Recent Developments in European Thought

by Unknown

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 passed 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 assumption 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 passed 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 mass 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 homogeneous mass 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 sex, 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 analogy, 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 sexual characters, such as the distinctive plumage of male birds, and also greatly influence the sexual 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 analysis 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.