PART I. Vitalism and Mechanism

VITALISM AND MECHANISM

SUMMARY

An uneasy recognition of the conflict between science and common sense in our generation has rekindled interest in the relation of science to moral philosophy. In this awakening the physicists have assumed the leading part. It will not be possible to predict the outcome, until the contribution of contemporary biology to natural philosophy is taken into consideration. Some writers have expressed the hope that the influence of biological concepts may assist to a reconciliation of the claims of natural science and moral philosophy. This hope is based on a failure to recognize that modern experimental biology is an ethically neutral body of enquiry. The merits of a mechanistic or vitalistic outlook in biology have been too often discussed from an ontological rather than an epistemological standpoint. Our estimate of the influence of biological concepts on the future of natural philosophy must be guided by a recognition of the essential similarity of method in biology and physics. This similarity is nowhere more evident than in those branches of physiology which lie most conspicuously outside the realm of applicability of physico-chemical hypotheses. Traditional mechanistic physiology has accepted the Cartesian dualism of mind and matter. The modern physiology of the conditioned reflex has undermined the distinction between reflex and voluntary behaviour. There is thus no nicely defined boundary at which physiology ends and philosophy begins. Biology is annexing regions of enquiry which have hitherto remained the province of moral philosophy. As a concept of biology Mind is replaced by Behaviour. Since modern biology claims to interpret the characteristics of conscious behaviour as properties of physical objects, the advance of biological science cannot be expected to reinforce the claims of moral philosophy. How far it is possible to reduce the interpretation of behaviour to purely physico-chemical hypotheses, we have no means of predicting. At present we can foresee no limit to progress in that direction. The significant issue is not the completeness of the mechanistic solution, but whether there exists any definable method of arriving at a more complete solution than the mechanistic outlook permits.

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“It is our happiness to live in one of those eventful periods of intellectual and moral history when the fast-closed gates of discovery and reform stand open at their widest. How long these good days may last we cannot tell. It may be that the increasing power and range of scientific method with its stringency of argument and constant check of fact may start the world in a more steady and continuous course of progress than it has moved on heretofore. But if history is to repeat itself according to precedent, we must look forward to stiffer, duller ages of traditionalists and commentators, when the great thinkers of our time will be appealed to by men who slavishly accept their tenets, yet cannot or dare not follow their methods through better evidence to higher ends. In either case it is for those among us whose minds are set on the advancement of civilization to make the most of present opportunities that, even when in future years progress is arrested, it may be arrested at a higher level.”

Tylor’s Primitive Culture

INTRODUCTION

No one who is familiar with contemporary thought can have failed to recognize two characteristics which have emerged into prominence during the past two decades. With increasing elaboration of its logical technique, science has been brought into apparently irreconcilable conflict with common sense. The result is that scientists, uneasy in the realization of this conflict, are seeking to establish a new working relation between science and philosophy. This rapprochement has been brought about especially through recent progress in physics. It will not be possible to predict its outcome so long as the physicist claims to speak for science as a whole. In this introductory essay I propose to discuss in a somewhat discursive and preliminary way how far the conflict between science and common sense is apparent rather than real, and to indicate the special need for reviewing the progress of modern biology in its philosophical bearings.

At the present time few biologists are anxious to court publicity in the field of philosophic controversy. Those who do so are rarely numbered among the ranks of those who are still actively contributing to contemporary progress in biological enquiry. Those who are actively contributing to the advancement of biological knowledge show little disposition to commit themselves to far-reaching generalizations. There has emerged from the morass of speculation associated with the rise of the evolutionary hypothesis a recognition of the paramount importance of painstaking quantitative study of limited aspects of vital phenomena. This attitude is a salutary one. It does not signify that biology is passing through a phase of stagnation. On the contrary current biological discoveries contain the germ of philosophical issues which may prove to be as revolutionary as Relativity and as repugnant to common sense. In the essays which follow I shall confine myself to accredited experimental data. I do not pretend that all or even a majority of biologists will agree with my interpretation of their philosophic significance.

There is no novelty in asserting the need for incorporating the contribution of biological science in a modern philosophical outlook. Herbert Spencer and the Evolutionists prepared the ground fifty years ago; but they failed to lay emphasis on the methodological aspect of biological enquiry. The methods and not the results of biological science are specially significant to philosophical discussion. In putting forward my own views upon the nature of life, it is not the results of biological enquiry, but the methods which I propose to discuss in the first series of essays in this volume. In contrasting the methods and concepts of physical and biological science, I shall sometimes draw inferences which will not commend themselves to the judgment of biologists for whose contributions I entertain a lively respect. I shall not be surprised to be told that my forecast of the outcome of biological enquiry is pretentious, and that my philosophical conclusions are in conflict with common sense.

At an early age I abandoned the conviction that scientific hypotheses must conform to the requirements of common sense. When I was a boy, there used to be in Portsmouth, the town of my nativity, a public figure by name Ebenezer Breach. Mr. Breach was a philosopher. To be precise he described himself as “Natural Astronomer and Poet.” In that he belittled his gifts. Of his poetry I shall say nothing, save that he stated the qualification “poet by Royal Patronage” in his fascinating brochure Twenty Reasons against Newtonianism or The Universal Challenge to Unnatural Science. This was sold for the modest price of twopence sterling. As his contribution to modern thought may be unfamiliar to many cultivated people who were not born in Portsmouth, I propose to quote the first of his twenty reasons as representative of the system he develops:

“Because the earth has no axis, therefore nothing on which to revolve, an imaginary mathematical line is substituted. But no solid body could revolve on an imaginary axis or line. It is an imaginary cause which can only produce an imaginary effect, so all that follows the cause must be imaginary. If anything be placed on the top of a revolving body it will fly off at a tangent.”

From this you might infer, wrongly it happens, that Mr. Ebenezer Breach earned a comfortable livelihood as Regius Professor of Moral Philosophy in an authentic University. He had in fact chosen to bear witness to the hope that was in him by the only alternative which a harsher economic destiny had imposed. Every Saturday night he addressed a handful of half-intoxicated seamen, tired commercial travellers, adventurous nursery maids and irreverent pupil teachers foregathered on the sea-front. There he occasionally succeeded in selling a copy of the Twenty Reasons, and beyond this obtained, as far as I am aware, no reward in the life that now is. In spite of his erudition and distinction of person, Mr. Breach, the prophet of common sense, did not make many converts. He was less successful in his popular appeal than an evangelical competitor who used to minister to Portsmouth beach before a banner whose legend stated, “the wages of sin is death.” This banner I can still recall as, in its way, a work of art. On the foreground were displayed the theatre, race-course, public-house, dancing saloon and gaming tables along the edge of a precipice over which poor folk in a semi-incandescent condition were tumbling into a lake of brimstone and fire. It invariably drew a large crowd. I had early imbibed the notion that science like Sunday travelling, whist and dramatic entertainment is worldly, so that the gospel of Mr. Breach, who condemned science on account of its essential unworldliness, presented a new and arresting point of view. On the whole the inhabitants of Portsmouth were more interested in their souls and what would become of them after death. Mr. Breach had another competitor with more peculiar views about the soul and about life. As far as I can remember he held that the brain secretes consciousness in much the same way as the liver secretes bile, and he asserted that the soul was the shadow cast by the machine. My nurse held very definite views about his domestic life. He was a materialist, and in all probability a polygamist, if not worse. Mr. Breach who was a bachelor, the evangelist who was certainly not a polygamist, and the Secularist who was undoubtedly a bad man all agreed in one particular. Each believed that the gospel he proclaimed was common sense.

Of the Flat Earth faith Mr. Ebenezer Breach is the only Confessor and, financially speaking, Martyr I have been privileged to encounter. I cherish the recollection of his secular ministrations for a reason which is eminently relevant to everything which I propose to say about the bearing of current biological concepts on philosophical discussion. At an age when, to my way of thinking, Punch and Judy were the only serious rivals to the magnetism of his stupendous intellectual gifts Mr. Breach stands out in the sharp relief of retrospect as the Forerunner of the coming conflict between science and common sense. I have already remarked that the uneasy recognition that science conflicts with common sense has been the keynote of philosophical controversy during the past decade. Curiously enough some scientists seem to regard this as a grave disability on the part of science. They feel compelled in consequence to adopt an apologetic attitude to the claims of traditional philosophy. Perhaps this is because the protagonists of science in the nineteenth century made it their proud boast that science is nothing more than organized common sense. They therefore felt that they had the man in the crowd on their side. Even Herbert Spencer, prophet of evolution, when evolution was still a subversive doctrine, could soberly declare that “the ultimate truth of a proposition is the inconceivableness of its negation.”

Neitzsche includes this quotation in the Will to Power as one of his “inscriptions over the porch of a modern lunatic asylum.” It is only necessary to mention the word Relativity to indicate how impossible it would be for a natural philosopher to express himself in similar terms to-day. The situation which has been created by progress in modern physics is not without parallel in human history. It is true that the new theories have employed an immensely elaborate and difficult logical technique. How far they can be simplified it is at present impossible to predict. Newton’s fluxions were unfamiliar to his contemporaries. The author of the Principia devoted a good deal of time to a geometrical presentation of his ideas, in order to make them accessible to his generation. For more than a century after Newton’s death the calculus remained a preserve for mathematical specialists. To-day a knowledge of the calculus requisite to an elementary understanding of the theory of elliptical orbits lies within the scope of the first year’s work at a university, if it has not been acquired in the higher forms of a good school. It is conceivable that the mathematical development of modern physical theories will be simplified in the course of time. In that sense the esoteric stage through which physics is now passing may be a temporary phase. The essential feature of the conflict between common sense and physical science in this generation lies in the unfamiliarity of the new concepts. The conflict between common sense and the new biological concepts shares the same characteristic.

In Bernard Shaw’s St. Joan, La Tremouille asks: “Who the deuce was Pythagoras?” “A sage,” replies the Archbishop, “who held that the earth is round and that it moves round the sun.” “What an utter fool,” says La Tremouille, “couldn’t he use his eyes?” La Tremouille here calls attention to a fact that was overlooked by Herbert Spencer, by Mr. Ebenezer Breach and by those Relativist philosophers, who, being unable to convince the man in the crowd, indulge in the luxury of wondering whether the claims of scientific method have been pushed too far. Common sense is another name for what good citizens are prepared to accept without argument. Scientific ideas only conflict with common sense so long as they are still new and unfamiliar. Mr. Breach was in advance of his time in daring to criticize the Newtonian system. He was behind his time in thinking that Newton’s position could be assailed successfully with the weapons of common sense. The essential rightness of the Newtonian system had already become incorporated in British middle-class respectability. To the rising generation suckled on Mr. Wells’ Outlines evolution is common sense. Two generations have elapsed since, as La Tremouille would say, any fool who used his eyes could see that a bishop was a product of special creation. The man in the crowd has no clearer notion of the logical status of the doctrine of descent than had his grandfathers who implicitly accepted the story of the Fall.

The phenomenal success of those who set out to popularize the Theory of Evolution makes it easy to overlook the circumstance that evolution was wholly repugnant to common sense within the memory of those who are still living. The outburst of public controversy which greeted its announcement has no parallel in this generation. In consequence its impact upon traditional philosophy has been far less apparent than its influence upon religious dogma and social theory. The younger generation of biologists cannot recapture the first fine raptures of enthusiasm which their elders experienced. The prevailing attitude is to welcome a return to the complacent dualism of pre-Darwinian days, when scientists did not meddle with philosophy and metaphysicians conceded to scientists the right to go to the devil in their own way. Although this view is widely held, I do not believe that the philosophical implications of evolution have ever been thoroughly explored; or that it was possible to do so, while the study of animal behaviour was still dominated by the language of introspective psychology. By explaining the secular origin of philosophers Darwin bequeathed to us the task of elucidating the anatomy of philosophy.

In the opening years of the present century, science had already lost that truculence which one associates with the generation of Huxley and Tyndall. It had surrendered its tradition of fearlessness and candour. Academic philosophy, liberal theology and utilitarian science went their placid ways without mutual interference. Bergson, a philosopher more widely known than Mr. Ebenezer Breach, had cast a pebble of belles lettres into the mill-pond of compromise. The indifference with which it was greeted by those engaged in the task of placing the evolutionary problem upon a secure foundation of experimental data is a measure of the esteem which they entertained for it. It has been interpreted as assent by some contemporary writers who are not themselves biologists. In Science and the Modern World Dr. Whitehead even speaks of Bergson’s “instinctive grasp of modern biology.” A modern biologist engaged in the study of behaviour would refer with greater caution to Madame Blavatsky’s instinctive grasp of modern astronomy or Mahatma Gandhi’s instinctive grasp of modern economics. He would regard the instinctive grasp of any branch of scientific knowledge with more suspicion than approval. No biologist has undertaken the task of examining the philosophical implications of Darwin’s doctrine in the light of contemporary progress in the experimental analysis of living matter.

Under the influence of Hegel, academic philosophy left the scientist to his own devices. To-day the physicist has again driven the idealist philosopher out of his retreat. He has compelled him to take account of a conceptual world which we all recognize whenever we consult a railway time-table or book a passage in an ocean liner. Secure in the prospect of fresh philosophical victories, the astronomer surveys the world with a blind eye to the microscope, and magnanimously dictates the new territorial frontiers of science and moral philosophy to the advantage of the latter. It has been customary in the past, and therefore common sense, to assume that the issues with which moral philosophy deals are more fundamental than those which fall within the scope of natural science. It is an assumption which, whatever its meaning, does not hamper the advance of pure physics; but the biologist is not bound to accept this convention when it restricts his own field of enquiry. A philosopher is a particular kind of organism. Philosophy itself might therefore be regarded as an aspect of the behaviour of a piece of living matter. The study of the properties of living matter is the province of the biologist. From this point of view the study of biology is more fundamental than the pursuit of moral philosophy.

The physicist brings to the discussion of philosophy the discipline of an older branch of enquiry with a more elaborate logical technique than that of biology. His claim to speak for the whole field of science should be scrutinized with a critical eye. I am sure that Professor Eddington will agree with me, when I say that the biologist has a specific contribution to make to what he has aptly called the “world symposium.” I am also confident that many biologists will agree with me, when I state that the contributions of the Relativist philosophers rarely display a profound understanding of the kind of problems biologists are now attempting to solve, and the way in which the modern biologist sets about his task. A quotation from Mr. Sullivan’s Bases of Modern Science, a stimulating and provocative book, will illustrate my meaning. Mr. Sullivan, whose physics I do not venture to criticize, states: “The primary concepts in terms of which the science of physics is constructed... have to be supplemented by others in the science of chemistry, and for the sciences of life and mind, are so far from being sufficient, that they have hardly yet been found to be relevant.” Half a century has passed since the concept of chemical affinity was annexed by thermodynamics, and the most conservative physiologist could hardly refrain from ridiculing the latter part of this quotation. Professor Eddington himself has adopted the “Principle of Indeterminacy” as an ad hoc hypothesis in a limited field of enquiry. From it he proceeds to draw conclusions about human responsibility and the doctrine of free will. These are topics which lie nearer to the province of biology than physics. It would be well to await the verdict of biological science before accepting inferences of so far reaching a character as those which Professor Eddington has advanced.

By emphasizing the conflict between science and common sense Relativity has engendered a new interest in the relation of science to moral philosophy. To view that relation in its proper perspective the concepts of modern biology must supplement the concepts of modern physics. I am not suggesting that this need is overlooked by those who are not biologists. Dr. Whitehead has gone so far as to advocate replacing the traditional physical idea of matter by the biological concept of organism, or as a modern biologist might prefer to say, behaviour. When he expresses the hope that this will assist to “end the divorce of science from the affirmations of æsthetic and ethical experiences,” it is clear that his conception of the nature of biological enquiry dates from Herbert Spencer and differs from that which contemporary biologists would generally be willing to accept. Owing to the separation of descriptive from experimental biology, a separation for which the evolutionists were pre-eminently to blame, a well-informed interest in the study of living matter is more rare among physicists of our period than it was in the days of Robert Hooke and Boyle or of Euler, Lavoisier, and Laplace. I am convinced that very few scientists who are not biologists--perhaps no professional philosophers--possess a clear notion of the way in which the modern experimental biologist approaches the study of the organism and the results at which he aims.

I have already suggested that there are special reasons why the concepts of biology stand in a more intimate relation to the scope of moral philosophy than do those of physical science in the restricted sense. It is difficult to define the meaning of philosophy without implying a particular point of view about the limitations of human knowledge. There are as many different definitions of philosophy as there are different schools of philosophical opinion. From the point of view of the materialist a Hegelian is a sea lawyer. From the point of view of the subjective idealist a materialist is not a philosopher at all. If there is anything which all the two and seventy jarring sects would agree to regard as a problem of philosophy, it is the Nature of Life. If we are to avoid making any unjustifiable assumptions about the nature of knowledge, we must for the present define a philosophical discussion of the Nature of Life as the most comprehensive treatment of the problem. It does not necessarily follow that there is any essential difference between a scientific and a philosophic enquiry in this sense.

When people first hear their own voices recorded by a gramophone, it is well known that they are often--like myself--a little humiliated, and generally somewhat surprised. I once had occasion to witness an instructive incident which occurred in the phonetics department of the University of Cape Town. A gramophone record of three men engaged in a conversation was prepared. None of the three participants had previously listened to a record of his own voice. When the record was completed each man agreed that the voices of the other two were faithfully recorded. Each man denied that his own voice had any semblance to its representation by the recording instrument. This simple experiment in human behaviour illustrates what I shall later call the distinction between the private worlds and the public world. It also illustrates a fundamental divergence of outlook which distinguishes two tendencies in philosophical discussion, and makes it difficult to give any definition of philosophy satisfactory to all parties. One school of philosophers defines a good record as a record which on the whole faithfully conveys the impression of human voices. The philosopher of the opposing school feels that it ought to be possible to manufacture a record which will faithfully represent the voice of his opponent, while at the same time registering his own voice as he hears it himself, when he is speaking, and would prefer other people to hear it.

This distinction has an interesting history which will be discussed in the third series of essays in this volume. Greek speculative philosophy had its first beginnings in a secular curiosity about Nature. Science and philosophy were thus one and the same thing to Thales, to Empedocles or to Democritus. In Greek thought speculation was not sufficiently disciplined by sustained observation of Nature. For that reason it gave birth to innumerable conflicting hypotheses which could never be made the subject of decisive tests. Out of this confusion of conflicting ideas was born a reaction against science. Philosophy turned from the slow and tedious task of examining the actual world to the more facile and pretentious pursuit of an ideal world. In the person of Plato it forfeited its secular temper. Science was introduced into modern Europe by the Arabs, who assimilated the secular curiosity of the Greeks. Ecclesiasticism seized upon the speculations of the later Greek philosophers to provide a rational basis for theological dogma. Since mediæval times scientists have submitted to an arrangement which gives to those who have not studied Nature the right to supervise the logical status of their conclusions. The stability of this arrangement has been maintained by the circumstance that human beings are far more interested in themselves than in any other material objects. Greek materialism declined, because it could not satisfy man’s curiosity about himself. The success of its rival was not due to its ability to settle the problems of human nature and social conduct. It succeeded because human nature demands a forum for the ventilation of its grievances. Science has been most successful in the past in dealing with inanimate things. Only in comparatively recent times has the phenomenal success of scientific method, fortified by the secular influence of Darwin’s teaching, encouraged the belief that it might be applied to the study of man’s behaviour and social organization. The belief that a philosophical discussion of the Nature of Life lies beyond the province of the biologist is due to centuries of subservience to a tradition which has identified philosophy with the interests of statesmanship and ecclesiasticism. If the method of science is applicable to the study of how statesmen and theologians behave, it is legitimate to undertake a discussion of the nature of life without assuming that the biological standpoint must be reinforced by the discipline of scholastic philosophy.

There is a further assumption which we need not make in our enquiries into the Nature of Life. We need not presume like Socrates that all questions about life are permissible. A proper respect for our own limitations is as essential to philosophy as to sanity and modesty in everyday life. It is only possible to formulate questions in the right way when we already have at our disposal a good deal of information relevant to the correct answer. This was not recognized by the materialists of the nineteenth century when they attempted to give a common-sense solution of the Riddle of Life. To-day it is customary to refer to materialism as an exploded fallacy. If instead of looking at the way in which the materialist attempted to answer the man in the crowd, we examine the way in which he attempted to answer the questions which he himself propounded, the explosion of the fallacy is not so encouraging to traditional beliefs. The term materialism, when it is not employed like Bolshevism as a term of abuse, is loosely applied to a constellation of beliefs, some of which concern the Nature of Life and some of which concern the Nature of Knowledge. In the latter sense materialism implies the conviction that the only genuine knowledge is that which can be gained by pursuing the method devised by scientists for the study of what are ordinarily called material objects. If this conviction is carried to its logical conclusion, a discussion of the Nature of Life in language which is intelligible to the audience of Mr. Breach or his Secularist competitor is impossible. The materialist who attempts a common-sense solution of the Riddle of Life is inconsistent with his materialism. It is his inconsistency and not his materialism which is an exploded fallacy. Common-sense materialism, the materialism which is an exploded fallacy to-day, was based on the belief that a plain answer to a plain question is the inalienable birthright of the plain man. Secularist rationalism was the offspring of Protestant democracy. Protestant democracy is suspicious of the expert, who is the person who knows that there is a technique of asking questions in the right way as well as a technique of answering them in the right way. Perhaps Xanthippe, who has become the symbol of a nagging wife, realized this profound truth more clearly than Socrates. Perhaps her short way with introspective philosophers was based on a considered recognition of human frailty, and experience of children.

An intelligent child of three once asked me to tell her the colour of Wednesday. To the more sophisticated adult the question is ridiculous, though I suppose the theosophist would regard it as permissible. Metaphorically speaking, the habit of asking the colour of Wednesday is not exclusively confined to children. Thousands of years ago human beings began to associate particular sounds with objects around them, so that these sounds became signals for activities. These activities became increasingly more complex as articulate speech became more elaborate. Gradually human beings ceased to employ a separate symbol for every object around them. They began to condense and economize, abstracting separate properties. It became no longer necessary to have separate words for white cow, black cow, white horse and black horse. In effecting this economy it was inevitable that new words with no clear relation to experience were often invented. Common language the world over is burdened with words which effect no economy of discourse. Anyone who has not realized this may perform the simple experiment of asking six educated people to define in writing on a folded slip of paper the meaning of the word sincerity. With the coming of civilization man invented a new form of symbolism, the language of science. In spite of immense social inertia this symbolism has become more and more important, because of the tremendous power for controlling nature which it has given us. In the invention of this new language not only sustained observation of nature, but active interference with nature, or experiment, is enlisted in the process of abstraction.

Because common language and the language of science are not the same thing, there never can be a plain answer for the plain question of the man in the crowd. There can only be a familiar one. In any restricted field of scientific enquiry confusion of thought is avoided by the introduction of new symbols to denote new experience, or a preliminary re-definition of old symbols, if these are employed. So long as the chemist is only concerned with the reducing power of a particular sugar, it is sufficient for him to describe it as dextrose. When he directs his attention to the optical properties of the sugars he finds this symbol no longer adequate to define a homogeneous class, and distinguishes between α dextrose, β dextrose and so forth. When scientific hypothesis so broadens its channels as to merge into the general current of human thought, the scientist finds himself dealing with matters for which there already exists a vocabulary, but one that has none of the precision of scientific nomenclature, one called into being by an approach to experience which has none of the disciplined restraint which scientific method imposes. That is why the practising scientist is sometimes compelled to treat the conundrums of humanistic philosophers like the question of the child who wanted to know the colour of Wednesday. Concerning those things about which we talk most our language is apt to be least definite.

One of the things about which we talk most is life itself. A discussion of the Nature of Life presupposes that we mean something quite definite, when we use the term life. The familiar lines of Mr. Belloc suggest a helpful analogy to illustrate the nature of a scientific definition:

“Here you may put with critical felicity The following question, ‘What is Electricity?’ ‘Molecular activity,’ say some. Others remain silent or are dumb.”

What is electricity?--is a plain question. It is only possible to give it an intelligible answer when we translate it into the form, what conditions determine electrical phenomena? A scientific concept is a label for a class of properties which can be investigated scientifically. Though this happens to be a cardinal doctrine of modern logicians, it is also a commonplace of scientific thought, when undisturbed by ulterior considerations. It is a commonplace which is constantly overlooked by biologists as well as laymen in a discussion concerning the nature of life. The temptation to overlook it is assisted by the custom of spelling nature and life with capital letters, a practice to which what the Melanesians call mana adheres. The only intelligible significance of the word Life in scientific discussion is to denote collectively the properties of living things.

The word life is variously employed in common language. To Mr. Mantalini life is one demmed {sic} thing after another. Every biological student is familiar with the experiment of removing a frog’s heart from its body, maintaining its beat by perfusing it with a suitable saline medium, arresting its rhythm and restarting it by changing the constituents of the medium. This can be performed repeatedly for many hours after the owner of the heart is, legally speaking, dead. The layman confronted with this commonplace of the laboratory invariably asks with some show of bewilderment, “Is it alive?” It is extremely difficult to answer him in words he will understand. He has been accustomed to think of an organism as a whole, just as we think of solid matter as a whole. Unaided common sense does not easily grasp the notion that the frog’s heart displays the characteristic properties of living matter, after the frog, considered as a whole, has ceased to display those characteristics of living matter which we associate with whole frogs, when we say that they are alive.

In biological discussion the nature of life can only be understood to mean the characteristic properties of living things, how they are related to one another and to the properties of non-living matter, how they have come into being. To those who are accustomed to thinking in abstract nouns and capital letters this way of defining life will seem rather like the well-known definition of an archdeacon as a man who discharges archidiaconal functions; but if life is only a convenient label for the properties of living matter, we have foreshadowed an important conclusion. Those who declare that materialism is an exploded fallacy are usually those who deplore the judicial separation of science and moral philosophy. If they entertain the hope that biology is likely to effect a restitution of conjugal rights, they evidently imply that life to the biologist means something more than the properties of living matter. They assume that a biological concept of life contains other implications of its use in common language.

The source of this confusion is easy to understand. The biologist can no more avoid using the word life than the physicist can avoid using the word matter in a loose and arbitrary sense in everyday conversation. Whatever meaning the biologist may attach to the term life, when he is exercising his domestic and political activities, there is only one legitimate manner in which he can employ it in his capacity as a scientist. Much discussion between the opposing schools of vitalists and mechanists is utterly barren, because this fundamental issue is not clearly defined at the outset. The vitalist can legitimately attack the mechanist by pointing out that living things have characteristic properties other than those which the mechanist attempts to analyse. If he does so, he must specify what such properties are. In the laboratory the biologist carries out his work on the same lines, whether he calls himself a vitalist or a mechanist. On the platform he may, and frequently does, overlook this. The layman may thus acquire a disproportionate estimate of the extent to which biologists differ among themselves about fundamental issues.

That biologists are still less unanimous than chemists in the hope of resolving, in more universal terms, concepts traditionally restricted to their own fields of enquiry, may be attributed to the complexity of their subject matter. Biology is a younger science, and a vast amount of purely descriptive work was necessary, before it was possible to formulate the mechanical problems which living matter presents. This task requiring considerable specialization in the descriptive study of the exclusively geometrical aspects of the configuration of living systems unhappily became divorced from the more fundamental issue of biological enquiry. The physical analysis of the properties of living matter is a problem necessarily spatio-temporal in its extension, experimental in its method, and quantitative in its grammar. The spectacular success of evolutionary speculation during the nineteenth century preceded the birth of quantitative and experimental researches on inheritance and variation, giving descriptive biology a reflected glory on account of the far-reaching cosmological consequences of the doctrine of descent.

By encouraging the hope of reconstructing the pedigree of mankind, Natural Selection widened the gulf between descriptive and experimental enquiry; and provided a satisfactory modus vivendi for two diverging and independent schools of research. Anatomy claimed the relation of one type of living being to another. Physiology concerned itself with the relation of living matter to inanimate objects. During the present generation evolutionary problems have emerged to the forefront of experimental enquiry. Heredity and variation are no longer axioms with which the taxidermist and the osteologist can conjure unchallenged. Experimental biologists are grateful to those who have compiled the Who’s Who of the Animal Kingdom. They refuse to concede that the execution of this task implies a profound understanding of the principles of political economy. Naturally the anatomist and the field naturalist view the change with a jealous and suspicious eye.

Biologists agree among themselves in recognizing that the approach to the organism as a physical object has led to many valuable discoveries; and that the application of physical methods to the study of the organism permits us to make many predictions about the behaviour of living systems with as much confidence as we have in predicting other secular events. In so far as recent investigation has probed into phenomena which it has been customary to place beyond the limit of applicability of physical methods and concepts to the analysis of the properties of living matter, it is not surprising that many biologists have failed to take stock of the situation. They may simply deny that certain aspects of the behaviour of organisms can be treated successfully by the traditional methods of experimental physiology. If they do, it should be sufficient to set forth the new evidence at our disposal. When they go further and assert that certain characteristics of living things properly belong to the sphere of traditional philosophy, it is permissible to entertain the suspicion that they share the all too human desire to be certain rather than to know.

The chief source of disagreement between different schools of opinion in any discussion of the Nature of Life arises from the difficulty of defining another concept which is intimately connected, but not necessarily co-extensive with, that of life itself. It has been customary in the past to assume that the concept of consciousness defines a field in which the methods of experimental physiology break down and require to be supplemented by the method of introspection. In so far as it bears on the Nature of Life this implies the possibility of identifying and specifying in living systems characteristics to which the term consciousness directs attention. It is impossible to avoid disagreement in connexion with this concept without recognizing a fruitful source of confusion. The statement “I (N or M) am a conscious being” has a formal relation to the statement “All men are conscious beings” like the analogous statement “Mr. Bertrand Russell is a conscious being,” so long as it is understood that I and Mr. Bertrand Russell are both single valued and members of the class “men.” From this it follows that any implication of the first proposition which is not implicit in the second defies logical analysis and therefore eludes philosophical enquiry. For the purpose of philosophical discussion “I am a conscious being” contains nothing that is not implied by saying that “all men are conscious beings.” I shall use the term public to signify this way of looking at the concept of consciousness. Any residuum of the first proposition which cannot be formally identified with the third and shown like it to be included in the second and more general proposition is a private affair of the individual. If we find that modern physiology has undertaken to investigate those characteristics of the behaviour of living systems associated with the term consciousness in its public sense, a new horizon of philosophical discussion is unfolded. If physiology is more successful than introspective philosophy in defining predictable conclusions about living behaviour, we have no need to go outside the data of physiology for the materials of a comprehensive discussion of the Nature of Life. A philosophical discussion of the Nature of Life will only be more comprehensive than a biological discussion of the Nature of Life in the sense that more attention will be paid to the methods of enquiry adopted.

Between two extreme schools of opinion existing at the present day the issue, in so far as it is a tangible one to the practising biologist, is thus defined by Dr. Haldane in his recent Gifford Lectures.

“We can of course leave the characteristic peculiarities of conscious behaviour out of account, and regard persons from a purely physical and chemical point, as weighing so much, as yielding certain amounts of various proteins and other chemical substances, distributed in a certain way, and as in various ways continually converting potential into kinetic energy. This mode of regarding persons is of great practical use for engineering and other purposes, but tells us nothing, however far we may extend it, regarding the distinctive characters of conscious behaviour...” (italics inserted).

In this passage Dr. Haldane is perfectly definite in stating where, as he believes, the methods of traditional physiology cease to be applicable. It is peculiarly felicitous that he uses the term conscious behaviour rather than consciousness in this connexion. If we find reason to believe that “conscious behaviour” can be analysed with reference to a space-time framework by the methods of physical science, Dr. Haldane’s attack on the mechanistic position falls to the ground except in so far as he can refuse to capitulate until the problem has been reduced to a question of pure physical chemistry. In the succeeding essay on The Mechanization of Consciousness I shall endeavour to show that in our generation the work of Pavlov’s school has successfully tackled, for the first time in history, the problem of what Dr. Haldane calls “conscious behaviour” in non-teleological terms. It has reduced it to the investigation of the conditions under which new reflex systems are brought into being.

In Science and the Modern World Professor Whitehead states that the “effect of physiology” on philosophical discussion has been “to put mind back into nature.” I presume that he is referring to the traditional distinction between reflex activity and voluntary behaviour. It is true that physiology has accepted this distinction which it inherited from the dualism of Descartes; but traditional physiology never attempted to probe deeply into the nature of voluntary behaviour. It was content to investigate reflex activity, and concede the prerogative of discussing the characteristics of conscious behaviour to moral philosophy. Experimental physiology like experimental physics is an ethically neutral science. If Pavlov has reduced the problem of conscious behaviour to the same level of discussion as the problems of reflex behaviour, the traditional distinction between reflex and voluntary activity has ceased to define the boundary at which physiology ends and moral philosophy begins. If the investigation of the characteristics of conscious behaviour can be brought within the scope of an ethically neutral method, we must abandon any hope that biology can assist to end “the divorce of science from the affirmations of æsthetic and ethical experiences.” If the Relativists can say that modern physics has given materialism its death blow by referring solid matter to an atomic nexus of conceptual fields of force which only exist in our consciousness, the physiologist can add that modern biological enquiry is disintegrating consciousness into an atomic nexus of reflex arcs. If modern physics has shown that we can no longer think profitably of solid matter as existing in the way in which it presents itself to common sense, modern biology is showing that for the purpose of profitable discourse mind itself does not exist as the essential unity which it assumes to common sense. If the advance of science has disposed of the older forms of materialism, it is also disposing of the traditional forms of idealism and dualism at the same time.

Biological science no less than physics is opening up new fields for exploration in philosophy. The new approach to the problem of “conscious behaviour” involves an intellectual effort no less repugnant than the non-Euclidean space of the relativist. The new, in preference to the traditional biological standpoint, owes its sanction to the same test as that by which relativistic theories must in the last resort be assayed. A disinclination to discuss “conscious behaviour” and a tendency to assert dogmatically the possibility of reducing it to purely chemical concepts has been characteristic of the mechanistic standpoint in the past. It would seem that to emphasize the applicability of physical methods to living matter in all its aspects, a new term, free from this taint, is now needed. Behaviourist has already acquired certain restricted implications, and for reasons which will be set forth in a succeeding essay I shall sometimes speak of the publicist in preference to the mechanistic standpoint. Unlike the Flat Earth doctrine of Mr. Breach the publicist standpoint in philosophy is not based on an appeal to common sense.

I began these introductory remarks by calling attention to two characteristic features of contemporary thought, the uneasy recognition of the opposition of science to common sense and the renewed search for a working agreement between the claims of science and of moral philosophy. In concluding, I would add a third to which I have not explicitly referred. The scientist brought into collision with common sense has for the time being lost his former air of self-confidence. We are told that science does not deal with reality, that the external world of physics is a shadow world, that the laws of physics are only statistical generalizations, that scientific hypotheses are no more than convenient devices to aid us in the practical business of living. I cannot but feel that the solemnity of these assertions is out of all proportion to their novelty. I cannot discover why recent developments in physics constitute a specially cogent reason for reiterating them at the present moment. I am disposed to believe that some of the younger generation who have been familiar with the writings of Mach, of Pearson, of William James and of Bergson, since they first began to think about the nature of scientific knowledge, must share with me a sentiment of surprise, when told that living scientists ever seriously put forward those extravagant claims which, as we are now assured, received their death blow from the theory of a relativity and the new quantum mechanics. The apologetic attitude so prevalent in science to-day is not a logical outcome of the introduction of new concepts. It is based upon the hope of reinstating traditional beliefs with which science was at one time in open conflict. This hope is not a by-product of scientific discovery. It has its roots in the social temper of the period. For half a decade the nations of Europe abandoned the exercise of reason in their relations with one another. Intellectual detachment was disloyalty. Criticism of traditional belief was treason. Philosophers and men of science bowed to the inexorable decree of herd suggestion. Compromise to traditional belief became the hall-mark of good citizenship. Contemporary philosophy has yet to find a way out of the intellectual discouragement which is the heritage of a World War.

The physicist has abandoned teleology in his own field. He has banished the spiritual values from the domain of his enquiries. He now looks to the biologist to shoulder the task of proving that the universe is consonant with our notions of ethical propriety. I shall endeavour to show that the progress of modern biology gives no justification for the belief that such a compromise is possible. In approaching this task my aim is not primarily to advocate the mechanistic conception of life or to criticise the vitalistic standpoint. Controversy between writers of the mechanistic and vitalistic schools has too often focused attention on whether a complete solution of the Nature of Life can be found within the mechanistic framework. The more significant question is whether any solution can be obtained outside the mechanistic framework. It may be interesting to know how far the biologist has progressed in his enquiry into the Nature of Life. Philosophically it is more significant to understand what methods of investigation have permitted him to advance towards an admittedly partial solution of his problem. To show that the mechanistic conception of life is inadequate is one thing. To show that any alternative and more comprehensive view can be gained by pursuing methods other than the traditional methods of experimental biology is a more difficult task. It appears to me that the mechanist can admit every criticism which the vitalist brings to bear upon his case without weakening its essential strength. If we commence our enquiries with the assumption that it is possible to know everything, we shall be disappointed to find that the mechanistic conception of life does not--and probably never will--find an answer to every question which we may be tempted to propound. In that disappointment lies the false security of the vitalistic standpoint. It was the peculiar merit of Hume’s philosophy that he rejected the necessity of making this assumption.

In assessing the respective contributions of the biological and physical sciences to the construction of a Public World, we are investigating the existence of certain characteristics common to all branches of natural science. The method of science is not static. It is ever growing and expanding, incorporating new territories within its empire. For this reason formal definitions unfortified by an examination of the historic past tend to be superficial and barren. The origins of even the most exact sciences are deeply rooted in the soil of magic. At any stage in the progress of human knowledge particular features may be evident in more than one branch of enquiry. As time goes on fresh similarities present themselves. The exact line of demarcation between the already scientific and what is not as yet scientific is therefore somewhat arbitrary. No definition of scientific method is adequate unless it implies the recognition of a developmental sequence in which new characteristics emerge successively into prominence.

In the natural sciences, as customarily defined, it is essential that the data shall be publicly accredited by the testimony of independent observers. The observation and recording of publicly accredited data is not in itself regarded as an adequate criterion of scientific study, unless the data are arranged or classified in a particular way. Such classification makes it possible to draw inferences which extend beyond the range of the original data. The validity of the relations implied in a particular classification is referred to their capacity to permit us to predict verifiable conclusions. In practice it is necessary to classify the data of a problem in a variety of ways before it is possible to arrive at the type of classification which yields relations satisfying this criterion of validity. This circumstance assists us to draw a rough distinction between a type of enquiry which is maturely scientific and one which is in process of becoming scientific. In the older and more firmly established branches of science, it is evident that severe economy in the initial assumptions promotes the construction of hypotheses which are valid in the sense defined. Ethical values have been eliminated altogether. The same characteristics are increasingly recognized in newer departments of scientific investigation.

When every criticism of the limitations of scientific method has been accepted, the belief that philosophy can provide a means of solving the problems which lie outside the realm of scientific enquiry still remains to be proved. In the essays which form the third series of this volume, I shall endeavour to discuss whether an enquiry into the nature of reality has any intelligible meaning. With Hume I doubt whether it is possible to attach any significance to deciding whether scientific beliefs are a faithful representation of “reality.” Scientific beliefs are specially characterized by their communicability, or, to use the term which I shall employ more frequently, their publicity. The fundamental problem of a philosophy which does not presuppose what it sets out to establish is to find what characteristics of beliefs make them communicable. It is by examining these characteristics that we can hope to decide whether the discussion of our ethical and æsthetic predilections can yield conclusions which have the same kind of communicability as scientific beliefs, and, if that is possible, in what manner such discussion must be conducted. I shall endeavour to show that there is a confusion of meaning involved in discussing whether the experiences with which science deals are more or less real than the experiences which moral philosophy has claimed for its parish. The more modest task of deciding whether the conclusions of science have more or less communicability than ethical and æsthetic beliefs is not a problem which necessarily eludes unprejudiced investigation.

I have indicated that an enquiry into the nature of life and the nature of consciousness presupposes the necessity of formulating the problem in the right way. This task is a necessary preliminary to the analogous question, what is philosophy? The firefly emits light. When we say that we understand what animal light is, we mean that we understand what processes are involved in the production of animal light. The method of science can lay bare the structures in which luminescent materials are secreted and the physical transformation of their chemical energy into visible radiation. Animal light is an unusual characteristic of a species of insect known as the firefly. The light of reason is a peculiar characteristic of certain human beings known as philosophers. We can only say that we truly understand philosophy or the light of reason, if we understand the processes which confer upon philosophers their unusual characteristics. A philosopher brought into being by the process of natural generation develops in an environment which includes inanimate objects and other human beings. He reacts to his physical environment by growth and to his social environment by learning. If the method of science can assist us to elucidate the processes of growth, learning and natural generation, science can assist us to understand what philosophy is. The anatomy of philosophy and the physiology of philosophers are inseparable. We need not be discouraged in pursuing this line of enquiry, because the answer which science can give us at present is incomplete. It is the chief glory of science that its answers are always incomplete. The pitiful failure of introspective philosophy resides in the finality of its answers. Perhaps the most permanent influence of Relativity in the history of philosophy will prove to be the challenge it issued to the finality with which Kant enunciated the concepts of space and time.

I. THE MECHANIZATION OF CONSCIOUSNESS

“Now, in conclusion, the Method which teaches adherence to the true order and an exact enumeration of the conditions of the thing sought includes all that gives certitude to the rules of arithmetic.”--Descartes, Discourse on Method

The onus of proving that all the properties of living matter can be reduced eventually to problems in physical chemistry or, on the other hand, of denying that such will ever be accomplished, may be laid on the shoulders of those who commit themselves to rash affirmations and denials. If this were the only matter to decide, a discussion of the merits of the mechanistic conception of life could reveal nothing more than a temperamental difference between the disputants. A temperamental difference does exist. The mechanist has a cheerful attitude to knowledge and refuses to capitulate to the fear of the Unknown: the vitalist, a sadder but not necessarily wiser type, finds balm in the limitations and failures of human effort. The average biologist, who has little sympathy either for the heroic or the desperate point of view, maintains a detached scepticism.

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.

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.

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.

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

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.

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.

In all animals the sperm is a microscopic entity. In all animals from the jellyfish to Man with very few exceptions its appearance is extraordinarily similar. It consists of a thicker portion to which is attached a long vibratile process, or flagellum. The eggs of different animals are of very different dimensions. Sometimes they contain immense stores of food material (yolk). Sometimes as they pass to the exterior by the female generative tract they are invested with an additional slimy coat and a leathery or calcareous shell, secreted by special glands. The immature egg of all animals is essentially similar. In the living condition it is a spherical or ellipsoidal body in which a clear spherical vesicle is seen; this vesicle present in all cells is called the nucleus. The thicker part or body of the sperm consists mainly of the nucleus of the cell from which it is derived. At fertilization it swells up and unites with the nucleus of the egg. The fertilized egg then divides into two separate segments or cells, and the process of dividing is repeated an indefinite number of times. The cells or segments into which the fertilized egg divides each contain a nucleus, and the process of segmentation which involves the division of cells into two is accompanied by the division of the nucleus of each dividing cell. Like the testis or ovary the substance of all the organs of the animal body is built up of the microscopic bricks which we have called cells. In some tissues like bone and cartilage the bricks are separated by a good deal of mortar. Others, such as the lining membranes of the body, consist simply of cells packed tightly together. At the beginning of embryonic existence all the cells are very much alike. In the course of development the cells of different tissues are considerably differentiated. Throughout all stages the process of cell division always involves the partition of the nucleus in a highly characteristic manner.

The details of this peculiar process, first elucidated by Flemming and others during the seventies, has proved to be of astonishing significance for the further understanding of Mendel’s hypothesis. When a cell is about to divide, the nucleus looks like a tangle of fine threads; and this tangle of fine threads resolves itself into a number of readily distinguishable filaments which become progressively shorter, assuming the appearance of stout rods staining deeply with basic dyes. These rods, visible only with high powers of the microscope, are the chromosomes, whose behaviour has provided us with a tangible basis for Mendel’s conception of inheritance, and have thereby permitted an extensive clarification and amplification of the original hypothesis. From one point of view they might be said to have done as much for the Mendelian conception of heredity as the discovery of alpha particles has done for our belief in the atomic structure of matter. As the dividing cell begins to constrict, the chromosomes arrange themselves at its equator, and split longitudinally into halves, each half travelling to opposite poles, where they spin out again into fine threads from which the nuclei of the daughter cells are built up. Thus each of the chromosomes in the nucleus of any cell in the body is structurally equivalent to a corresponding chromosome in the preceding or succeeding cell generation. About the year 1875 it was recognized that this numerical constancy extends beyond the life of a single individual. In every species of animal or plant the number of chromosomes which can be counted in dividing nuclei is a constant for the species.

With the discovery of this fact a new problem arose so soon as the essential features of fertilization were appreciated. How is this constancy maintained from generation to generation of new individuals? Two investigators, Van Beneden and Boveri (1881-3), who worked on the horse threadworm, a form which has only four chromosomes in the dividing cells of the segmenting egg, showed that the egg and sperm each contain only half the number of chromosomes characteristic of the cells of the embryo. This conclusion turned out to be a perfectly general one. Attention was immediately directed to the nuclear changes which happen in the formation of the gametes. Innumerable cell divisions occur in the testis or ovary of an animal. These are at first similar in all respects to those which occur in the segmentation of the developing embryo; but cell division goes on in the testis or ovary throughout life. If we trace backwards the history of an individual sperm or egg in the testis or ovary in which it originates, we find a reduction of the number of chromosomes effected during the last division but one, leading up to the formation of a sperm or ripe egg. This penultimate division of the germ nuclei is preceded by the fusion of the chromosomes lengthwise in pairs. When the division actually takes place, each pair behaves like a single chromosome, splitting in such a way that one member of each pair goes to form each daughter nucleus. The succeeding division being normal, each gamete receives half the number of chromosomes present in ordinary cell division. At fertilization the normal number is restored. Thus each ordinary cell of the body has a chromosome set of which half the components are paternal and half maternal in origin.

In many animals and plants the chromosomes are very distinctly of different sizes and shapes, and can be sorted out into corresponding pairs. Such arrangements are constant for the species, and could only be maintained constant, if each gamete contains one representative of each pair. This means that the maternal and paternal constituents of a pair are distributed in the reduction division to different cells. The chromosomes therefore exist in pairs of which one element is of maternal origin and one of paternal origin. Each gamete receives one element of each pair, just as Mendel supposed that each gamete contained either the paternal or maternal element of his paired “factors.” By a curious coincidence this far-reaching conclusion was first established in the very year which witnessed the application of Mendel’s principles to animals by Bateson in England and Cuenot in France (1902). Its recognition accompanied the elucidation of another peculiarity of nuclear division, also destined to have important theoretical consequences. In many animals there is found to be an unequally mated pair of chromosomes, the XY pair. When this occurs, it occurs in one sex only. In the alternate sex there is a corresponding equal pair (XX). In birds and moths the female is the XY, the male the XX individual. In other animals the male is usually found with sufficiently careful measurement to have an unequal (XY) pair which is equally mated in the female (XX). During the nineties it was found that some animals had in one sex an odd number of chromosomes, a fact which at first sight seemed to conflict with the numerical constancy of the chromosomes. In the early years of the present century American zoologists provided the key to an understanding of the discrepancy. In all such cases the alternate sex has one more chromosome. The case of the large cockroach will serve as an illustration. The male of Periplaneta americana (its technical name) has 33, the female 34 chromosomes. The eggs will all have 17 chromosomes. One-half of the sperm will have 17, the other half 16 chromosomes. If a sperm of the former class fertilizes an egg, the individual produced will be a female (17 + 17 = 34); and if a sperm of the second type fertilizes an egg, the individual produced will be a male (17 + 16 = 33). In an animal with an unequally mated (XY) pair of chromosomes in the male reduction will result in one-half of the sperm carrying the X and one-half the Y chromosome. The eggs will all have the X, since this chromosome is equally paired in the female. Thus an egg fertilized by a Y-bearing sperm will become a male, while an egg fertilized by an X-bearing sperm will develop into a female.

By statistical reasoning Mendel had deduced from his experimental data the existence of entities which behave just as the chromosomes do. He had no direct evidence that his factors had any material basis in the architecture of the germ cells. The new cell anatomy provided independent confirmation of his predictions from an unexpected quarter; but it was not immediately recognized that this was so. Antagonism to the belief that the chromosomes fulfilled the requirements of Mendel’s hypothesis is easily explicable. To Mendel’s first disciples his second and first laws were equally sacrosanct. Mendel’s second law implies that different pairs of hereditary factors behave quite independently of one another. On such an assumption one of two deductions is inevitable. Either the applicability of Mendel’s first law is extremely restricted; or the number of factors is too large to permit of their localization in the chromosomes. The sweet-pea, for instance, has only seven pairs of chromosomes. If Mendel’s second law were as general as the first, only seven pairs of factors could be accounted for by the behaviour of the chromosomes. From this dilemma further development of the atomistic view of heredity was rescued, when it was discovered that Mendel’s second law is only a particular case of the possibilities inherent in the first.

In 1910 Bateson and Punnet first discovered in the sweet-pea what they then called “coupling and repulsion,” or as we now say, linkage. Without going into the experimental data, we may define the phenomenon of linkage in the following way. Suppose that these are two varieties A and B which obey Mendel’s first law and two other varieties C and D which likewise conform to its requirements, when crossed with one another. Mendel’s second law stated that in a cross between AC and BD the second generation will consist of the types AC, AD, BC and BD in numerical proportions agreeable to the assumption that it is equally likely that the factor determining A will be present in the same gamete as the factor determining C or the factor determining D. Bateson and Punnet found that this does not always happen. There is another category of cases in which the factor which determines A sticks more or less completely to the factor for C in preference to the factor for D. The detailed analysis of these cases was at first made difficult by Mendel’s literal symbolism, and his way of thinking of factors in pairs. But the discovery of linkage at once led Lock to formulate the fruitful suggestion that factors located on the same chromosome pair would satisfy the requirements of linkage, while factors located on different pairs of chromosomes would fit in with Mendel’s second law.

From this point onwards the most spectacular development came from the study of inheritance in animals, and the significance of the chromosomes was immensely reinforced by newly gained knowledge of sex determination. Almost contemporaneously with the discovery of the sex chromosomes or XY mechanism, as we now say, Leonard Doncaster had elucidated in moths the phenomenon of sex-linked inheritance. This was soon found to be of common occurrence in animals. Till this discovery, which was made in 1905, the same results had always been obtained in crosses of pure-bred varieties, whether the male or the female parent displayed one or the other characteristic distinguishing them. Doncaster’s work on the currant moth showed that there is a category of cases which at first sight obey Mendel’s first law in its simplest form when the cross is made in one way, but yield a different type of result when the cross is carried out reciprocally with respect to the sex of the parents. In such cases one sex is only able to transmit certain characters to its offspring of the opposite sex. It was already known that the XY sex (male in Man and most animals) can only transmit its X chromosome to the XX type. The facts did not dovetail at first sight, because sex-linked inheritance was originally elucidated in birds and moths of which the female is the XY type. There was still an attitude of hesitancy towards accepting Lock’s hypothesis, strengthened by the persistence of an incorrect interpretation of the process of reduction which had been made the basis of Weismann’s metaphysical speculations concerning “germinal selection.”

When in 1914 Doncaster summed up the case for regarding the chromosomes as the material basis of Mendel’s first law, a new era had already dawned. Thomas Hunt Morgan, the central figure of a group of ardent investigators at Columbia, had initiated a body of enquiries which within half a decade eclipsed all other achievements that had succeeded Mendel’s pioneer labours. About the time when Bateson first encountered the phenomenon of linked inheritance Morgan began to rear the fruit-fly Drosophila for breeding experiments. Till then genetic experiment had been held in check by the slow rate at which most convenient animals and plants reproduce and the expense entailed in breeding them in sufficiently large numbers to permit statistical inference. The fruit-fly completes its life cycle, if kept in warm laboratory conditions, in a period of ten days. It is prolific. It feeds on rotten banana skins. It therefore costs little to breed. To these immense advantages it adds two others of supreme importance. It has only four pairs of chromosomes readily distinguishable from one another by size and shape; and it has produced in the laboratory a crop of several hundreds of sports or mutants. Each mutant type differs from the wild parent stock in some well-defined characteristic inherited in crosses with the wild type in accordance with Mendel’s first law. The mutant characters are extremely varied. One is distinguished from the red-eyed parent by having white eyes, another by having purple eyes, another by having no eyes at all. One is distinguished by having wings that are practically vestiges, another by wings that turn up at the tips, another by wings that are truncated at their extremities. From the wild type which has a greyish body, one mutant is distinguished by a deep black, another by yellow coloration. The mutant characters are thus in general clear-cut differences lending themselves to easy identification. With an animal that breeds so rapidly and prolifically information accumulated with astonishing rapidity. From data based on the study of a large assemblage of mutant characters there soon emerged the precise requirements of Lock’s hypothesis. All the mutant characters of Drosophila fall into four groups. Members of the same group always tend to stick together in hereditary transmission. Members of different groups like Mendel’s dihybrids behave independently of one another. Of several hundred mutant characters in Drosophila every one belongs to one of these four linkage groups; and the number of chromosome pairs in Drosophila is four.

This discovery was only the beginning of what might well be called one of the faery tales of modern scientific research. In the way of accepting Lock’s hypothesis there were still difficulties. It was in evading the principal difficulty that Morgan’s school extended the atomistic concept of heredity much further than his predecessors had done. Till then the main outcome of experiments on breeding had been to show that Mendel’s principle was of vastly wider applicability than was at first supposed, and to engender the suspicion that the patient unravelling of difficult and elusive cases would establish its universal validity. As yet the world of Mendel’s atoms was without form. Morgan and his colleagues gave it a map. Not content with showing that Mendel’s atoms of heredity have their material basis in the chromosomes, nor with actually identifying which chromosome is significantly associated with a particular mutant character, Morgan went further and localized the region of an individual chromosome in which a particular Mendelian factor resides. He thus gave to Mendel’s factors spatial co-ordinates in the living cell.

At the outset the study of linkage upon which the chromosome map is based was facilitated in the case of Drosophila, because the varieties dealt with were all known to be mutants from a fixed wild type. Thus it was possible to break away from Mendel’s conception of “pairs” of hybridizing characters. The Mendelian factor was replaced by the mutant gene, by saying which is implied that a mutant arises because at some point on a particular chromosome a physical change has taken place. The gene is the Mendelian factor for the mutant condition, but no assumptions are made about what determines the wild-type condition. The inter-relationship of different characters is greatly simplified by thinking only of the relation of one mutant gene to another. The discovery that all the genes fall into four groups corresponding to the four groups of chromosomes presented one stumbling-block. Members of the same group in general do not invariably stick together. When two mutants are crossed the numerical proportions of the various types of offspring give a definite value for the probability that the gene A and the gene B will stick together or separate apart. This is a constant for A and B. The constant used in practice is the tendency for A and B to separate. Expressed as a percentage, it is called the cross-over value. Some additional information was necessary to explain why A and B do not always stick together, if they are associated with the same chromosome. It was from the solution of this problem that the chromosome map took shape.

Here the sex chromosomes came to the rescue. One very interesting type of sport which has turned up in breeding the fruit-fly is not recognizable by any discrete bodily peculiarity but merely by an abnormality in the number of chromosomes. Of these the first to be discovered was a type of female which has in addition to its usual four pairs of chromosomes an additional Y chromosome. The XXY females yield very extraordinary numerical results both as regards the sex ratio and other characteristics, when used in making crosses involving mutant characters. There is a class of mutant characters in Drosophila, more than a hundred in all, distinguished by the fact that they are not inherited symmetrically with regard to sex. They display linkage inter se. They behave as “sex-linked” characters. The introduction of XXY females into crosses involving such mutant characters results in numerical ratios which are inexplicable on any assumption other than the view that the sex-linked gene is referable to the X chromosome alone. Yet, although the sex-linked genes are all borne on the same chromosome, they do not invariably stick together in crossing. The holistic chromosome clearly would not do. An atomistic chromosome had to be put in its place.

The clue to this was provided by studying more closely the extent to which the different genes stick together. Taking all the genes located on the X chromosome this remarkable generalization emerged from Morgan’s researches. If A, B, and C are three sex-linked genes; if the probability that A and B will not stick together is x, and the probability that B and C will not stick together is y, the probability that A and C will not stick together is either the sum or the difference of x and y. The correspondence here stated is, of course, subject to the margin of error permitted by the theory of probability. To interpret this new law of the linear alignment of the genes Morgan made use of a structural peculiarity of the reduction process. When the chromosomes pair in the reduction division, they appear to become twisted. The appearance suggests that in the ensuing split corresponding lengths of the original pair are interchanged. It is very natural to assume that the likelihood that two points will be separated from one another in such a manner is proportional to their distance apart. So if the sex-linked genes are arranged in a series along the length of the chromosome, the probability that A and C will not stick together must be the sum of the probabilities that A and B and B and C will be separated. This is just what experiment had shown to be true. Thus all the genes on the X chromosome can be arranged in a linear series. The intervals between consecutive genes in such a series represents a space dimension.

The law of the linear alignment of the genes was soon found to apply to the other groups of linked characters. Abnormalities in the number of chromosomes have made it possible to identify each of the remaining three linkage groups of the fruit-fly with its corresponding pair of chromosomes. The first chromosome map of Drosophila was constructed in 1916. It revealed the suggestive coincidence that the number of ascertained points on each pair of chromosomes is roughly proportional to its size. There is now very little doubt that the work of the Columbia school has revealed an aspect of inheritance which is of general significance. After years of patient work with the relatively slow breeding sweet-pea, Punnet has at length elucidated seven linkage groups corresponding to its seven pairs of chromosomes. He has constructed a chromosome map of a seed plant on the basis of the principle first established for the fruit-fly. A law which holds good for two organisms so far apart in the evolutionary scale can hardly be supposed to be lacking in universal validity. The chromosome hypothesis may now take its place as one of the major generalizations of biological science. The law of linear alignment has transformed Mendel’s original conception of inheritance in a way which might be compared with the elaboration of Dalton’s hypothesis after the discovery of the law of combination of gases by volume. Mendel’s atoms of heredity are now units spatially localized in larger units of microscopically visible dimensions. These supermolecules are the chromosomes.

Being a portion of living matter the chromosome is constantly undergoing chemical change. Some critics of the chromosome hypothesis have based objections upon this circumstance. The difficulty is more apparent than real. Like the individuality of the modern atom the individuality of the chromosome must be conceived in statistical terms. For the discussion of the more familiar chemical reactions the statical atom of traditional chemistry is adequate. For the interpretation of hybridization experiments the diagrammatic chromosome of the text-book suffices. In the field of radioactivity the statical atom makes way for a dynamical model. So also in the domain of cell physiology we conceive the chromosome as an ever-changing entity. The logical situation is analogous in the two cases. Those who hold with Dr. Haldane that the biologist must interpret his data in a manner different from that in which the chemist or physicist interpret theirs have now to fall back on the contention that the Mendelian view is only a partial picture of heredity transmission. Anything which might have been said in favour of this contention ten years ago has been weakened by recent work on the inheritance of size. The pioneers of Mendelism selected clear-cut hereditary differences which ordinarily manifest themselves in any environment in which the animal or plant can live. They succeeded in showing that a vast number of hereditary differences involving a great variety of anatomical and physiological features conform to the requirements of Mendel’s hypothesis. There was one category of phenomena which remained obscure till quite recently. Differences in size, height, body weight and the like vary greatly with environmental conditions. Two stocks may be distinguished from one another by the fact that the average member of one is measurably different from the average member of another; but any given individual of one stock may be indistinguishable from another individual of the other, because, even when the environment is standardized as much as is practicable, the range of variability of the two stocks overlaps. The analysis of such cases cannot be undertaken by the ordinary technique of Mendelian experiments; but certain statistical requirements of Mendel’s laws may nevertheless be verified. By elementary statistical reasoning we can deduce that the coefficient of variability of the progeny of a cross between two inbred stocks must be a minimum in the first generation and a maximum in the second. This has been shown to be true in a number of crosses in which it is impossible to distinguish individual genetic types by direct observation.

There is no longer any adequate reason to support the contention that Mendel’s atomistic concept leads us to an incomplete understanding of biparental inheritance in animals and plants. Those who assert that it is so are now forced to fall back upon the last resort of obscurantism by appealing to the magnitude of our ignorance. The modern theory of the gene is a statistical construction consistently developed by a logical interpretation similar to that adopted in elaborating the great generalizations of physical science. The mechanist is often accused of attributing vital processes to “chance” combinations of phenomena. If the word chance is used to imply that we do not know the precise conditions which determine such combinations, the statement is hardly exceptionable. It might also imply that the phenomena which biologists study can be successfully interpreted in terms of the mathematical laws of chance. The history of Mendelism shows that these laws provide a fruitful basis for predicting the behaviour of living systems, even where physico-chemical hypotheses at present fail to throw light on the phenomena which the biologist studies. The biologist is able to progress to greater certainty of prediction only when he interprets his data with the same logical method employed by the chemist and physicist to deduce physical “laws.” Whatever the future holds in store for further interpretation of heredity and variation on physico-chemical lines, the progress already achieved has at every stage involved elimination of holistic concepts by the ruthless application of mechanistic logic. To the application of physico-chemical hypotheses no branch of physiology has proved more recalcitrant than the physiology of inheritance. No branch of physiology might more suitably be chosen to cast doubt on Dr. Haldane’s recent statement that “anything which can properly be called scientific physiology is impossible apart from the assumption of holism.”

III. THE NATURE OF LIFE

“I am sorry then, I have pretended to be a philosopher: for I find your questions very perplexing; and am in danger, if my answer be too rigid and severe, of passing for a pedant and scholastic: if it be too easy and free, of being taken for a preacher of vice and immorality. However, to satisfy you, I shall deliver my opinion upon the matter, and shall only desire you to esteem it of as little consequence as I do myself. By that means you will neither think it worthy of your ridicule nor your anger.”--David Hume, The Sceptic

Since a man must needs live before he can be a philosopher, no problem of philosophy is more fundamental than the nature of life. There is also no issue which provides more scope for vague, barren and undisciplined discussion. A Regius Professor of Moral Philosophy, whether he accepts the fact with resignation or refuses to do so, is a piece of living matter. Perhaps this is why physicists are more vocal than biologists in promoting a pacific solution of the territorial dispute between science and traditional philosophy. At the present moment it is the fashion among those who are writing on scientific philosophy either to neglect the contribution of the biologist to the world symposium, or to assume that the biologist in dealing with living matter operates with different methods and different concepts from those employed in physics. No phase in the history of biology is more fitted than the present to illustrate the fundamental unity of scientific method. In no branch of science is the limit of applicability of scientific method a more significant issue.

Since a scientific concept is only a way of describing a class of properties, the nature of life cannot refer to anything but the nature of the properties of living things. Having arrived at some general classification of the characteristic properties of living things, a discussion of the nature of life in the light of modern biological science presents two issues of pre-eminent interest. One is how far the methods employed in physical science have been successful and are likely to continue to prove successful in dealing with the properties of living matter. The other is how the increasing measure of success which attends the utilization of purely physical concepts to interpret the properties of living matter is calculated to influence our evaluation of the place of science in human thought. Whatever differences of interpretation may exist among biologists on matters of detail, it should at least be possible to infer from a survey of the progress of biology whether the study of living matter is progressing satisfactorily along the lines of quantitative analysis of experimental data towards greater certainty of prediction, and whether there is good reason to believe that the preservation of the teleological standpoint in dealing with living matter is likely to ensure conspicuous success in the same direction.

It may be admitted that there exists among biologists more unanimity with reference to the first than towards the second issue. Every infant science makes use of notions peculiar to its own province. Chemistry has but lately passed beyond the stage when the concept of affinity first became amenable to interpretation in thermodynamical quantities. There are still many biologists who would assert that the concept of adaptation demarcates the province of biology from that of physics and chemistry by an impassable gulf. There are others, fewer in number, who, surveying the teleological growing pains of the more exact sciences and bearing in mind that only 300 years have passed since the properties of familiar chemical compounds were literally personified as spirits of wood, spirits of salt and the like, do not feel compelled to regard the concept of adaptation as final. They are able to entertain the possibility that those properties which enable an organism to maintain its continued existence as an organism are not permanently more incapable of physical interpretation than the polarization of a voltaic battery, a phenomenon which the consistent teleologist would presumably regard as an attempt on the part of the latter to save its own life. Clearly the onus of defining what precisely is implied in the concept of adaptation lies on those who assert its uniqueness. Until the vitalist is more definite on this issue, the mechanist is under no obligation to refrain from classifying the properties of living matter in the light of his own experience. The mechanist denies that anything is to be gained by clinging to the teleological standpoint with its implication of some extra- or intra-mundane purpose which has been abandoned in all branches of science that lay claim to exactitude. He refuses to deal with living matter except in as far as it is considered as a series of “events” whose characteristics must be interpreted with rigid economy of hypothesis.

In approaching any lump of living matter, let us say the author of these essays, as an object of the external world, the maintenance of economy of hypothesis compels the enquirer to seek as far as possible a common basis for the characteristic properties of living and non-living systems. This necessitates a clear definition of the distinction between the two. Taking a comparatively complex organism, as, for instance, the common frog, a distinction might be attempted along the following lines. In the first place, its possibilities of behaviour are more varied than those of any machine which can be manufactured by man; yet, while possessing a greater range of reversible response than any non-living system, it would be difficult to specify in a living system any single activity which could not be reproduced by a mechanical system. Apart from this diversity, which we may refer to under the generic term reactivity, living matter is characterized in general by the wide range of external influences which are significant in determining its characteristic reversible responses. This peculiarity, in view of the subjective preconceptions implicit in the older terms irritability, sensation, etc., may be denoted by the term receptivity. Here again it is impossible to isolate any single agency (or “stimulus”) capable of evoking reversible change in any living system and incapable of evoking reversible change in any non-living system. Finally--and at first sight--a more diagnostic difference between living and non-living matter is seen in the property of reproduction (taken in the broader sense of the term, to include growth). A given piece of living matter comes into being in our experience only through the agency of other pieces of living matter closely resembling itself.

Were the more obscure process of sexual reproduction universally characteristic of living matter, this distinction would appear especially fundamental. Experimental biology is far from the achievement of a complete physico-chemical analysis of asexual reproduction in any type of organism. On the other hand, in the life cycles of those multitudes of micro-organisms which multiply by simple fission after attaining a certain limit of growth, there is nothing which compels an unprejudiced investigator to regard the process as more intrinsically incapable of physical interpretation than the splitting into two of a liquid drop. Although our knowledge of the nature of sexual reproduction is fragmentary, in this very field some particularly spectacular advances have been registered in substituting physical agencies as effective instruments for initiating processes which at one time were only amenable to the influences of living matter itself. Thirty years have now passed since Loeb’s discovery that changes in the osmotic pressure of the external medium or alteration of the permeability of the egg itself, leading to changes in its own internal osmotic pressure, can initiate without any assistance from the sperm the development of the ovum into a new and complete organism. That discovery was the starting-point of a body of investigations whose influence has radiated into many other fields of biological enquiry. Especially noteworthy in this connexion is the work of Warburg during the last decade. Warburg was able to show that sea-urchin eggs, and later animal cells in general, if rapidly dehydrated and ground to a powder, will, like the intact cell, absorb oxygen for some time when moistened. He showed also that this property, like respiration in the intact cell, can be abolished by the action of cyanides and other classes of tissue poisons. By doing so, Warburg has taken a characteristic and highly complex property of living matter out of the realm of vitalism into that of physical chemistry. His analysis went further. Experiment showed that three classes of poisons which inhibit tissue respiration can be distinguished by their quantitative relations. Of these, the efficacy of one class, the cyanides, was shown by Warburg to be correlated with the iron content of the cell. On the hypothesis that iron catalysis is the main factor in the oxidation of organic material in the cell, Warburg manufactured suspensions of charcoal with a high iron content capable of catalysing the auto-oxidation of sugars, fats, etc. The catalytic activity of these suspensions was found to be related quantitatively to the three categories of respiratory poisons in a manner closely parallel to the action of the latter on tissue respiration.

Though reproduction is, in some respects, to the biologist at least, the most fundamental of all the three features which I have defined above, the ever-changing reactivity and manifold receptivity to external influences so characteristic of living matter pre-eminently engage our attention in connexion with the more intimate and subtle issues of a field of enquiry which biology may yet claim. I refer to the analysis of human behaviour. In this connexion I shall mention progress in three directions as illustrating the transition from teleological to quantitative treatment during the last half century; namely, the physical analysis of the events which constitute an isolated unit of response or reflex, the integration of reflexes in the normal behaviour of animals, and the determination of new behaviour patterns along the lines laid down by Pavlov’s school. With regard to the first, we will consider the effect of flashing a bright light upon an animal that has been previously kept in the dark. The characteristic response, let us say, blinking of the eyelids, and the intervening events involved are, first, a physical change in a receptive area, namely, the retina; secondly, the propagation of the disturbance there set up along a certain path, the nervous system; thirdly, the liberation of a considerable quantity of energy at the seat of response or effector organ, that is to say, the muscles of the eyelid.

Our knowledge of the nature of receptivity is least complete. That it is a measurable physical event is beyond dispute. When light impinges upon a given area of the retina there follows a characteristic series of changes in electrical potential of the excited area with reference to a non-excited area. Through the work of Jolly, Adrian and others the sequence, the time relations and the magnitude of these changes are being related to the intensity and duration of the stimulus within predictable limits for a given species. These events initiate the propagation of the disturbance known as the nervous impulse. The nervous impulse is a physical event whose space-time relations can be defined as concretely as the passage of an electric current through a wire. Three-quarters of a century ago Helmholtz showed that the time which elapses between the application of a stimulus to a nerve and contraction of its attached muscle is a linear function of the distance between the latter and the point of application of the stimulus. The conception of the nervous impulse as a physical event had been, till this discovery, entirely repugnant to scientific thought. We now know not only, as Helmholtz showed, that the nervous impulse has definite space-time co-ordinates, but that it has the dimensions of energy. Its passage corresponds to the rate of propagation of an electrical change of an analogous character to the electrical response of the excited retina. The total energy of its propagation has been recently measured by Gerrard and A. V. Hill from determinations of heat production during its passage. Its mass relations are attested by a measurable increase in the carbon dioxide production of stimulated nerve. The rate of propagation of the nervous impulse varies like all chemical reactions in a characteristic way with increase in temperature. The goal of the nervous impulse after it has traversed one or more synapses in the central nervous system is the effecter organ itself--in the case of blinking of the eyelids, a muscle fibre. During the past two decades a series of brilliant researches based on calorimetric methods have revolutionized our knowledge of the final component of the reflex. A. V. Hill and Meyerhof have correlated the chemical and energetic changes accompanying muscular contraction with a precision of the order expected in purely physico-chemical determinations. They have shown that the total energy of muscular contraction can be quantitatively related to the energy liberated in vitro by the breakdown into lactic acid of an amount of glycogen equivalent to that which is converted into lactic acid in the actual contractile process.

Passing from the analysis of the constituent events of the reflex to the integration of reflexes in normal behaviour, we are faced with a striking change in the attitude of enquiry adopted in the study of those aspects of behaviour determined by generalized stimuli such as light and gravity and denoted by the term tropisms. Three-quarters of a century ago, after Helmholtz had dispelled the belief that identified the nervous impulse with an imponderable psychical principle, biologists like Lubbock were content for the most part with the statement that the moth flies towards the candle because it likes the light. The work of Loeb and others has shown that the state of contraction of particular groups of muscles is reflexly determined by the stimulation of particular areas of the retina. It is a mechanical necessity that when different areas are unequally stimulated, differences in tension of different groups of muscles will bring the body into such a position that symmetrically opposite areas will be equally illuminated. The animal must move, as in fact it does, along the path of the incident beam, whether by so doing it brings itself into a brighter, or, as can easily be arranged experimentally, a darker situation. Whereas the older and purely teleological attitude permits us to predict nothing of consequence, the objective interpretation of tropisms by experimental methods permits us to make many verifiable predictions, as, for instance, the fact that the moth will move in circles, if one eye is blackened, owing to the fact that the muscles on that side will be more relaxed.

By the end of the nineteenth century, experimental biologists were generally disposed to the belief that the analysis of the reflex and the integration of reflex systems were problems not of apologetics but of energetics. Investigation had been confined to those aspects of behaviour which are for practical purposes invariable responses to a particular situation. From the human standpoint the most fascinating feature of the behaviour of an organism is, after all, the extent to which its behaviour is conditioned not by the immediate but by the antecedent situation. In the opening years of this century the researches of Sherrington were elucidating the integration of reflexes in normal behaviour. Restricted as they were to the decerebrate animal, the traditional distinction between reflex and voluntary activity remained as a defeatist formula in biological nomenclature. The distinction was not a gratuitous olive branch to introspective philosophy. It had its objective basis in the domain of behaviour which is not uniquely determined by the immediate stimulus, when all synchronous conditions have been standardized. That distinction has been superseded to-day by the work of Pavlov’s school, which has shown, first, that in the higher animals with the cerebrum intact, new reflex systems can be built up experimentally under perfectly definable and reproducible conditions; that the relations between such conditioned reflexes can be defined in the language of space and time; and that the concept of sensation can be externalized by reference to the ability of a given stimulus to become a specific agent in the building up of a new reflex system. In short, it is legitimate to anticipate the possibility of giving a complete specification of how such an animal as a dog will behave in a given situation without recourse to the traditional nomenclature of memory, consciousness, sensation, etc.

Pavlov’s work is now accessible to the English reader through two translations of the Russian physiologist’s own writings and several excellent résumés, such as the one given in Lovatt Evans’ Recent Advances in Physiology. How far-reaching are its consequences has not been widely recognized even by biologists themselves. Experimental biology, during its brief career, has attempted to accommodate itself to the introspective temper of traditional philosophy by a compromise explicitly formulated in the writings of Descartes, who bequeathed to physiology the dualism of mind and matter. In conformity with the Chaldæan mythos, many philosophers, Descartes among them, have endowed Man alone with soulfulness. The coming of the Evolutionary hypothesis has broken down so inflexible a distinction between Man and other forms of living matter. Evolutionists in the nineteenth century, like Haeckel, were prepared to equip the Amœba with a soul. In our time the Cartesian compromise has again shifted its boundaries. By the beginning of this century the moth once more had gone to join the candle. Still Man stood with a little family of mammals around him, each with one leg on either side of the frontier that separates the universe of space and time from the Platonic world of universals. Pavlov has taken those aspects of behaviour which would have been referred twenty years ago to exclusively introspective concepts, and has treated them successfully as predictable configurations in a space-time framework. The little family of mammals has been let through the tollgate of the Cartesian compromise. A new school of psychologists has come into being with the express object of making psychology a physical science, relieving Man, the celestial pilgrim, of his burden of soul.

Philosophers have always had a legitimate cause for complaint that biologists were unable to deal with those aspects of human life which interest people most. The distinction between reflex and voluntary activity provided the fullest absolution for that amiable libertarianism which we all entertain under the influence of alcohol and love. Because that distinction was implicit in the outlook of the most radical mechanists of the last generation, Loeb among them, Dr. Haldane finds it so easy to point out the inadequacy of the mechanistic outlook. In the light of Pavlov’s work we can now envisage the possibility that the methods of physical science will one day claim the whole field of what can be properly called knowledge. If I am right in cherishing such an opinion, it would thus appear that the investigation of the conditioned reflex initiates a new epoch in biology, pregnant with more far-reaching philosophical implications than the evolutionary speculations of the nineteenth century. The fact that no reference to the conditioned reflex is contained in Dr. Haldane’s Gifford Lectures may in part account for the fact that he can so easily dispose of the mechanistic position. The modern mechanist does not say that thought and love and heroism do not exist. He says, show me behaviour to which you apply the adjectives thoughtful or loving or heroic, and we will, one fine day, endeavour to arrive at predictable conclusions with reference to it by following the only method of enquiry which we have learned by experience to trust. When Dr. Haldane goes out of his way to dispose of the puerile formula that thought is a secretion of the brain, as bile is a secretion of the liver, and does so, I gather, under the impression that mechanists either believe it to mean something or alternatively shut their eyes to the major problems of existence, I can only respectfully suggest that he is flogging a dead horse, while the living ones are getting out of the vitalistic stables.

I have endeavoured so far to indicate the increasing measure of success that has crowned the application of physical methods and the use of physical concepts in modern biological investigation. I have attempted to illustrate the continuous retreat from teleological concepts that has accompanied this advance. In asking what progress may be anticipated from encouraging the teleological attitude to the nature of life, I wish now to urge that the important advances of biological science during the last hundred years have not only involved continual abandonment of teleological concepts, but have consistently been made in the teeth of opposition from the vitalists, organicists and holists of their time. A century ago, in the same year that witnessed Wöhler’s announcement of the successful synthesis of Urea, the great chemist Henry wrote (1827) concerning organic compounds: “It is not probable that we shall ever attain the power of imitating Nature in these operations. For in the functions of a living plant a directing principle appears to be concerned peculiar to animated bodies and superior to and differing from the cause which has been termed chemical affinity.” Only six years before Helmholtz’s determination of the velocity of the nervous impulse in 1851, Johannes Müller had declared that to measure the propagation of that imponderable psychical principle was a theoretical absurdity.

It is not unlikely that before another celebration of the centenary of Wöhler’s achievement, Fischer’s synthesis of an octadecapeptide will have been surpassed by the manufacture of complex proteins in the laboratory. Looking forward a little in the light of what success has crowned the construction of physical models of vital processes, it is, as Sir Edward Sharpey Schafer scandalously suggested at a meeting of the British Association some years ago, perfectly legitimate to entertain the possibility, even the likelihood, that scientists will one day construct from artificially synthesized organic materials, systems with so wide a range of reversible reactivity and receptivity to external influences that they would be called organisms, if met with in Nature. While taking a more hopeful view in this matter than some biologists, I would remark that the validity of the mechanistic outlook is quite independent of this possibility. The security of any dynamical system of treating the motions of the heavenly bodies is independent of the possibility that human effort could manufacture a new satellite for Jupiter.

If we can assert that the present phase of biological enquiry is a peculiarly fruitful one, and that there is no reason to see any immediate cessation of progress in the use of physical concepts as the basis of our analysis of the properties of living matter, can we not go further and state that we have absolutely no encouragement for entertaining the hope that any deeper knowledge will accrue from apostrophizing under the sobriquets of entelechy, life force, élan vital that elusive entity to which, perhaps, the poet William Blake referred as Old Nobodaddy? It is doubtful whether we shall see a recrudescence of such frankly animistic devices as these. As biology becomes more technical and more exact, an aptitude for rehabilitating oriental mysticism in somewhat unusual verbiage will be regarded as an insufficient equipment for entering the field of biological controversy. The investigator who abandons physics for the pursuit of biology will contribute new ideas. Fruitful contributions need no longer be expected from those who combine the pursuit of literature with an amiable interest in natural history. The days of Butler and Bergson are passed.

Dr. Haldane, the most vigorous contemporary critic of the mechanistic standpoint, is very anxious to avoid any suspicion of being tainted with the cruder forms of vitalism. He disowns any allegiance to the life force, élan vital et hoc genus omne, except in so far as he, somewhat mysteriously, contrives to introduce an adventitious deity into the latter portion of his Gifford Lectures. This does not make its appearance until his major thesis is complete. Anxious as is Dr. Haldane to disclaim adherence to the tenets of vitalism, he is very definite in denying the possibility that atomistic concepts will ever successfully deal with the problem of what he calls “conscious behaviour.” In the light of Pavlov’s work we see that the problem of what is usually called conscious behaviour, or as we should rather say conditioned behaviour, can now be approached as a problem in the study of those conditions which determine whether a new reflex will, or will not, be brought into being. We may state this in other words by enquiring how the passage of impulses along particular tracts in the central nervous system influences the facility with which the nervous impulse will pass across a particular type of synapse. Since the problem of the conductivity of the synapse is, as we have seen, an essentially physical problem, it is not overstating the case to say that the work of Pavlov’s school has brought the study of what Dr. Haldane calls “conscious behaviour” within the realm of physical enquiry. Once this is fully grasped it no longer seems incredible that the interpretation of conditioned behaviour will eventually come within the scope of physico-chemical analysis. Contrary to the holistic standpoint, we are thus led to an atomistic concept of individuality. This I shall venture to formulate as the statistical probability that in an immensely elaborate system of reversible reactions a certain number of states characteristic of any given moment will be reproducible at another moment.

In his Lowell lectures Professor A. V. Hill lays down two general conclusions derived from the extension of modern biological enquiry. First, as I have endeavoured to show, there is no limit to the extent to which the mechanisms of life can be elucidated with the aid of physical methods and concepts. Second, that, however far we get, we shall still find function, adaptation, organization and purpose in the processes we explore. I would venture to suggest that, however alluring such a compromise between vitalism and mechanism may appear, these two conclusions, though formally in nowise inconsistent, are, nevertheless, in practice incompatible. As Henderson points out in his Fitness of the Environment, if we wish to indulge in teleological phantasies, we can find as much scope in physics and chemistry as in biology. We do not dismiss the hypothesis that thunderstorms occur when a blue unicorn sneezes on Uranus, because it is actually possible to disprove so engaging a fancy, but simply because other ways of treating thunderstorms lead to more useful conclusions.

Hence it seems to me that as we come to understand more and more about the mechanics of living systems by using methods of which Professor Hill is so brilliant an exponent, we shall inevitably find ourselves talking less and less about purpose and function. In consequence many of the problems which now engage the attention of philosophers will be relegated to the same status as the philosopher’s stone. No doubt such a change will come very gradually, so gradually that we shall hardly notice it. Nevertheless, one may venture to predict that philosophers, already forced by the developments of modern physics to divert their attention from the pretentious crossword puzzles of the Hegelian tradition, will sooner or later be driven to take account of the post-evolutionary developments in biology, and more especially those which have their starting-point in Pavlov’s researches.

By undertaking the analysis of the characteristics of conscious behaviour without departing from the methods of the traditional physiology of reflex action, biological science in our generation has shown that there is no nicely defined boundary at which physiology ends and moral philosophy begins. Hitherto physiology and academic philosophy have developed independently, because physiologists themselves have accepted the common sense dualism of mind and matter. Moral philosophy can no longer claim that there is any distinctive aspect of the Nature of Life, which lies beyond the province of physiological enquiry. If any fundamental distinction between mind and matter remains, that distinction henceforth defines the antinomy of a public world of common beliefs which all can share, the conceptual world of science in which ethical neutrality and economy of hypothesis reign supreme, and, in contradistinction to that public world, many private worlds which for the present remain impenetrable through the medium of discourse. Biological science is continually socializing our beliefs. What seems irrevocably part of the private worlds of one generation becomes irrevocably part of the public world of its grandchildren. Thus the new pluralism will not be, like the Cartesian system, static, but dynamic. It is ever tending towards a monistic outlook as a limiting case. Such a monism, unlike traditional materialism and traditional idealism will be regarded not as a formula but as an asymptote. It is evidently immaterial to public discourse whether we privately entertain the view that the public world is more or less real than our private worlds. It would thus seem that as biological science invades the province of human behaviour the concept of publicity, as I venture to call the communicability of beliefs, will come to occupy the status of importance which reality has held in the systems of egocentric philosophers.

The public world, as I have conceived it, is a construction based on the continuous extension of the principle of mechanism. The principle of mechanism, that a complex system is interpretable only by reference to the properties of its constituent parts, is not urged in the spirit of dogmatic assertion, but because it has served us well in the past. We still await any single verifiable conclusion that is uniquely developed from any alternative principle. Holism, the newest form of Vitalism, claims to have found an alternative or supplementary principle that is essentially teleological. The holist does not specify by reference to any single concrete situation how he proposes to use his principle. It is admitted even by the mechanist that we are not in a position to construct a symbolic relation which will completely describe the vagaries of a Ford car in terms of the field equations of the proton and electron. Does the holist wish us to believe that we can help anyone to drive a car by assuring him that at every level of complexity between the internal structure of the atom and the newly licensed automobile there emerges an ever-increasing urge to a wholeness or unity which is somehow indefinably different from the interaction of the parts? Verily the mechanist of all people knows that we know in part and we prophesy in part. For this very reason, because he is prepared to await with patience the slow advance of science, he refuses to subscribe to high-sounding pseudonyms for ignorance and principles that are never seriously intended to be put into practice.

In the recent symposium on The Nature of Life before the British Association both General Smuts in his exposition of the holistic standpoint and Dr. Haldane who supported him dwelt upon the supposed collapse of mechanistic principles in physics itself. The former cited in support of his point a somewhat rhetorical remark by Dr. Whitehead in this sense. It is of course evident that if our mechanical principles undergo modification our biological interpretations must share in the general change of outlook. It is, therefore, beside the point to criticize the mechanistic standpoint on the ground that our mechanical principles are undergoing revision. Let us examine this objection a little more closely. Experimental biology, we are told, has been directed towards the attempt to describe the properties of living matter in terms of the traditional physical concepts of mass, length, time, energy, etc. Since these concepts now appear to be less fundamental than we once believed, the hope that a complete mathematical description of the universe is realizable, has, as Mr. Sullivan asserts with triumphant naïveté, “no longer any plausibility.” Surely it is evident that a signal advance towards a more monistic interpretation of nature has been made, when the analysis of any biological phenomenon has been achieved with the aid of traditional physical concepts, and when concepts once peculiar to biology, as affinity was once peculiar to chemistry, have been translated into the traditional language of physics. Physics to-day is seeking a new synthesis to take into one system of equations all the old data, and many new ones which have lately accumulated. This is not a new situation. The old mechanics remains as valid as ever for the realm in which it was developed to operate. To effect a more comprehensive scheme it has been necessary to examine many of the old postulates. In the meantime we have to recognize that we are not so near to a single unifying hypothesis as the rise of energetics led the physicists of Kelvin’s generation to hope. What does this signify? Certainly not that mechanics has abandoned the principles of mechanism. Is it not rather a fact that the modern physicist is complaining that the inadequacy of Newtonian principles is in part attributable to teleological implications insufficiently recognized till now? The very hope of finality which Kelvin’s generation entertained seems from the new mechanistic standpoint, as I have stated it, to savour of scholasticism.

In taking this line General Smuts and Dr. Haldane seem to me to have laid bare the source of a misunderstanding that lies at the root of most of the criticism which vitalists old or new direct against the new or the old mechanistic standpoint. Those who have the scholastic predilection for finality and the scholastic predilection for the abstract noun, do not seem to be able to believe in the existence of people who are not like themselves. They cannot, it appears, understand that unless one starts off with the obsession that the universe can be summed up in a monosyllable, one is under no imperative necessity on the one hand to be resentful towards or disappointed with science because it lays no claim to the finality of religious dogma, nor on the other to make the assumption that such finality ought to be obtainable. The mechanist does not claim that his system is, or ever will be, complete in the sense that science will one day find an answer for all the conundrums which the scholastic temperament dictates. On the contrary, it is the essence of the mechanistic position that there is a technique of asking questions profitably as well as a way of answering them satisfactorily. All the mechanist claims is that as far as we can see at present his way of dealing with things leads to the most complete unanimity which it is possible to attain. Against the old vitalism, that of Dr. Haldane, who denies that the principle of mechanism can ever deal with conscious behaviour the older mechanistic outlook was secure in the assurance that, if the principle of mechanism failed at such a level, no other principle led to verifiable predictions in the same field. Against the new vitalism or holistic standpoint of General Smuts which no longer asserts dogmatically that the principle of mechanism is inapplicable at any specific level of existence, but contends that it does not anywhere give a complete account, the new mechanistic or publicist standpoint which I have outlined contends that if the principle of mechanism fails to give a complete account at any level no alternative or supplementary principle has been discovered. The reply of the mechanist old or new to the vitalist old or new is that of Mr. W. B. Yeats’ faeries:

“Is anything better, anything better Tell us it then...”

It follows that, in any discussion between the two, the combatants are generally at cross-purposes. The mechanist is primarily concerned with an epistemological issue. His critic has always an ontological axe to grind. The mechanist is concerned with how to proceed to a construction which will represent as much about the universe as human beings with their limited range of receptor organs can agree to accept. The vitalist or holist has an incorrigible urge to get behind the limitations of our receptor organs and discover what the universe is really like. What we mean by really in this connexion evidently depends upon whether we view the question socially or individually. In our relation to other human beings the nearest approach to what the universe is really like is found in the schematization of our common experiences. If there is any other reality its sanction is non-social. Thus in contradistinction to the reality of traditional philosophy which is an individualistic concept, the concept of publicity, which it is proposed to substitute as the goal of synthetic philosophy, is an essentially social one.

IV. THE CONCEPT OF ADAPTATION

“No philosopher who is rational and modest has ever pretended to assign the ultimate cause of any natural operation, or to show distinctly the action of that power which produces single effect in the universe. It is confessed that the utmost effort of human reason is to reduce the principles productive of natural phenomena to a greater simplicity.... The most perfect philosophy of the natural kind only staves off our ignorance a little longer, as perhaps the most perfect philosophy of the moral or metaphysical kind serves only to discover larger portions of it.”--David Hume, Sceptical Doubts

By those who hesitate to commit themselves to an explicit advocacy of either the vitalistic or mechanistic views about the Nature of Life it has often been urged that the concept of adaptation is fundamental to biological science. Professor A. V. Hill is perhaps the most brilliant physiologist now living. He adopts a hopeful attitude to the progress which awaits further analysis of the properties of living matter in physico-chemical terms. He also thinks that, however far mechanistic principles are extended, the biologist will always encounter “adaptation” in the phenomena which he studies. Another distinguished physiologist, Professor Lovatt Evans has expressed himself in rather more emphatic terms.

“Physiologists,” he states, “in attempting to know what life is, have in my opinion attempted too much, and I think that a new point of view is essential.... The idea of adaptation, urged by Claude Bernard, should be adopted by physiology as its basal principle, as the chemist accepts the conservation of matter or the physicist the conservation of energy. We need not seek to know why it is so, that is the province of the philosopher.... It is not a definition of what life is, but a brief statement of its way.... Life is conserved by adaptation.”

When I first read these words I was not sure that I agreed with them. I was not quite certain that I knew what they meant. I had already come to the conclusion that the word adaptation is frequently used by biologists without a very clear agreement as to its content. I cannot subscribe to the view that there is a sort of trade union of philosophers to which physiologists are ineligible, unless they can show their articles of apprenticeship. Nor can I conceive what is meant by a concept of life except such as is implied in a statement of its way. A scientific concept defines a class of properties. A scientific concept of life or adaptation must conform to this requirement. In this essay my object is not to criticize Professor Lovatt Evans for whose breadth of view I entertain a very sincere respect. I have quoted his words, because they focus attention on some significant and controversial issues. They serve to reveal how imperative it has become that biologists should agree about the sense in which they intend to use the word adaptation.

The quotation given above might be interpreted to mean two very different things. If the term adaptation is used to define certain very general characteristics of living systems, it becomes almost co-extensive with a scientific concept of life itself. If we use principle in a somewhat archaic sense to indicate a field for investigation, like the principle of affinity or the active principle of the thyroid gland, there can be no question that the idea of adaptation is the basic principle of physiology. The comparison of the biologist with the chemist or physicist seems to go beyond this, and imply that adaptation is not something to be explored and interpreted, but part of the logical procedure of biology, something by the aid of which we can predict conclusions of universal validity in the field of biological enquiry. I do not think that Professor Lovatt Evans really means this. I do urge that biologists continually confuse within the compass of the concept of adaptation the notion of a problem for solution and of a vera causa. This in everything but verbiage is precisely what the cruder type of vitalist does, when he invokes the vital principle. He first introduces a term to describe a large number of things about which we are ignorant and wish that we knew more. He then falls into the trap of imagining that the invention of a new term has solved the problem.

Quite apart from this difference which, if it is to define the scope of our scientific enquiries, cannot be dismissed as metaphysics, biologists differ a good deal concerning the extent of the phenomena and the kinds of phenomena they are dealing with, when they speak of adaptation. The physiologist--in the restricted sense of the term--is usually referring to something which might be called the self-regulating characteristic of the body. The evolutionary biologist--who to-day is a physiologist in the broader sense of the term--is usually thinking of “a change in the structure, and by implication also in the habits of an animal which render it better fitted” for life. I here quote Professor D. M. S. Watson’s suggestive address on adaptation from the evolutionary standpoint. Sometimes the word adaptation has a more comprehensive significance and includes both definitions which I have distinguished. It then amounts to saying that living systems are self-regulating and self-propagating, which is one way of defining the nature of life as a scientific concept. None of these technical uses of the word adaptation imply anything that the most dogmatic mechanist could decry. If we define adaptation as the self-regulating processes by which living matter retains its recognizable characteristics, it is a truism to say that life is preserved by adaptation. In that case, if adaptation is to be made the paramount issue for biological enquiry, we can hardly upbraid our predecessors for presumptuously seeking to know what life is. If we are to reach any agreement about the use of the word adaptation we must therefore retrace our steps, and examine more closely what are the characteristics of a living system. It is useless to define the goal of biological enquiry in terms of a concept which is as vague as life itself. I suggest that when, in its various uses, the term adaptation has any objective utility, it refers to these two more or less distinct categories of characteristics which living beings display, i.e. self-regulating and self-propagating. They are separable issues inasmuch as a worker bee and a Dominican friar are self-regulating but not self-propagating systems. There is no particular reason to object to the use of the prefix, so long as no personalistic implications of the word self are imposed upon it without further discussion.

Of the two ways in which the word adaptation is used in biological discussion, that which implies the notion of self-regulation is most fundamental. A living organism is an extremely complex system in dynamic equilibrium with its environment. The idea of dynamical equilibrium is not peculiar to biology. The atom, which for traditional chemistry was a statical concept, is no longer regarded in that way by the modern physicist. What is more peculiar about living matter is its amazing complexity, and the idea of adaptation in the sense of self-regulation calls attention to the fact that a system of such extreme complexity, a system with so many characteristics, continues to maintain its individuality, i.e., its manifold characteristics, in spite of all the changes that are taking place within it and without. The recognition of this complexity is common ground. If the mechanist underrates the difficulty of the problem, he is certainly to be discouraged, except in so far as the scientist in attacking any problem must always focus his attention on a limited range of data and rule out certain things as insignificant for his present purpose. It may be true, as Professor Lovatt Evans opines, that mechanistic interpretations tend to become arrogant and superficial. Is he on surer ground in holding that “it is unthinkable that a chance encounter of physico-chemical phenomena can be the explanation”? Might we not reflect with David Hume that “our own mind being narrow and contracted, we cannot extend our conception to the variety and extent of nature, but imagine that she is as much bounded in her operations, as we are in our speculation”?

Scientific hypotheses are not always thinkable, if by that we mean pleasant, easy or conformable to common sense. Our grandfathers thought it “inconceivable” that her Gracious Majesty, Queen of Great Britain and Ireland, Empress of India and Defender of the Faith, could be descended from an ape. The atomic structure of matter was unthinkable to many people little more than a century ago. To-day the quantum atomic model is unthinkable; but we think it is the best way of interpreting the data. Given this amazingly complex system in dynamically stable equilibrium with its environment, we have to decide consistently with the fullest requirements of the problems what is the most economical way in which we can envisage its existence. Seeing that a mechanistic interpretation is evidently the most economical one, the real issue is to decide whether there are any characteristics of the complex which are inconsistent with such an attitude.

From the modern standpoint the individuality of the atom is a statistical concept. The atom is in dynamically stable equilibrium with its surroundings. It might, therefore, be argued that a Ford car is an example of a complex mechanism which is in dynamical equilibrium with its environment. This would be a superficial analogy for the order of complexity which we encounter in living matter. The molecular constitution of the parts of a Ford car is comparatively static. In the minutest parts of an organism new molecules are being built up and replacing others that have been broken down. Nevertheless, in all this astonishing panorama of microscopic revolutions which underlie the microscopic continuity of the organism we know of no events which are in conflict with the great generalizations of physical science.

On this point Professor Hill speaks with special authority, when he declares:

“Fortunately for physiology several of the generalizations of science appear to be fairly strictly true, even when applied to the living organism. Although such exact experiments are not possible on man, or animals, or plants, as may be made on non-living objects, there is little evidence--indeed, I would be bold and say there is no evidence--that such living creatures can, in any manner or degree, evade the ordinary laws of mechanics, chemistry and physics, the principles of the Conservation of Energy and Mass.... There really is no evidence that momentum and kinetic energy, that chemical transformations, that electrical and magnetic phenomena, occur in the living body in any manner, or to any extent, which differs from that obtaining in the more readily investigated non-living world.”

In the same lecture Professor Hill replies to a statement which has been frequently reiterated by vitalistic writers including General Smuts and Professor Julian Huxley. Referring to the Second Law of Thermodynamics, he says:

“Philosophically speaking, the Second Law of Thermodynamics, dealing with the limitations of the availability of Energy, is more liable to doubt. It is known to rest on a statistical basis, and when we are dealing with units, complete, self-producing, yet as invisible and intangible as the filter-passing or other micro-organisms, it is, theoretically speaking, possible that some means may be available of evading the statistical relations which govern the behaviour of larger systems. But here again we must ask for evidence--and there is none of a precise or definite character which suggests, in the least degree, that the living cell can escape the jurisdiction of the Second Law.”

We are thus forced to consider the order of complexity of the living system maintained in dynamical equilibrium with its surroundings as the essential feature which distinguishes it from non-living things. This complexity can be arbitrarily divided into many levels; but for convenience we may confine ourselves to two, the macroscopic and the microscopic. Let us be explicit about the meaning of this distinction. In the more familiar animals, we are accustomed to recognize a variety of responses to a variety of external agencies. Generally speaking in the more complex animals each kind of reactivity and each kind of receptivity is spatially localized. For instance, light impinging upon the retina evokes contraction of the pigment cells in the toes of a frog. From this macroscopic complexity of the gross architecture of the animal body arise two types of problems: first, the problems of co-ordination dealing with the way in which a disturbance recurring in some receptive area is propagated to an effector organ (gland, muscle, etc.) in some other region; and second, the problems of metabolic exchange, dealing with how the supply and distribution of sources of energy for all this display of activity is maintained. The first involves the study of the nervous impulse along the peripheral nerve fibres and through the central nervous system; it also involves the study of the internal secretions. The second involves the study of digestion and assimilation of foodstuffs, the intake of oxygen to burn up the waste products of chemical activity, and the removal of carbon dioxide, water and other products of oxidation. In contradistinction to the gross complexity of organs or populations of cells, we have to take into account the microscopic complexity of the cell itself. This presents a more general issue, because there exist many organisms whose complexity is of the same order as that of the separate cells which make up the bodies of familiar animals of visible dimensions. Two of the major problems of cell physiology concern the way in which the cell maintains its semi-permeability, and the way in which it maintains a constant renewal of chemical materials by utilizing the energy liberated in certain organic oxidations.

If we remove the magneto from a car, we can keep it intact for an indefinite period: it is fundamental to our idea of a mechanism that it can be taken to pieces and put together again. We are so accustomed to think of a leg or an arm as dependent for their activity on the rest of the body, that the conception of a living mechanism is repugnant to common sense. In the laboratory it is possible to study properties of nerve, muscle, the cell membrane, absorption of food in the gut, oxidation of nitrogenous materials in the liver, etc., as isolated events. A person who is not a biologist almost invariably expresses bewilderment when he sees the isolated heart of an animal beating regularly in a perfusion apparatus. There exists the idea that the living organism differs from a mechanical system in that the parts cannot persist without the whole. Behind this illusion of common sense the holistic concept of adaptation stands securely entrenched. The holistic conception implies that for living systems the part must be interpreted in relation to the whole, and not the whole from the interaction of parts. We have seen that the ultimate non-biological constituents of living matter, molecules, atoms, etc., do not behave differently when united to form a living system. In transcending this level of organization we are faced with an equally striking conclusion. The contraction of an isolated muscle preparation is essentially the same as the contraction of a muscle considered as an isolated aspect of the behaviour of the intact organism. The passage of the nervous impulse along an isolated nerve is not fundamentally different from the passage of the nervous impulse in the normal animal. The conversion of sugar into alcohol by the isolated enzyme zymase obtained from crushed yeast cells is a process like that of the conversion of sugar into alcohol by the living yeast fungus. The whole development of physiology, from the time when Haller first made an isolated muscle preparation and Spallanzani produced animal light by moistening a dessicated powder of luminescent jelly fishes, bears witness to the conclusion that the separated constituents of a living whole do not at any level of complexity behave differently from the way in which they behave as parts of a more complex order. Thus, when the fullest recognition is given to the extreme complexity of living systems, the problem of self-regulation submitted to experimental analysis does not bring forward any confirmation for the holistic view of adaptation as the creative interpolation of new irreducible properties at different levels of complexity. The holist may reply that it is one thing to take the living machine to pieces, and another thing to put it together again. Even here the analogy with the machine holds good. To graft the eye of one salamander tadpole on to the head of another individual is now a commonplace of experimental embryology. Five-legged and two-headed newts are now manufactured in the laboratory.

Self-regulation, the way in which an organism maintains a seeming continuity of arrangement in spite of the uninterrupted and ubiquitous flux of macroscopic and microscopic changes which its existence implies, defines the sense in which the term adaptation is ordinarily used by the physiologist. In contradistinction to this physiological and individual use, adaptation is employed in biological discussion in a morphological and specific sense, when we consider how one animal comes to be distinguished from another by some architectural arrangement appropriate to a particular kind of environment. In this sense the problem of adaptation has played a prominent part in the evolutionary speculations of the past century. Given the fact that organisms are not only self-regulating but self-propagating, the evolutionary theory sets out to explain how living systems come to exist in so many specific forms, and how it is that these specific forms are on the whole fitted or adapted to their respective surroundings. The qualification on the whole is highly significant. Organisms display many peculiarities of architecture which by no stretch of imagination can be regarded as necessarily fitting them better for their conditions of life. To assume that every peculiarity of structure in an animal is useful to it in the struggle for existence is a pure assumption unfounded on anything but teleological prejudice.

Adaptation in the morphological sense really includes two ideas which to some extent coalesce, and are therefore all the more readily confused. At times the word implies nothing more than viability. In this sense adaptation is the whole problem of evolution. Up to a certain point an organism must be “suited” to its environment in order to live at all. At other times adaptation is extended to mean an essential utility in every detail of the structure of an organism. This is a mischievous implication which, as will be seen later, has hindered the formation of a clear conception of the evolutionary process. Even if we could justify the belief that female peafowl are as much impressed by peacocks as are some male biologists, we have still failed to supply a criterion of survival value which has any satisfactory significance. The enthusiast who describes an adaptation is often like the advertizing manager who tells us how many customers we shall get, if we advertize with him, but is inclined to be reticent about whether the profit derived from more customers is commensurate with the fees he proposes to exact for his services. Bateson, who first applied Mendel’s principles to animals, wrote five years before the Mendelian Renaissance in terms which may still be commended to the thoughtful examination of every student of the evolutionary problems:

“Whereas the only possible test of the utility of a structure is a quantitative one, such a quantitative method of assessment is entirely beyond our powers. To know that the presence of a certain organ may lead to the preservation of the race is useless, if we cannot tell how much preservation it can effect... unless we know also the degree to which its presence is harmful, unless, in fact, we know how its presence affects the profit and loss account of the organism.” (Materials for the Study of Variation).

That animals do in fact display many structural characteristics which are in no sense useful to them is generally admitted to-day. It thus becomes as much the function of any theory of the evolutionary process to explain the origin of useless as to explain the origin of useful devices. There is a practical limit to the use of the concept of adaptation in morphology. There is a no less obvious limit to the use of the concept of adaptation in physiology. An animal is a self-regulating system up to a point; but it cannot in every contingency take arms against a sea of troubles and by opposing end them. If we could define in some general terms where this limit lies, we should be justified in speaking of a principle of adaptation in the sense that we speak of a principle of conservation of matter. The ideally self-regulating unit of living matter endowed with the secret of perpetual youth is as imaginary as the Economic Man. At present the fact that organisms cease to regulate themselves and die is as fundamental a problem of biology as the converse fact that they regulate themselves and thereby continue to live. The fact that the organism has a good deal of useless anatomical equipment seems to be as true as the fact that on the whole its anatomy is suited to the requirements of its surroundings. In whichever way we employ the term adaptation we are forced to the conclusion that it is only legitimate to speak of a principle of adaptation in the sense in which we speak of the active principle of the thyroid gland. Adaptation defines a field of problems which await solution. In that sense the concept of adaptation is as fundamental to mechanistic as to any other theories of the organism.

This is not what is generally meant when it is said that adaptation is a fundamental principle of biological enquiry. I believe that it is the only legitimate sense in which it can be said that there is a biological principle of adaptation. It seems to me that, when we go further and put more than this into our concept of adaptation, we are driven to formulating the problems of biology in a wrong way. By inventing hypotheses to explain facts which do not exist, we then proceed to give false interpretations of the significance of facts that do exist.

When the principle of adaptation is treated as a principle which enables us to predict conclusions, it constantly leads us to fantastic distortions of what really happens. If I wished to illustrate this in connexion with the self-regulating aspect of the concept of adaptation, I could not do better than refer to current speculations about the rôle of the ductless glands in the economy of the organism. The physiologist who interprets his field of observation in a manner analogous to that of the physicist and chemist realizes that we have no reason to believe that every chemical entity found in the animal body is necessary or even useful to its owner. He will not therefore draw any conclusions of a far-reaching nature from the discovery that a certain tissue extract has highly specific physiological properties, unless he can show that the removal of the tissue itself produces effects of an opposite nature to those which ensue on injecting its active constituent. The student of ductless glands who regards adaptation as a principle to be applied rather than as field to be explored will not be held back by such restraint. We must thank the “principle” of adaptation in endocrinology for the romantic guess-work of that school which undertakes to interpret the whole of human history in terms of a glandular explanation of temperament. Most speculations on which the glandular theory of temperament are based have their only experimental basis in the presence of supposedly specific active substances in one or other tissue extract. A “principle of adaptation” does not assist us to understand why the pituitary gland of a fish should contain one specific constituent which produces expansion of the black pigment cells in the skin of a frog, another specific constituent which causes the uterus of the mammal to contract, and yet a third which produces a specific fall of blood pressure in the bird and a specific rise of blood pressure in the mammal.

It is especially in the field of evolutionary biology that we must look for most guidance, because the concept of adaptation has occupied such a prominent part in the evolutionary controversy. As an example of how “the principle of adaptation” leads to incorrect conclusions I need cite only one example from an exceedingly able and provocative address of Professor D. M. S. Watson.

“It is not unusual for a student of fossils to discuss the habits of an extinct animal on the basis of a structural resemblance of its ‘adaptive features’ with those of a living animal and then to pass on to make use of his conclusions as if they were facts in the discussion of an evolutionary history or of the mode of origin of a series of sediments. In extreme cases such evidence may be absolutely reliable: no man faced with an ichthyosaur so perfectly preserved that the outlines of its fins are visible can possibly doubt that it is an aquatic animal, and such a conclusion based on structure is supported by the entire absence of ichthyosaurs in continental deposits of appropriate ages and their abundance in marine beds. But if extremes give good evidence, ordinary cases are always disputable. For example, there is, so far as I know, not the least evidence in the post-cranial skeleton that the hippopotamus is aquatic; its limbs show no swimming modification whatsoever, and the dorsal position of the eyes would be a small point on which to base assumptions. Most palæontologists believe that the dentition of a mammal, and by inference also that of a reptile or fish, is highly adaptive, that its character will be closely correlated with the animal’s food, and that from it the habits of an extinct animal can be inferred with safety. Here again the extreme cases are justified, the flesh-eating teeth of a cat and the grinding battery of the horse are clearly related to diet. Crushing dentitions, with the modification of skull and jaw shape and of musculature which go with them, seem equally characteristic. I had always believed that the horny plates and the jaws of Platypus were adapted to hard food, and that that animal possessed them, whilst the closely allied Echidna was toothless, because it was aquatic and lived in rivers which might be expected to have a rich molluscan fauna which could serve as food. But the half-dozen specimens whose stomachs I have opened contained no molluscs whatsoever, and seem to have fed on insect larvæ, the ordinary soft bottom fauna of a stream.” (Op. cit.)

We are now beginning to see that the evolutionists of the nineteenth century focused their attention far too exclusively on adaptation. In other words, they regarded adaptation as a principle like the principle of conservation of matter, one of universal validity within the field of biology. Any theory of evolution has to explain why non-adaptive, as well as adaptive, features arise. In that sense the fundamental problem of evolution is not the origin of adaptation but the origin of species. Both the theories of Lamarck and Darwin implicitly assumed that the differences between species, in the traditional, i.e. Linnæan sense, are mainly utilitarian. Having started with an incorrect apprehension of the facts they proceeded to elaborate hypotheses to account for them. Thence inevitably they drew from these hypotheses an unsatisfactory account of the way in which new species do arise. From the modern standpoint analysis of the species problem does not demand a recognition that species differences are even in the main utilitarian, though such a statement would probably be true of differences between larger units such as genera. Nor from the modern standpoint do the hypotheses of either Lamarck or Darwin give us any clue to the way in which the species barrier, i.e. inability to breed with other species successfully, can have arisen. Anything which remains of the Lamarckian principle in the light of modern research has no special relevance to the origin of adaptations. Whatever remains of the theory of natural selection has been completely divested of the implication that non-adaptive characters were necessarily adaptive at their inception.

A discussion of the fate of the Lamarckian and Darwinian theories must be undertaken elsewhere. Here it is sufficient to point out that both, more particularly the latter, had a peculiarly sterilizing influence on the growth of experimental biology. Obsessed with the principle of universal adaptation which Natural Selection had secularized, zoology, from the publication of the Origin of Species to the rediscovery of Mendel’s Laws, wandered for forty years in a wilderness of phylogenetic speculation. Biological research in the words of Professor Punnett became

“devoted to the construction of hypothetical pedigrees suggesting the various tracks of evolution.... The result of such work may be said to have shown that the diverse forms under which living things exist to-day, and have existed in the past, so far as palæontology can tell us, are consistent with the view that they are all related by the community of descent.... It is obvious that all this work has little or nothing to do with the manner in which species are formed.”

According to the Selectionist doctrine in its original form, characters originated and persisted in virtue of their utility and what Darwin somewhat vaguely called “the strong principle of inheritance.” To explain any peculiarity of structure or habit, it became necessary only to show one of two things, either it was useful to its owner or was once useful to an ancestor of its owner. Everything was or had been an adaptation. This resulted in a complete divorce of comparative anatomy from comparative physiology. The morphologist and systematic zoologist regarded it as an impertinence of the physiologist to seek for experimental evidence, where a perfectly good case of adaptation was evident to anyone who would accept their premisses.

I will illustrate this from a field in which I have myself carried out experimental investigations for twelve years. Writing of colour change in frogs Dr. Hans Gadow makes the following remarks in the Cambridge Natural History (Amphibia and Reptiles, p. 36):

“Biedermann concludes that the chromatic function of frogs in general depends chiefly upon the sensory impressions received by the skin, while that of fishes depends upon the eye. All this sounds very well, but the observations and experiments are such as are usual in physiological laboratories, and the frogs, when absorbed in their native haunts, or even when kept under proper conditions, do not always behave as the physiologist thinks they should. There is no doubt that in many cases the changes of colour are not voluntary but reflex actions. It is quite conceivable that the sensation of sitting on a rough surface starts a whole train of processes: roughness means bark, bark is brown, change into brown; but one and the same tree frog does not always assume the colour of the bark, when it rests or when it sleeps upon such a piece. He will if it suits him remain grass green on a yellow stone or on a white window frame.... The sensory impression received through the skin of the belly is the same, no matter if the board be painted white, black or green, and how does it then come to pass that the frog adjusts its colour to a nicety to the general hue or tone of its surroundings.”

It is safe to say that no one, unless at the outset prejudiced by the principle of adaptation, could be led to entertain the view that frogs as a rule are able to adjust themselves “to a nicety” to the general hue and tone of their surroundings. The state of the pigment cells in the skin is influenced independently by a number of diverse factors, including moisture, temperature, diffuse light acting on the skin and reflected light acting on the retina of the eye in the opposite sense. Individual frogs differ in basic pattern, but the range of hue between the dark and pale condition for any frog is fixed, as is also true of the proverbial chameleon. When the conditions affecting colour change in a frog are defined, it is possible to predict the pigmentary response of a frog and its time relations with as much confidence as any other physical event in nature. It is, on the other hand, quite impossible to draw any far-reaching conclusions about colour change from uncontrolled observation of the frog in its native haunts, because the number of significant variables is far too numerous to handle in this way. I have quoted this passage to show the attitude which zoologists under the influence of the post-Darwinian tradition adopted towards experimental enquiry of any description. Dr. Gadow applies the “principle of adaptation,” as it was then used in morphology, to the self-regulating aspect of the organism with results which show what might well happen to physiology if the physiologist employed the principle of adaptation as the chemist employs the principle of conservation of matter.

Ecology, or the study of the relation of species to particular types of environment, provides a clear illustration of the progress that has been achieved by detailed study of physiological mechanisms in place of the speculative application of the principle of “adaptation.” Krogh and his pupils have made a special study of the physico-chemical properties of the blood pigments of the lower organisms, and have thereby thrown a good deal of light on the conditions which determine their ecological distribution. Let us take the case of two common bony fishes, the carp and the trout. It is a matter of common experience that in nature the trout will only live in running streams. It can be kept with great difficulty in aquaria, if special precautions for aerating the water are taken. The carp will live in still water, where the oxygen content is low, and like its ally the goldfish accommodates itself to the aquarium with great ease. The difference between the two types is at once understood, when we know that the hæmoglobin of the carp has a much higher affinity for oxygen than the hæmoglobin of the trout. In consequence the blood of the carp is completely saturated with oxygen when the oxygen content of the water in which it swims is far below that which is in equilibrium with the oxygen pressure of the atmosphere. The blood of the trout on the other hand is only fully saturated with oxygen when the water is itself nearly saturated.

The concentration of salts in the blood of fishes like the trout and carp is kept constant at a level below that of sea water. The concentration of dissolved substances in the blood of sharks and dogfishes which are all marine is in equilibrium with the osmotic pressure of the sea. The respiratory centre of the wrasse is paralysed at 60° C. {sic}, while the heart of the English dogfish shows irreversible changes above 18° C. Taking these facts together we can deduce a good deal about the viability of a species in a given locality. A fish like the skate placed near the estuary of a large river is forced to remain where the salt concentration is above a certain level. A salmon is not subject to this restraint. Assuming that the fish can pass the estuarine boundary and proceed upstream, two alternatives present themselves. He can remain in the swiftly moving main stream or take to backwaters and stagnant lakes connected with it. If he has the hæmoglobin of a trout, he is committed irretrievably to the former alternative. Being compelled to remain in the swiftly running part of the river bed, he might stay in the lowlands or make for the source, which in general will be much colder. In the case of a fish like the wrasse, whose respiratory centre is paralysed at a temperature of 6° C., the latter course is impossible, if the river rises in a high range. Thus in place of vaguely speculating about how an organism is specially “adapted” to live in some particular place, experimental biology is gathering clearly defined ideas about why an organism cannot live in any place other than that in which it does live.

The idea that a problem can be solved by invoking the principle of adaptation assumes its most grotesque form in Haeckel’s discussion of Recapitulation. The classical example of what is called recapitulation is provided by the gill clefts of vertebrates. All vertebrate embryos have pits or clefts at the sides of the throat, supplied by a characteristic arrangement of blood-vessels. In fishes the clefts acquire filaments richly supplied with blood-vessels, and act as gills. Both the gill clefts and the characteristic arrangement of blood-vessels associated with them persist throughout life. In frogs and salamanders gill filaments are developed in the tadpole stage, but the clefts disappear in adult life and the characteristic arrangement of their blood supply becomes profoundly changed. In Man and most land vertebrates the gill clefts are never used as respiratory organs, and with their blood-vessels disappear at an early stage in development. During the first half of the last century Van Baer, the pioneer embryologist, propounded a generalization which may be stated thus: embryos of different species of animals of the same group are more alike than the adults, and the younger the embryo the greater are the resemblances. This generalization, well illustrated by the gill clefts, was later extended by Haeckel with the sonorous title “Biogenetische Grundgesetz.” It is thus defined by its author: “The history of the fœtus is a recapitulation of the history of the race, or in other words, ontogeny is a recapitulation of phylogeny.”

The way in which the modern geneticist handles the problem of development offers a striking contrast to the attitude of Haeckel and a generation of zoologists unduly preoccupied with the concept of adaptation. A recent investigation from the laboratory of Professor Julian Huxley will illustrate the difference. In the little crustacean Gammarus there are a number of varieties distinguished by the colour of their eyes. All coloured eyes are at an early stage of development colourless. They then become scarlet owing to the formation of a red pigment. They may subsequently darken owing to the deposition of the black substance known as melanin. Varieties with eye colour from a dark red through various grades of chocolate to dark brown and black are distinguished by the time at which the deposition of melanin begins and the rate at which it occurs. Here there is no difficulty in seeing what conditions must be fulfilled in order that a new variety should or should not recapitulate the characteristic of the ancestral stock from which it arises. If a red-eyed variety of Gammarus arose from a white-eyed stock, it would necessarily exhibit the ancestral condition at the beginning of development, because all eyes are at first colourless. If a black-eyed form arose as a sport from a red-eyed stock, it would also recapitulate the ancestral characteristic, because all black eyes are at first red. If a white-eyed form arose as a sport in a red-eyed stock, or a red-eyed form emerged from a black-eyed stock, in neither case would the ancestral condition be manifest at any stage of development. There is no question of the intrinsic usefulness of a new character involved in this. Whether recapitulation does or does not occur here depends upon whether the Mendelian factor which distinguishes a new variety hastens or retards some feature of the developmental process.

Now Haeckel’s “Grundgesetz” implies an additional statement to that contained in Van Baer’s Law. It signifies that the embryonic stages of one form are to be compared with adult rather than embryonic stages of another. This in fact is not correct, as the classical cases of recapitulatory phenomena demonstrate most clearly. The mammalian embryo never possesses true gills. It goes through a stage at which it has the characteristic clefts and arterial arches which in the fish embryo precede the development of functional gills. This is also true of crustacean larvæ. Sacculina, the crab gall, passes through the two characteristic larval forms of the true barnacles. It has no resemblance to an adult barnacle in any stage. A more serious objection to Haeckel’s way of stating the idea of recapitulation in development is the vagueness it assumes when brought face to face with the exceptions that are as numerous as the applications of the rule. An illustration of the exceptions is provided by eye colour in the human species. It is fairly certain that the blue-eyed condition has arisen as a mutant in a brown-eyed stock; yet the eyes of brown-eyed adults are often blue in the newly born.

It is not difficult to discover in Haeckel’s own writings the train of reasoning which led him to distort the facts of development in stating the law which is often associated with his name.

“The evolution of the fœtus (or ontogenesis),” states Haeckel, “is a condensed and abbreviated recapitulation of the evolution of the stem (or phylogenesis); is preserved by a constant heredity; on the other hand, it becomes less complete in proportion as a varying adaptation to new conditions increases the disturbing factors in the development (or cenogenesis). The cenogenetic alterations or distortions of the original paligenetic course of development take the form, as a rule, of a gradual displacement of the phenomena, which is slowly effected by adaptation to the changed conditions of embryonic existence during the course of thousands of years. This displacement may take place as regards either the locality or the time of the phenomenon. The first is called heterotopism, the second heterochronism.”

So naïve a combination of garrulous teleology and self-contradiction is characteristic of the hopeless confusion of thought which existed in evolutionary biology, while it remained dominated by the principle of adaptation. The larval “adaptations” should on the face of it recapitulate their ancestral story--and so on in endless regression. There is no intelligible meaning in Haeckel’s explanation of the admittedly ubiquitous exceptions to his rule.

Haeckel’s so-called Biogenetische Grundgesetz exerted a profound influence on biology during the second half of the nineteenth century, and perhaps did more than anything else to divert zoologists from the study of activity to the pursuit of insignificant details of no conceivable physiological interest. Instead of furthering the development of zoology as an exact science, it substituted the construction of architectural mnemonics for the search after quantitative laws. With Haeckel’s law is associated an interesting logical fallacy in the development of the argument for evolution. Huxley made a good debating point when he disclosed the embarrassing information that a bishop at one stage of the episcopal life cycle has gill structures like those of a fish. From the standpoint of formal logic the point is worthless. Only the atmosphere of religious propaganda which surrounds the birth of the evolutionary doctrine can explain the perennial reappearance of the contention that recapitulation constitutes an argument sui generis in favour of the doctrine of descent. If experimental breeding taught us that mutant forms recapitulate the characteristics of the stock from which they originate, the resemblance of developmental stages of present-day forms to adult organisms which existed in the geological past would constitute a special consideration in favour of regarding fossil remains as ancestral to contemporary animals. As yet experimental breeding teaches us no such thing. We do not find that a white-eyed fly originating as a sport in a red-eyed stock invariably has red eyes at any prior stage of development. Recapitulatory phenomena are difficult to explain on a theological basis, but they do not constitute a special argument in favour of the evolutionary alternative. To-day biologists are beginning to realize that evolution must furnish an explanation of specific differences which are not adaptive as much as specific differences which are adaptive. With this change of outlook it is becoming possible to discuss the logical status of the evolutionary hypothesis without recourse to arguments which belong more properly to propaganda than to science.