Recent Developments in European Thought Part 15

FOR REFERENCE

Bragg, _Rays and Crystals_ (Ball & Sons).

FOOTNOTES:

[Footnote 70: Since this address was given, the results of the Eclipse Expedition to Brazil are considered to have confirmed in a satisfactory manner one of the most remarkable deductions made by Einstein from the principles which he maintains. The matter has roused so much interest that some of the leading exponents of the relativity principle have published careful accounts intended for students not familiar with it: it would therefore be superfluous to discuss the matter here.]

IX

PROGRESS IN BIOLOGY DURING THE LAST SIXTY YEARS

PROFESSOR LEONARD DONCASTER, F.R.S.

On November 24, 1859, _The Origin of Species_ was published, and this date marks the beginning of an epoch in every branch of biology. Before it, Biology had been almost entirely a descriptive science, but within a few years after the publication of the _Origin_ its effects began to colour all aspects of biological research. A co-ordinating and unifying principle had been found, and the leading idea of biologists ceased to be to describe living things as they are, and became transformed into the attempt to discover how they are related to one another. The first effect of this change of att.i.tude was chiefly to turn biologists towards the task of tracing phylogenetic or evolutionary relations.h.i.+ps between different groups of animals--the drawing up of probable or possible genealogical trees and the explanation of natural cla.s.sification on an evolutionary basis. When once, however, the notion of cause and effect, or more correctly of relations.h.i.+p, between the phenomena seen in living beings had become familiar to biologists, it spread far beyond the limits of tracing genealogical connexions between different animals and plants. It made possible the conception of a true Science of Life, in which every phenomenon seen in a living organism should fall into its true place in relation to the rest, and in which also the phenomena of life should be correlated with those discovered in the inorganic sciences of Chemistry and Physics.

The history of the various branches of biological science in the past sixty years reflects the general course of these tendencies. Until shortly after 1859, the study of morphology, or the comparative structure of animals (and of plants) was intimately related with that of physiology, that is, with the study of function. In the years following the appearance of the _Origin_, however, anatomists and morphologists were seized with a new interest. For the time at least, the chief aim in studying structure was no longer to explain function, but rather to explain how that structure had come into being in the course of evolution, and how it was related with h.o.m.ologous but different structures in other forms. The result was a tendency to a divorce between morphology and physiology, or at least between morphologists and physiologists, which led to the division into two more or less distinct sciences of what had hitherto been regarded as closely inter-related branches of one. The greater men of the early part of the period, such as Huxley, remained both morphologists and physiologists, but most of their followers fell inevitably into one or the other group, and in discussing the later phases of biological progress it will be necessary to keep them separate.

Apart from its effect on the systematic and anatomical side of Biology, the idea of Evolution, and especially of Darwin's theory of Natural Selection, had important consequences on that side of the science which may be described as Natural History. Before the appearance of Darwin's work, Natural History consisted chiefly in the observation and collection of facts about the habits and life-history of animals and plants, which as a rule had no unifying principle unless they were used, as in the Bridgewater Treatises, to ill.u.s.trate 'the power, wisdom, and goodness of G.o.d'. Now, however, a new motive was provided--that of discovering the uses to the organism of its various colours, structures, and habits, and the application of the principle of natural selection to show how these characters conduced to the preservation and further evolution of the species. And out of this interest in the theory of natural selection grew in the last twenty years of the nineteenth century the greatly increased attention to the facts and theories of heredity, which was stimulated by Darwin's hypothesis of Pangenesis and especially by Weismann's speculations about the nature and behaviour of the 'germ-plasm'. Before the appearance of Weismann's work, the germ-cells, which bear somehow or other the hereditary characters that appear in the offspring, were supposed to be produced directly from the body of the parent. Darwin provisionally suggested that every cell of every organism gives off minute particles which become congregated in the germ-cells, and that these cells thus contain representative portions of all parts of the parent's body. Weismann, on the basis of his work on the origin of the germ-cells in Medusae and Insects, maintained that these cells are not derived from the body, but only from pre-existing germ-cells stored within it--that, in fact, although an egg gives rise to a hen, a hen does not give rise to an egg, but only keeps inside her a store of embryonic eggs which mature and are laid as the time comes round. The theory had to be modified to suit the facts of regeneration and vegetative reproduction, but in essence it was accepted by the biological world and is the orthodox opinion (if such a word may be used in Science) at the present day. The difference between the two views is not only of theoretical interest, for it involves the whole question of whether characteristics acquired by an individual during its life in response to external conditions can or cannot be transmitted to offspring. If the germ-cells contain representatives of all parts of the body, modifications impressed on the body during its life may at least possibly be transmitted to offspring born after the modifications have taken place. If, however, the germ-cells are independent of the rest of the body, and only stored within it for safe-keeping like a deed-box in the vaults of a bank, it would seem impossible for any environmental influence, whether for good or ill, to take effect on the offspring.

This controversy on the heritability of 'acquired characters' was one of the most important towards the end of last century, and although the majority of biologists now follow Weismann in so far as they deny that 'acquired' characters are transmissible, the question is not yet completely settled; all that can be said is that, in spite of many attempts to prove the contrary, there is no satisfactory evidence of the transmission to offspring of effects impressed on the body of the parent, unless the germ-cells themselves have been affected by the same cause--as for example in some cases of long-continued poisoning by alcohol or similar drugs.

While the problem of the transmission of acquired characters, and of the cause of variation and its relation to evolution, was occupying much of the attention of biologists, the whole problem entered upon a new phase in the year 1900 with the re-discovery of Mendel's work on heredity.

Mendel worked with plants, and published his results in 1865, but at that time the biological world was too much occupied with the fierce controversy which raged over _The Origin of Species_ to take much notice of a paper the bearing of which upon it was not appreciated. Mendel's discovery never came to the notice of Darwin, was buried in an obscure periodical, and remained unknown until many years after the death of its author. In 1900 it was unearthed, and, largely owing to the work of Bateson, it rapidly became known as one of the most important contributions to Biology made during the period under review.

This is not the place to describe in detail the nature of Mendel's theory. Its essence is, firstly, that the various characteristics of an organism are in general inherited quite independently of one another; and, secondly, that the germ-cells of a hybrid are pure in respect of any one character, that is to say, that any one germ-cell can only transmit any unit character as it was received from one parent or the other, and not a combination of the two. This leads to a conception of the organism as something like a mosaic, in which each piece of the pattern is transmitted in inheritance independently of the rest, and in which any piece cannot be modified by a.s.sociation with a different but corresponding piece derived from another ancestor. It is impossible to say as yet whether this conception at all completely represents the nature of the living organism, but it is one which is exercising considerable influence in biological thought, and if established it will mark a revolution in Biology hardly inferior to that brought about in Physics and Chemistry by the discovery of radio-activity.

An important consequence of the advance in our knowledge of heredity a.s.sociated with the work of Mendel and his successors is a tendency to doubt whether natural selection is of such fundamental importance in shaping the course of evolution as was supposed in the years of the first enthusiasm which followed the publication of the _Origin_.

Darwin based his theory of Natural Selection on the belief which he derived from breeders of plants and animals, that the kind of variation used by them to produce new breeds was the small and apparently unimportant differences which distinguish a 'fine' from a 'poor'

specimen. He supposed that the skilled breeder picked out as parents of his stock those individuals which were slightly superior in one feature or another, and that by the acc.u.mulative effect of these successive selections not only was the breed steadily improved, but also, by divergent selection, new breeds were produced. Experience shows, however, that although this method is used to keep breeds up to the required standard, it is rarely, if ever, the means by which new breeds arise. New breeds commonly come into existence either by a 'sport' or mutation, or by crossing two already distinct races, and by selecting from among the heterogeneous descendants of the cross those individuals which show the required combination of characters. And it is further found that most of the distinguis.h.i.+ng features of various breeds of domestic animals and plants are inherited according to Mendel's Law, suggesting that each of these characters is a unit, like one piece of a mosaic, independent of the rest. Now it is easy to see how the selection of small, continuously varying characters could take place in Nature by the destruction of all those individuals which failed to reach a certain standard, but it is much more difficult to understand how natural selection could act on comparatively large, sporadic, unco-ordinated 'sports'. There is thus a distinct tendency at present to regard natural selection as less omnipotent in directing the course of evolution than was formerly supposed, but it must be admitted that no very satisfactory alternative hypothesis has been suggested. Some have supposed that there is a kind of organic momentum which causes evolution to continue in those directions in which it has already proceeded, while others have postulated, like Bergson, an _elan vital_ as a kind of directive agency.

Others again have reverted towards the older belief in the inherited effects of environment--a belief which, in spite of the arguments of Weismann and his followers, has never been without its supporters. The present condition of this part of biology, as of many others, is one of open-mindedness approaching agnosticism. There is dissatisfaction with the beliefs which satisfied the preceding generation, and which were held up almost as dogmas, but there is no clear vision of the direction in which a truer view may be sought.

Before leaving this side of the subject, reference must be made to one important aspect of modern work on heredity--that of the inheritance of 'mental and moral' characteristics. As a result of the work of the biometric school founded by Galton and Pearson, it has been shown that the so-called mental and moral characteristics of man are inherited in the same manner and to the same extent as his physical features. Of the theoretical importance of this demonstration this is not the place to speak; its practical value is unquestionable, and may in the future have important effects on sociological problems.

Another notable line of advance, entirely belonging to the period under review, and chiefly the product of the present century, is seen in the science of Cytology--the investigation of the microscopic structure of the cells of which the body is composed. The marvellous phenomena of cell and nuclear division have revealed much of the formerly unsuspected complexity of living things, while the universality of the processes shows how fundamentally alike is life in all its forms. In recent years great progress has been made in correlating the phenomena of heredity and of the determination of s.e.x with the visible structural features of the germ-cells. Weismann attempted a beginning of this over thirty years ago, but the detailed knowledge of the facts was then insufficient.

Since the discovery of Mendel's Law, a great amount of work has been done, chiefly in America, by E.B. Wilson and T.H. Morgan and their pupils, on tracing the actual physical basis of hereditary transmission.

Although the matter is far from being completely known, the results obtained make it almost indubitable that inherited characters are in some way borne by the _chromosomes_ in the nuclei of the germ-cells.

The work of Morgan and his school has shown that the actual order in which these inherited 'factors' are arranged in the chromosomes can almost certainly be demonstrated, and his results go far to support the conception of the organism, referred to above, as a combination or mosaic of independently inherited features.

It was said at the beginning of this sketch that most of the more notable lines of advance in Biology could be traced back to the impetus given by the acceptance of the theory of Evolution, and the desire to test and prove that theory in every biological field. It is most convenient, therefore, to take this root-idea as a starting-point, and to see how the various branches of study have diverged from it and have themselves branched out in various ways, and how these branches have often again become intertwined and united in the later development of the science.

Perhaps the most obvious method of testing the theory of evolution is by the study of fossil forms, and our knowledge of these has progressed enormously during the period under review. Not only have a number of new and strange types of ancient life come to light, but in some cases, e.g.

in that of the horse and elephant, a very complete series of evolutionary stages has been discovered. In this branch, however, as in almost all others, the results have not exactly fulfilled the expectations of the early enthusiasts. On the one hand, evolution has been shown to be a much more complex thing than at first seemed probable; and on the other, many of the gaps which it was most hoped to fill still remain. A number of most remarkable 'missing links' have been discovered, such as, for example, _Archaeopteryx_, the stepping-stone between the Reptiles and the Birds, and the faith of the palaeontologist in the truth of evolution is everywhere confirmed. But the hope of finding all the stages, especially in the ancestry of Man, has not been realized, and it has been found that what at one time were regarded as direct ancestors are collaterals, and that the problem of human evolution is much less simple than was once supposed.

A second important piece of evidence in favour of evolution is provided by the study of the geographical distribution of animals, on which much work was done in the earlier part of the period under review. And in this connexion mention must be made of the science of Oceanography, for our whole knowledge of life in the abysses of the ocean, and almost all that we know of the conditions of life in the sea in general, has been gained in the last fifty years.

Another of the chief lines of evidence for the truth of the evolution theory is based on the study of embryology, and this also was followed with great vigour by the zoologists of the last thirty years of the nineteenth century. It is found that in many instances animals recapitulate in their early development the stages through which their ancestors pa.s.sed in the course of evolution. Land Vertebrates, including man, have in their early embryonic life gill-clefts, heart and circulation, and in some respects skeleton and other organs of the type found in fishes, and this can only be explained on the a.s.sumption that they are descended from aquatic fish-like ancestors. On the basis of such facts as these, the theory was formulated that every animal recapitulates in ontogeny (development) the stages pa.s.sed through in its phylogeny (evolution), and great hopes were founded upon this principle of discovering the systematic position and evolutionary history of isolated and aberrant forms. In many cases the search has led to brilliant results, but, as in the case of palaeontology, in many others the light that was hoped for has not been forthcoming. For it soon became evident that the majority of animals show adaptation to their environment not only in their adult stages but also in their larval or embryonic period, and these adaptations have led to modifications of the course of development which are often so great as to mask, or obscure altogether, the ancestral structure which may once have existed.

Although, therefore, the results of embryological research have provided most convincing proof of the truth of the theory of evolution in general, they have not completely justified the hopes of the early embryologists that by this method all the outstanding phylogenetic problems might be solved.

The detailed study of embryology, however, has led to most important results apart from the particular purpose for which most of the earlier investigations in this field were originally undertaken. For the study of embryology, at first purely descriptive and comparative, was soon found to involve fundamental problems concerning the factors which control development. An egg consists of a single cell, and it develops by the division of this cell into two, then into four, eight, and so forth, until a ma.s.s of cells is produced. In some cases all these cells are to all appearance alike, or nearly alike; in others the included yolk is from the first segregated more or less completely into some cells, leaving the other cells without it. But in any case, after this process of cell-division has proceeded for a certain time, differentiation begins to set in--some cells become modified in one way, others in another, and from what was a relatively h.o.m.ogeneous ma.s.s an organized embryo, with highly differentiated parts, appears. The problem immediately propounds itself--what are the factors which control this differentiation? This problem is essentially a physiological one, and yet, since it arises most conspicuously in a field which has been worked by professed zoologists rather than physiologists, it has been studied more by those trained in zoology and botany than by those who have specialized in physiology. In this way, as in many other directions, such as in the study of heredity, of s.e.x, and of the effects of the environment on the colours and structure of animals, the trend of zoology in recent years has returned towards the physiological side, and the old division which separated the sciences (but which has never so seriously affected students of plant life) is being obliterated.

Hence we are led back to consider the progress of Physiology as a whole--a subject with which the present writer hesitates to deal except in a very superficial manner. Physiology as an organized science has inevitably been deeply influenced by its close relation with medicine, with the result that through a large portion of the period under review it has concerned itself chiefly with the functions of the human body in particular, or at least chiefly with Vertebrates from which, by a.n.a.logy, the human functions may be inferred. In this field it has made enormous progress, and a vast amount of knowledge has been gained with regard to the function and mechanism of all the parts and organs of the body. It may perhaps be suggested, however, that in the pursuit of this detailed (and in practice absolutely necessary) knowledge, physiologists have to some extent lost sight of the wood in their preoccupation with the trees. That is to say, while they have advanced an immense distance in their knowledge of organs, they have not yet got as far as might be hoped in the understanding of the organism--which is to say no more than that the great and fundamental problem of Biology, the nature and meaning of Life, is apparently almost as far from solution as ever. To this further reference will be made below.

The progress of Physiology has been so great in all its branches that it is difficult to decide which most deserve mention; perhaps the most important advances are those connected with the nervous system and with internal secretions. Little or nothing was known fifty years ago of the minute structure of the nervous system, nor of the special functions of its different parts. Now the main functions of the various parts of the brain, and the relation of these parts to the activities of the other organs of the body, are well known, although much remains to be discovered with regard to the more detailed localization of function.

The study of the microscopic structure of brain and nerve, and experiment on the conduction of nervous impulse, have given us some insight into the mechanism of the nervous system, but the fundamental nature of nervous action still remains unsolved.

The nervous system is the chief co-ordinating link between the various organs of the body, but in recent years it has been discovered that the relations of the different parts to one another are greatly influenced by substances known as internal secretions or 'hormones'. These substances are produced by ductless glands (the thyroid, suprarenals, &c.), from which they diffuse into the blood-stream and exercise a remarkable influence either on particular organs or systems, or on the body as a whole. Some of these secretions act specifically on the involuntary muscles of the body, others control growth, others the development of the secondary s.e.xual characters, such as the distinctive plumage of male birds, and also greatly influence the s.e.xual instinct.

Much still remains to be discovered with regard to them, but it seems clear that they are of immense importance in the economy of the body. It has been suggested, without much experimental support, however, that if a part of the body becomes modified by use or environment, it may produce a modified hormone, and that so, by the action of this on the germ-cells, the modification may be transmitted to subsequent generations.

Before leaving the subject of physiology in the more special or technical application of the term, reference must be made to another science the growth of which has been largely under the influence of medicine. This is bacteriology, one of the newest branches of biology, and yet one which both from its practical importance and from the theoretical interest of its discoveries is rapidly taking a foremost place. Of its practical achievements in connexion with disease, and with the part played by bacteria and other minute organisms in the life and affairs of man, it is not necessary to speak. Every one knows the great advances that have been made in recent years in identifying (and to a less extent in controlling) disease-producing organisms, whether bacteria, protozoa (such as the organisms causing malaria, dysentery, etc.), or more highly organized parasites. The attempt, however, to combat these pathogenic bacteria has led to discoveries of the highest importance with regard to the production of immunity, not only against specific germs, but against many organic poisons such as snake venom and various vegetable toxins. That an attack of certain diseases leaves the patient immune to that disease for a longer or shorter time has of course been known for centuries, but it is a modern discovery that a specific poison induces the body to produce a specific antidote which neutralizes it, and the detailed working out of this principle and the study of the means by which the immunity is brought about promise to lead us a long way towards the central problem of the nature and activities of life itself.

We have seen how zoology has been led back into physiological channels of research, and how the study of bacteria is opening up some of the deepest problems of the reaction of living things to environmental stimuli, and just as the various branches of these sciences interlace and influence one another, so all of them, in recent years, have been coming into contact with the inorganic sciences of chemistry and physics. One of the noteworthy features of science in all its branches in recent years has been the tendency of subjects which were at one time regarded as distinct to come together again and to find that the problems of each can only be successfully attacked by the co-operation of the others. In their earlier days the biological sciences were in most respects far removed from chemistry and physics; it was recognized, of course, that organisms were in one sense at least physico-chemical mechanisms, consisting of chemical elements and subject to the fundamental laws of matter and energy. With the advent of the theory of evolution this conception of the organism as a mechanism took more definite shape, and among many biologists the belief was held that in no very long time all the phenomena of life would be explicable by known physico-chemical laws. Hence arose the scientific materialism which was so widespread in the years following the general acceptance of Darwin's theory. It was recognized, of course, that our knowledge of organic chemistry was at the time entirely inadequate to place this belief upon a proved scientific basis, but the expectation of proving it gave a great impetus to the study of the physical and chemical phenomena of life. This attempt was still further stimulated by the investigation of the factors controlling development referred to in a preceding paragraph, for it is evident that to a great extent at least these factors are chemical and physical in nature. And concurrently, the great advances in organic chemistry, resulting in the a.n.a.lysis and in many cases in the artificial synthesis of substances previously regarded as capable of production only in the tissues of living organisms, made possible a much more thorough investigation of the chemical and physical basis of vital phenomena. The result of this has been that to a quite considerable extent the factors, hitherto mysterious, which control the fertilization, division, and differentiation of the egg, the digestion and absorption of food, the conduction of nervous impulses, and many of the changes undergone in the normal or pathological functioning of the organs and tissues, can be ascribed to chemical and physical causes which are well known in the inorganic world.

As in other instances, however, some of which have been mentioned above, the elucidation of the organism from this point of view has turned out to be a much less simple process than the more sanguine of the early investigators supposed. The more knowledge has progressed, the more complex and intricate has even the simplest organism shown itself to be, and although the mechanism of the parts is gradually becoming understood, the fundamental mystery of life remains as elusive as ever.

The chief reason for this failure to penetrate appreciably nearer to the central mystery of life appears to be the fact that an organism is something more than the sum of its various parts and functions. In tracing the behaviour of any one part or function, whether it be the conduction of a nervous impulse, the supply of oxygen to the tissues by the blood, or the transmission of inherited characters by the germ-cells, we may be able to give a more or less complete physico-chemical or mechanical account of the process. But we seem to get little or no nearer to an explanation of the fact that although every one of these processes may be explicable by laws familiar in the non-living, in the living organism they are co-ordinated in such a way that none of them is complete in itself; they are parts of a whole, but the whole is not simply a sum of its parts, but is in itself a unity, in which all the parts are subject to the controlling influence of the whole. An organism, alone among the material bodies which we know, is constantly and necessarily in a state of unstable equilibrium, and yet has a condition of _normality_ which is maintained by the harmonious interaction of all its parts. Every function of the body, if not thus co-ordinated with the rest, would very quickly destroy this condition of normality, but in consequence of the co-ordination each is subject to the needs of the whole, and normality is maintained. When the normality is artificially disturbed, all the functions of the body adapt themselves to the change, and, if the disturbance be not too great, co-operate in the restoration of the normal condition. It is in these phenomena of adaptation and organic unity and co-ordination that up to the present time the efforts to reduce the phenomena of living things to the operation of physico-chemical laws have most conspicuously failed.

From what has been said it will be evident that, fundamentally, all biological research, whether its authors are conscious of it or not, is directed towards the solution of one central problem--the problem of the real and ultimate nature of life. And the main outcome of the work of sixty years has been that this problem has begun clearly to emerge as the central aim of the science. The theory of evolution made the problem a reality, for without evolution the mystery of life must for ever be insoluble, but whatever direction biological investigation has taken, it has led, often by devious paths, to the borderland between the living and the inorganic, and in that borderland the central problem inevitably faces us.

Many suggestions for its solution have been made. On the one hand there is still, as there always has been, a considerable body of opinion that the solution will be a mechanical one--using the word mechanical in the widest sense--and that the living differs from the non-living not in kind, but only in degree of complexity. The upholders of the mechanistic or materialist theory, however, are perhaps less confident than their predecessors of the last century, for the solution in this direction has to face not only the problem of organic co-ordination already referred to, but also that of consciousness and mind. For although the study of psychology on physiological lines has made similar progress to that of other branches of physiology, it seems to approach little nearer to a discovery of the nature of the relation between consciousness in its various aspects and the material body with which it is a.s.sociated.

So long as this gulf remains unbridged, the possibility of a satisfactory mechanistic explanation of life seems far away.

On the other hand, there has been a revival of the ancient tendency towards what is called a vitalistic solution. A certain number of biologists, impressed by the apparent similarity between the control and co-ordination exercised by the organism over its functions and the conscious control of voluntary activity with which we are familiar in ourselves, have supposed that these things are not merely superficially similar but have a real and fundamental affinity. This does not mean that organic control is always conscious, but that there is a controlling ent.i.ty, non-material in nature, which is similar in kind to the 'ego' of a self-conscious human being. They suppose that the organism is not simply material, but is a material mechanism controlled by a non-material ent.i.ty the nature of which is more akin to what we mean by the word spirit than anything else of which we are accustomed to think. They are in fact dualists, and divide reality into the material and spatial on the one hand, and non-material principle or ent.i.ty which may fairly be called spiritual on the other.

And, in the third place, there are those who seek a solution which denies the truth of both the preceding, and which is metaphysically idealist or monist in character. To them, if the present writer understands their att.i.tude, matter and spirit are different aspects of one reality. In the inorganic and non-living, phenomena appear which are generalized under the laws of physics and chemistry, but the phenomena of life fall into a different category which includes the conception of co-ordination or individuality, while a still higher category is required to include the phenomena of consciousness and mind.

It is evident from this brief review that Biology in the period considered has pa.s.sed through three main stages. The first of these was the acceptance of a new illuminating and unifying idea, which led to enthusiastic research in many directions for the purpose of proving and amplifying it. Very rapidly new facts, or new interpretations of facts already known, were shown to fall into line, and the evolution theory became converted from a hypothesis into something approaching a dogma.

Not only the idea of organic evolution itself, but all the current beliefs about the method of evolution, and the larger speculations to which it gave rise, were widely regarded as almost indisputable, and where difficulties and inconsistencies appeared, these were supposed to be due solely to the insufficiency of our knowledge, which would soon be remedied. Then, however, as detailed knowledge increased, the voice of criticism and doubt was more frequently heard. The various branches of Biology began once more to overlap, and to join hands with chemistry and physics, and it became clear that the interpretation of life was very far from being a simple problem. And so, as with the Atomic Theory in chemistry, the present position is one of dissolution of the older ideas and of hesitation to express a fixed belief, for while Biology has a clearer vision of the problem before it than ever it had, its wider knowledge reveals the fact that the problem is far from being solved.

Perhaps one of the chief results of the great increase of knowledge during the past sixty years has been to show us the immensity of the field still remaining to be explored.

FOR REFERENCE

Centenary volume on Darwin (Cambridge University Press).

X

ART

A. CLUTTON-BROCK

My subject is art and thought about art. I deal with aesthetics only so far as they concern art, that is to say I shall not attempt any purely philosophic speculations about the nature of art and I shall speak of the speculations of others, such as Croce and Tolstoy, only so far as they seem to me likely to have a practical effect upon art. My subject is the art of to-day and our ideas about it. We are beginning at last to connect aesthetics with our own experience of art and to see that our beliefs about the nature and value of art will affect the art we produce. Hence a new aesthetic is very slowly appearing; but I have to confess it has not yet appeared.