncement:

“Pavlov states that he was struck by the fact that when the
physiologist leaves the study of the simpler parts of the central
nervous system which he has investigated by the observation of
reflexes, and proceeds to the higher parts, his methods suddenly
change. He gives up the observation of the relation between
external phenomena and the reaction of the organism to them and
introduces psychological ideas, derived from his own internal
consciousness. To extend to the higher centres the method of
observing what changes in the organism are correlated with external
changes might appear too difficult, but Pavlov has succeeded in
doing so to a remarkable degree” (_General Principles_, 1914, p.
502).

In denouncing the mechanistic view of life as set forth by Professor Donnan at the meeting of the British Association in 1928, Dr. Haldane states:

“I regard this view as now entirely obsolete, since it ignores
the facts, and this is far more evident now than it was a few
years ago, before physiology had become to so large an extent a
_quantitative science_” (italics inserted) “... The fact that
Professor Donnan, though his work in physical chemistry commands
universal respect among those who know it, is not a physiologist,
may partly account for his opinions.”

Perhaps also the fact that Dr. Haldane, whose work on the physiology of respiration and excretion commands universal respect among those who know it, neglects in his Gifford Lectures to make any reference to the work of Pavlov may partly account for his belief that “a biologist interprets his observations in a different manner from a physicist” (p. 97). It is certainly permissible to state that Dr. Haldane is not speaking for biologists as a whole, when he denies that the problem of conscious behaviour can ever be attacked successfully by the traditional method of the physiologist.

Biologists may be expected to differ in the hopes they may entertain as to the progress of further investigation. We can at least envisage the possibility that biology will advance towards a comprehensive account of the properties of living matter without interpreting its observations in a manner different from that adopted in physics. The work of Pavlov’s school shows that it is not necessary to introduce concepts foreign to other parts of biology in dealing with conscious behaviour. Of late years the notion of matter which is so fundamental to common sense has been disintegrated by the advance of the physical sciences. The notion of mind or consciousness so fundamental to common sense is being disintegrated by contemporary biology in an analogous way. If materialism in the traditional sense is dead, idealism in its traditional form is dead. Like traditional dualism they are dead because they never contained within themselves the capacity for growth. The success of biology in attacking the problem of “conscious behaviour” in Haldane’s terminology has been consistent with the attitude of treating _conditioned behaviour_ as an aspect of the properties of a peculiar kind of matter, living matter. In that sense the new philosophical outlook which emerges from Pavlov’s work is a materialistic one.

Physiology has at length discovered a neutral ground for the investigation of the problem of learning. If it is too early to predict the final outcome of this advance, it is permissible to proffer some tentative suggestions concerning its influence on the future of philosophical discussion. From Plato to modern times philosophical enquiry has mainly occupied itself with what Kant calls “the problems of mere pure reason.” Of these Kant enumerates God, Freedom and Immortality as the three principal objects of philosophical enquiry. For the final solution of these problems, Kant asserted that “philosophy stands in need of a science which shall determine the possibility, principles and extent of human knowledge _a priori_.” Introspective psychology was the “science” to which he assigned this task. Introspective psychology has failed to fulfil the expectations which Kant entertained, when he concluded the _Critique_ by expressing the hope that it “would bring reason to perfect contentment in regard to that which has always, but without permanent results, occupied her powers and engaged her ardent desire for knowledge.” The type of psychology which Kant promoted had already begun to sever its connexion with moral philosophy before the emergence of the Behaviourist tendency in an explicit form. Kant did not refute Hume’s arguments when he proposed the question, “whence could our experience acquire certainty, if all the rules on which it depends were themselves empirical and fortuitous”? He stated a problem. For its solution he lacked a method. For its discussion he lacked a vocabulary. If the physiology of human learning continues to progress under the Behaviourist influence to which Pavlov’s work has given birth, Kant’s solution of the problem, which he himself propounded, must eventually be relegated to the same status as astrology and palmistry in the history of human knowledge.

The strength of Kant’s case against Hume’s empiricism lay in the immature state of physiological knowledge, when the _Critique of Pure Reason_ was published. Kant’s views on Space and Time were circumscribed by the biological limitations of his period. The Kantian conception of experience was defined by the influence of light, sound, chemical stimuli, mechanical pressure and temperature affecting the eye, the ear, the nose, the mouth and the skin--the only receptor organs recognized by the physiologists of the eighteenth century. Two of the most important instruments of receptivity in the human body, the labyrinthine organ and the proprioceptors which respond to the state of tone of the muscles, were not studied till the nineteenth century. If Kant had been familiar with the physiology of the labyrinthine organ, he would not have argued with the same cogency that the concept of space is essentially different from the concept of weight. The _a priori_ necessity of the proposition that “space has only three dimensions” was determined, according to Kant, by the existence of an “external sense” which is “a property of the mind.” If he had lived fifty years later he would have realized that the “necessity” of the Cartesian frame work is a material consequence of the structure of the internal ear. If Kant had been familiar with Sherrington’s work on the proprioceptor organs, he would have seen a deeper significance in the experiment which Galileo performed, when he used his own pulse to measure the period of a swinging lamp. Kant was compelled to attribute the “_a priori_ necessity” of the proposition that “time has only one dimension” to “the internal sense by which the mind contemplates itself.” The time conditioned reflexes which Pavlov has demonstrated are intelligible to modern physiology without recourse to a “faculty of pure _a priori_ cognition.” The human body is itself a clock from whose tickings we can never escape. Periodic changes in tone of the body muscles influence the proprioceptor organs in a manner essentially analogous to the way in which light exerts its effect on the eye.[2]

Kant’s physiology calls for more detailed treatment elsewhere. In concluding this essay, I must remove one source of misunderstanding. I do not assert that all aspects of conscious behaviour will eventually be explained in terms of Pavlov’s conditioned reflexes. I do affirm that Pavlov has successfully applied the methods of traditional physiology to the study of processes presumably included in Dr. Haldane’s definition of conscious behaviour. The strength of Dr. Haldane’s position lies in the fact that behaviour ceases to be called conscious so soon as it is possible to bring it within the range of scientific prediction. I can well believe that the vitalists of fifty years hence will be assuring their opponents that they never regarded the process of learning, the phenomenon of attention or sensory discrimination as characteristics of the conscious state.

II. THE ATOMISTIC VIEW OF PARENTHOOD

“When you can measure what you are talking about, and express
it in numbers, you know something about it; but when you cannot
measure it, when you cannot express it in numbers, your knowledge
is of a meagre and unsatisfactory kind; it may be the beginning
of knowledge, but you have scarcely in your thoughts advanced to
the stage of science whatever the matter may be...”--Lord Kelvin,
_Addresses_

§1

The future progress of biological science depends upon a large number of unpredictable contingencies, some political, others meteorological. The collision of the earth with a comet may leave the fate of the argument between the mechanist and the vitalist for ever unsettled. There is therefore no justification for a dogmatic assertion that all the properties of living matter will eventually be reduced to the same hypotheses as are adopted in physical chemistry. But it is doubtful whether any biologists of the mechanistic persuasion have on any occasion explicitly committed themselves to so rash a statement. The vitalistic Sarah Gamp has invented a mechanistic Mrs. Harris with the express object of giving her a piece of her mind. As a polemical device this is most valuable, especially in political propaganda. It does not help the mechanist to understand what vitalism can offer as a guide to further biological enquiry. His perplexity is increased by the circumstance that so many vitalists of the platform behave themselves with mechanistic propriety in the laboratory. Dogmatism is at least as frequent among those who call themselves vitalists as among mechanists. The vitalist does not qualify his denial that a complete solution of the riddle of life can be obtained in physico-chemical terms. The mechanist is usually content to state that he knows of no other terms in which an intelligible solution could be found. The vitalist even goes further, and, quite inconsistently with his laboratory practice, if he is a competent biologist, asserts, that in its very methodology, biology is an _independent_ science. A biologist, says Dr. Haldane in his Gifford Lectures, “interprets his observations in a different manner from that of the physicist.”

This I think is the main bone of contention between the two attitudes which are generically denoted by the terms mechanistic and vitalistic. The real issue has shifted from deciding whether the hypotheses of physics and chemistry suffice for the interpretation of vital phenomena to deciding whether there is an essential difference between the logical structure of those branches of science that deal with living matter and those which deal with inanimate objects. This is a welcome change, because it presents a much more genuine and concrete problem for solution. It is somewhat surprising that the controversy should undergo such a metamorphosis at the present moment. The recent development of evolutionary biology is especially calculated to reinforce the belief that biological theory only progresses, when the biologist adopts towards the subject matter of his investigations the same attitude as that which the chemist and physicist adopt towards the objects which they study. In our generation it is possible to find in those aspects of biology which are most recalcitrant to the application of physico-chemical hypotheses the most conspicuous examples of a fundamental similarity in the logical procedure which the biologist on the one hand and the physicist or chemist on the other employ in constructing their hypotheses. It would not be possible to select from the whole field of biological science a more striking illustration of the success of quantitative and experimental methods than the recent extension of Mendel’s hypothesis by Morgan’s school. This advance has entailed an extensive elimination of teleological concepts in the interpretation of the evolutionary process. Yet the phenomena of heredity and variation at present lie completely outside the scope of physico-chemical analysis in the ordinary sense of the term; and any attempt to formulate the problems of genetics in physico-chemical terms is still a matter of pure conjecture.

In this sense we may agree with one writer of the vitalistic school in saying that to speak of the “mechanism of heredity” is a meaningless collocation of words. But if our interest is primarily directed not to the end product itself but towards the way in which the scientist proceeds to elaborate his hypotheses, the study of heredity provides a particularly clear example of how a hypothesis developed without any departure from the _principle of mechanism_ can yield verifiable conclusions about the behaviour of living systems. From this point of view it is both legitimate and intelligible to speak of the mechanisms of heredity and variation; and the expression is as permissible as the analogous phrase, the mechanism of chemical reaction. A comparison of the growth of the Mendelian principle with Dalton’s atomic theory of the structure of matter will help us to see whether the biologist does actually interpret his observations in a manner different from that adopted by the student of non-living matter, and whether the biologist has recourse to a kind of logic which is different from the logic which the physicist and chemist employ in framing their own generalizations.

When Mendel took up the problem of hybridization, the nature of fertilization in plants was known in a general way. Just a century before Mendel began his work Kolreuter by painting pollen from one individual on to the stigmas of another variety, and vice versa, had shown that hybrids inherit equally from the pollen and seed plant. At the end of the eighteenth century and the beginning of the nineteenth, Knight and Goss in England had made further progress in crossing pure bred varieties by calling attention to the “splitting” of hybrids, or reappearance of parental types when intercrossing hybrid offspring. Contemporaneously with Mendel, Naudin in France studied this phenomenon more closely, and came very near to formulating Mendel’s principle. His results were published in 1862. These pioneers in hybridization laid down the necessity of working with what to the geneticist is like pure chemicals to the chemist, pure breeding stock. They fell short of arriving at far-reaching results, because their attitude to heredity was dominated by the holistic standpoint. They could only think of the plant in terms of a preconceived notion of individuality. They refrained from focusing their attention on the separate parts, and following out the fate of discrete characteristics in their crosses.

We must not overlook the debt which Mendel owed to the pioneers of hybridization. There would have been no modern chemistry if the Arabs and alchemists had not devoted years of laborious study to the clarification of our idea of a pure substance; and there would have been no genetics, if the idea of pure breeding stock had not been laid down by Mendel’s predecessors. Chemistry failed to progress beyond the stage of describing new compounds so long as it remained entangled in the vitalistic “phlogiston” concept; and genetics, the study of heredity and variation, remained purely descriptive, until it was emancipated by Mendel from the holistic tendency to concentrate upon the organism as a whole. Naudin did in fact envisage less definitely than Mendel the atomistic concept of heredity, just as William Higgins had partly visualized the chemical possibilities of atoms before Dalton published his theory.

Mendel used in his researches pure breeding stocks differing only in well-defined particulars, employing single characteristics as units of study, and recording the progeny of every cross separately for comparative observation. In his original work Mendel chiefly dealt with the common pea, which possesses two advantages which recommend it for such experimentation, namely, that its flowers are capable of self fertilization (i.e., the pistil can be pollinated from the stamens of the same flower) and that it has a number of well-marked varieties distinguished by tangible characteristics such as the shape (round or wrinkled) and colour (green or yellow) of the seeds, or the stature (tall or dwarf) of the shoot, etc. In all his crosses involving a single difference of this kind he found that the first generation of the cross resembled one of the parents. When these crossbreeds were self fertilized, they produced offspring resembling the original parents in the constant ratio of three to one. One-quarter of the offspring of the crossbreeds resembled one parent and bred true; one-quarter resembled the other parent and bred true; and the remaining half being like the “dominant” parent, which the first generation of hybrids resembled, behaved exactly like the latter, when self fertilized.

An investigator who had not the attitude which makes a capable chemist might have been distracted by the peculiar circumstance of dominance, or the resemblance of the impure individuals to one of the parents exclusively. Mendel rightly judged this to be insignificant. A chemical analogy will perhaps assist to make this clear. Sodium and potassium yield colourless salts with most common acids, but the permanganates of both are purple in solution. The salts of copper are generally of a bluish or greenish tint in solution. In the one case the anion, in the other case the kation, is the dominant factor in determining the physical property of colour; but in both cases the other component behaves in any reaction with no less characteristic efficacy, because its presence is seemingly masked. So likewise Mendel looked beyond the bodily resemblance of the dominant parental and hybrid individuals to their hereditary makeup; and recognized in his experimental data two general conclusions which prompted special consideration. One was the fact that the original parental types can be recovered in all their purity. The other was the fact that the various hereditary types produced by hybridization regularly appear in the same numerical ratios. Both conclusions are of universal validity, though Mendel had the very good fortune to select materials which yield the simplest type of numerical results which occur in crosses between pure strains. When Dalton formulated the atomic hypothesis two fundamental empirical generalisations of chemistry were fully accredited. The law of the conservation of matter and the law of constant proportions had been established. Mendel found in his data the proof of what we might call the principle of the conservation of genetic materials and the law of constant genetic proportions. To the recognition of these empirical generalizations he added a conceptualization of the basis of their existence in terms of discrete factors. These factors were according to Mendel’s hypothesis (or Mendel’s “first law”) units of hereditary combination, just as Dalton’s atoms were units of chemical combination.

Each character involved in his crosses was regarded by Mendel as determined by a factor derived from the maternal and one derived from the paternal parent. A pure individual was thus represented by _aa_ or _bb_, and an impure individual by _ab_. Mendel assumed that _a_ and _b_ are atoms of heredity in the sense that they retain their separate entities through the whole course of development. Having introduced this conception, he showed that all his numerical data followed from the laws of chance, if the maternal and paternal factors which determine a particular character separate in the formation of the gametes (pollen and ovules) so that one-half of the gametes contain only the maternal and one-half only the paternal factor for the character considered. The combinations which may occur as the result of fertilization are compatible with the assumption that any given male gamete (pollen or sperm) may fertilize any given female gamete (ovule or egg cell). Mendel’s first law may then be stated thus: characters distinguishing different hereditary strains depend upon factors which are inherited from both parents and _segregate_ in the formation of the gametes, so that one-half contain the paternal and one-half the maternal factor. Mendel tested the implications of this hypothesis by crossing his hybrids to pure types with verifiable results. He then proceeded to make crosses involving two or three character differences. This led him to enunciate a second law which might be compared with the law of multiple proportions in chemistry, for its validity is of less general significance than the first law. It served eventually to direct attention to the much more complicated numerical results which arise in dealing with character differences attributable not to one but several pairs of factors. The analysis of such cases was left to Mendel’s successors.

There is internal evidence in Mendel’s writings to support the view that Mendel himself realized that the atomistic conception of inheritance would demand a drastic revision of the prevailing notion of variation. To Mendel’s generation, to Darwin and the pioneers of Natural Selection, variation and heredity were co-extensive terms. Offspring were always on the whole like their parents, but always on the other hand a little different. So the species in conformity with sound liberal principles broadened down from precedent to precedent. But on the atomistic view heredity is essentially conservative, and variation essentially revolutionary. For an indefinite number of generations the atoms of heredity remain unchanged. But times come, when the political barometer falls, and the change when it happens is a discontinuous one. Something new has been brought into being, as when lead is produced from the disintegration of radium or another allotropic modification of an element is formed. The full implications of this were not destined to be realized till forty years had elapsed. Meanwhile the evolutionary ship drifted upon an uncharted ocean of speculation without the compass of experiment to direct its course.

§2

Mendel’s work published in an obscure horticultural journal remained neglected for forty years, till in 1900 his principle was independently rediscovered by three continental workers--de Vries, Tschermak, and Correns. During that period the study of the reproductive process had progressed rapidly. The way was being paved for new and spectacular developments of the atomistic standpoint in heredity. To appreciate the subsequent elaboration of Mendel’s hypothesis in its historical perspective a brief digression into the anatomy of the cell is necessary.

Mendel’s researches were confined to plants. When he started his work the nature of fertilization in animals was still obscure. The bodies of animals like plants were known to be built up of microscopic bricks, or _cells_ as Robert Hooke had called them. With the use of more powerful microscopes this had gained general recognition during the thirties and forties. Two centuries had elapsed, since Leeuwenhoek with the first microscope had seen seminal fluid teeming with minute vibratile bodies, the _spermatozoa_. At the end of the eighteenth century that inquisitive ecclesiastic Spallanzani had shown that the sperm is the essential constituent of the seminal fluid. By 1841 Kolliker had traced the development of the spermatozoa from single cells of the testis. It was not until 1875-9 that Hertwig and Fol working on sea urchins independently observed for the first time in history the penetration of the egg by the sperm, and established the universal rule that fertilization involves the union of a single sperm with a single egg cell. All modern discussion of genetic differences takes its starting point from the fact that anything which is implied by the word inheritance has its material basis in the microscopic sperm contributed by the father or in the egg cell with which it unites.