INTRODUCTORY T HE general problem of development is without question one of the most difficult and intriguing in the whole field of knowledge. That from a minute germ of relatively simple structure there should be gradually built up, by a series of processes beautifully co-ordinated in space and time, the complex organization of the adult is a fact that has never ceased to excite the wonder of mankind. It has provided a constant challenge to the intellect of man, and many and various have been the theories invented to explain it. It ranks as one of the major problems of biology.
A convenient classification of biological problems is that which divides them into problems of the individual and problems of diversity. The problems of the second group relate to the differences between organic forms'and the mode of origin of these differences; they comprise such problems as the origin of species, and generally the factors and causes of organic evolution. The problems of the individual are those which are common to all living things and may be studied in any individual whatsoever. They relate to such organic activities as behaviour, general physiological functioning, reproduction, and development.
I propose in this book to discuss the general interpretation of development, heredity, and reproduction, considered as essential and fundamental functions common to all living things. No attempt will therefore be made to deal with the specific modalities of these functions. The discussion will relate almost exclusively to the development and reproduction of animals. What is the best way of envisaging the problems of development and reproduction; which will give us the deepest insight into them, and enable us to understand them most fully and adequately ? This is the principal question with which we shall deal. Particular attention will accordingly be paid to questions of method, that is to say to the various ways in which the problems may be and have been
approached, and to the fundamental assumptions and modes of thought which lead to these various lines of approach.
It will be necessary to treat of the matter to some extent historically, in order to be able to envisage modern theories in their proper perspective, to understand their mode of origin, and generally to follow out the filiation of ideas. We shall find that, in spite of the vast accumulation of detailed knowledge, which is, in some quarters, supposed by itself to constitute science, there is much less difference than one would expect between the fundamental hypotheses or modes of explanation adopted, say, by the Greeks and those in vogue at the present day. This is because there are — apparently — only one or two possible ways of interpreting development open to the human intelligence, and these few alternative methods tend to recur again and again throughout the whole history of biological science. One is accordingly forced to the conclusion that on its constructive or theoretical side biology (and perhaps the other sciences as well) is by no means a simple transcript of fact, but in large measure a construction of the mind, a conceptual edifice, the lines and plan of which may vary according to the type of mind of its architect. The relative parts played by fact and hypothesis in the building up of biology would make a fascinating subject for detailed treatment; we shall deal with it to some extent in this book, apropos of theories of development.
We shall find in the course of our historical survey of the main types of theory regarding development and heredity 1 that one cardinal problem of method continually arises, namely, the relation of the parts to the whole. Is development to be treated as essentially an activity of the organism as a whole, or can its full explanation be found by analysing the process into its constituent elements ? Is heredity essentially the reappearance and realization of the functional potentialities of the whole, or are the separate characters of the organism transmitted piecemeal, being represented separately
1 We regard heredity as one of the main characteristics of development — see below, p. S.
in material form in the germ ? Are development and heredity functions of the organism as a whole, or functions of its cells, or of still smaller constituent units ? In general, is the organism a real unity or individual, not completely reducible to its constituents, or is it a mere composite, built up as a hierarchy of independent units ? Can the whole be fully explained in terms of its parts, or must the parts ultimately be explained in terms of the whole ? This fundamental question, which may be thus variously and somewhat loosely formulated, is one which will occupy us throughout the book. We shall find that according to the answer given to this question theories may be classified as ‘unity’ theories or ‘particulate’ theories. We shall find the antithesis between the two views expressed quite clearly by Aristotle in his criticism of the pangenesis theory of Hippocrates and Democritus. It recurs again in the disputes between the preformationists and the epigenesists in the seventeenth and eighteenth centuries, and the battle continues even to the present day.
After our historical sketch of typical theories we shall accordingly devote some time to elucidating the contrast between these two main types of theory, to laying bare the fundamental premisses on which they are respectively based. We shall here come to grips with the fundamental problem of the relation of the parts to the whole, and with the use and misuse of the method of abstraction.
We shall find reason to conclude that although the analytic- method has added greatly to our knowledge of heredity and development and indeed remains indispensable, it must be corrected and supplemented by the integrative view which is essential for the full understanding of these processes. Accordingly, after this general discussion of the two main types of theory, we shall attempt to work out and justify a particular form of the integrative view, which may best be described as the ‘organismal’ 1 theory of the living thing. This is a development (and I hope an improvement) of certain views on biological method which I have already
1 The word is borrowed from Ritter; see p. 176 below.
published. It is claimed for it that it gives a less abstract and schematic account of the living thing than that offered by the mechanistic conception, with which it is contrasted, and that it opens up the possibility of a real and autonomous biology.
The remainder of the book is devoted to testing, and illustrating the use of, this organismal method, by applying it to the study of the very earliest stages of development — with particular and critical reference to the cell-theory and the chromosome-theory — and to the general interpretation of reproduction, both sexual and asexual. No attempt is made to deal in terms of the organismal theory with the special problems of development and reproduction ; the great wealth of fact disclosed by the experimental work of the last few decades on the physiology of development 1 has been left for the most part untouched, though I am convinced that the application of the organismal conception to these results would be fruitful and illuminating. My primary object has been to describe a point of view, to sketch a method of approach, and to indicate the general conception of development and reproduction to which it leads. This is outlined in the final chapter, which draws together the threads of the argument which run through the whole book.
As a prelude to our historical survey of theories (Chapters II— VIII), it is proposed to enumerate here the main characteristics of development and reproduction. There is a certain advantage in doing this, even in a crude and elementary way, for we can only judge of the success or adequacy of a theory if we bear in mind the main and outstanding facts for which that theory must account. Furthermore, in these days of specialization one is very apt to forget or overlook the broader and simpler aspects of a complex phenomenon like development; it is worth while then to correct our too microscopic vision by taking a bird’s-eye view of the problems as a whole.
1 A useful summary of an important section of these results is given by G. R. de Beer, ‘The Mechanics of Vertebrate Development*, Biological Reviews , ii, 2, pp. 137-97, Cambridge, 1927.
(1) The most general characteristic of development is clearly the fact of increasing differentiation of structure and function. Development is essentially a progression from a simpler to a more complex organization, with a concurrent s pecialization and limitation of function. T his holds good botlTwith unicellular and with multicellular organisms.
Differentiation does not mean quite the same thing applied to structure as it does applied to function. There is formation of new structure during development, an actual increase in material complexity, bringing into being structures not there before. There is no such new formation of function, for all essential functions are present already in the living egg, though in an undeveloped state. The egg breathes, absorbs nourishment, grows, excretes waste products, exhibits irritability and movement of its parts ; the grown organism does nothing essentially different, exception made of the function of reproduction. What happens in the differentiation of function is not new creation, but the specialization and intensification of powers already present, and their localization in special organs which permit of their full development. With this specialization of function there necessarily goes hand in hand a limitation of function, according to the principle of the division of labour.
Furthermore, the relation of function to structure differs at different periods of development, and we may follow W. Roux (see below, p. 106) in distinguishing in many forms a first period, in which the parts and organs are differentiated in advance of, and as it were, in preparation for, functioning, and a second period in which differentiation is dependent on functioning, in which functional activity, if it does not create, at least shapes and polishes the organ. We may distinguish then non-functional differentiation from functional differentiation.
While development is normally an increasing differentiation — a formation of new organized structure and an accentuation and localization of functions already present — there occurs under special conditions and in special cases the reverse process of dedifferentiation, as in the simplification of
structure shown by a starved Planarian or an encysted Ciliate. So, too, retrograde or degenerative development may follow initial differentiation, as in many sessile and parasitic groups. Again, the process of differentiation may restart after a phase of dedifferentiation. It is convenient then to distinguish as a first group of characteristics, differentiation, dedifferentiation, and redifferentiation, and differentiation we may subdivide into functional and nonfunctional.
(2) Differentiation is essentially a formation of organized structure, and an unfolding of co-ordinated functional activity. There is therefore throughout the whole course of development a unity or harmony , which expresses itself in the spatio-temporal co-ordination of its processes. The organism developes essentially as a whole, as a unitary individual, persisting in time. There is an orderly succession of events in ontogeny, one stage leading to the next, the whole series showing a deep-seated rhythm which it is very difficult to upset or reverse. At every cross-section of this spatiotemporal ‘event’, which is development, the organism is a complete and co-ordinated whole.
We shall see later that this fact of the unity, in a spatiotemporal sense, of the developing organism constitutes not so much a problem as a postulate. A continuing unity is part of the concept of organism at which we shall later arrive. It may be possible to investigate and explain the ways in which this fundamental unity of the developing organism is maintained and restored, but the fact itself is not susceptible of complete explanation, and must be accepted as an irreducible postulate of biology. It is in discussing the problem of the relation of the parts to the whole that we shall discover the inevitability of this postulate.
(3) Another striking characteristic of the developing organism is its autonomy. By this I mean its relative independence of environment, its self-containedness, its steady persistence towards the goal of the finished form. The developing organism acts as if it were fulfilling an end or purpose — that of arriving at the typical form and modes of
activity of the species; it tends towards this goal in spite of difficulties, and the end is more constant than the way of attaining it. The environment supplies the conditions for development, provides the means, and also acts as a limiting factor, but the developing organism reaches its definitive form as it were in spite of environment, utilizing environment where it can, and seeking other conditions when the environment becomes unfavourable to its development. It is impossible by experimental means to alter substantially the specificity of development ; an embryo will, if the conditions are possible at all, reproduce its typical form, or as much of it as it can. Absence of essential environmental factors may prevent development altogether, or lead to the production of monstrous and non-viable forms, but alteration of environmental conditions will not produce an essentially different embryo.
The word autonomy well expresses this characteristic of development, for the embryo in a very real sense embodies and fulfils the law of its own being.
With this character of autonomy one may associate the allied characteristic of regulation. If the conditions do not permit of a straightforward normal development, if for instance the developing organism suffers deformation or loss of parts, it has to a considerable degree the power of so modifying the course of its development as to cope with the unusual situation, replacing, for example, the missing parts. Innumerable examples could be given of this remarkable power possessed by the developing organism of so regulating its structure and activities as to counteract adverse influences and arrive at an approximately normal result; the phenomena of regeneration occur naturally to the mind.
(4) Not only does the developing organism arrive at a typical form of organization if development is possible at all, but this typical form is an amazingly exact replica of the form of its parent or parents. This is the fact of heredity.
The broad fact of repetition of type has tended of recent years to become lost from sight, because of the excessive attention paid to the laws of transmission of such slight
variations or differences as are no bar to successful interbreeding; the study of heredity has come to mean in practice the study of the modes of inheritance of minor differences.] But clearly there is this major problem which is practically untouched by genetic or statistical studies, and equally clearly, repetition of type must be regarded as one of the main characteristics of development, not as a separate and independent problem.
We shall accordingly treat of heredity as being primarily a feature of development ; it must, however, be given almost undue prominence in our historical survey, on account of the methodological interest which attaches to the many theories purporting to explain it.
We note in passing that hereditary resemblance applies throughout the course of development ; the cycle of changes undergone by the offspring is the same as it was for the parents; there is a point-for-point correspondence between equivalent stages in the development of both.
(5) Not only is the developing organism compelled to retrace step by step the course of development followed by its immediate ancestors, but it is influenced by the evolutionary history of its race, so that its development repeats in a certain measure, and in a general way, the evolution of its race. This is the fact of recapitulation. It has been denied by some, and there is in fact considerable uncertainty in many cases as to how far interpretation in terms of ancestral history is valid, but one has only to think of such striking examples as the transitory appearance and subsequent transformation of the gill-slits and gill-arches in Amniota to realize that recapitulation does take place, and that it must be reckoned as a general feature of development. It accounts for the curiously indirect course which development frequently takes.
Much of the uncertainty as to the extent and significance of recapitulatory processes in development arises from the circumstance that we do not know, except in relatively few cases, the actual evolutionary history of any group, and we are thus thrown back upon probabilities. The theory of
recapitulation has in the past been employed much too freely as a means of deducing probable lines of descent, and such speculations have tended to bring the theory into disrepute. But recapitulation remains all the same an important characteristic of development, and as such requires explanation.
Repetition of type and recapitulation have both to do with the all-important historical aspect of development. The organism is above all a historical being; in some way, which we do not as yet clearly understand, the past experience of the race lives on in the present activities of the individual; nowhere is this more clearly exemplified than in the course of individual development.
The above may be regarded as the main characteristics of development, which every comprehensive theory must take into account and explain if it can. There are others which might be mentioned, as for instance the fact of sexual differentiation, which is so widespread in the organic realm. A complete theory of development would have to take into account also the life-cycle as a whole, with its successive phases of youth, maturity, and old age, and not limit itself merely to the phenomena of embryonic development.
In this connexion the preparations which the organism makes for reproduction would require particular study — including the investigation of the later developmental processes leading to maturity, the seasonal and cyclical changes in structure and behaviour associated with reproduction, and the formation, growth, and ripening of the germ-cells.
Though good work has been done from the physiological side, reproduction has ceased to be taken seriously as a primary biological problem, ever since the general acceptance of the germ-plasm theory. In particular, the relation between sexual and asexual reproduction has rarely of late been considered with any thoroughness, nor the rationale of the long process of ripening which the germ-cells undergo. Yet reproduction is one of the master-functions of the organism, in a sense the crown and completion of individual development, and its influence on behaviour is enormous. We shall
accordingly in the last chapter devote some attention to outlining, as well as a theory of early development, the general theory of reproduction to which the organismal conception of the living thing naturally leads.
In the historical chapters which follow no attempt will be made to give a complete survey of theories; this has already been done in a thoroughly adequate way by Delage; 1 we shall consider only certain typical theories, which present points of special methodological interest.
1 Yves Delage, La Structure du Protoplasma et les Theories sur I'Heredite et les grands Problcmes de la Biologie generalc i Paris, 1895, 2nd edition, 1903.