Читать книгу The Biological Problem of To-day: Preformation Or Epigenesis? - Oscar Hertwig - Страница 6

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What is development? Does it imply preformation or epigenesis? This perplexing question of biology has reappeared recently as a problem of the day. Of late years there have been set forth contradictory doctrines, each seeking to explain the process by which the fertilised egg-cell, an apparently simple beginning, gives rise to the adult organism, which often is exceedingly complicated, and which has the capacity of producing new beginnings like that from which it itself arose.

The opposing views of to-day were in existence centuries ago, and they are known in the history of science as the theory of preformation or evolution, and the theory of epigenesis. That most of the great biologists of the seventeenth and eighteenth centuries were decided upholders of evolution was the natural result of the contemporary knowledge of facts. For they knew only the external signs of the process of development. All they saw was the embryo becoming adult, the bud growing out into a blossom, as the result of a process in which nutrition transformed smaller to greater parts. And so they regarded development as a simple process of growth resulting from nutrition. Their mental picture of the germ or beginning of an organism was an exceedingly reduced image of the organism, an image requiring for its development nothing but nutrition and growth. That the material eye failed to recognise the miniature they attributed to the imperfection of our senses, and to the extreme minuteness and resulting opacity of the object.

That it might satisfy our human craving for final causes, the theory of preformation had to be accompanied by a corresponding explanation of the origin of the miniatures. Biologists had already abandoned the error of such spontaneous generation as the origin of flies from decaying meat, and, in its place, had accepted the doctrine of the continuity of life, formulating it in the phrase, Omne vivum e vivo (Each life from a life), and in the similar phrase, Omne vivum ex ovo (Each life from an egg). One creature issued from another, within which it had lain as a germ, and the series was continuous. Thus, the theory of preformation gave rise to the conception that living things were a series of cases or wrappings, germ folded within germ. The origin of life was relegated to the beginning, at the creation of the world: it became the work of a supernatural Creator, who, when He formed the first creatures, formed with them, and placed within them, the germs of all subsequent creatures.

To reckon at their proper value the theory of preformation, and, still more, the doctrine of enfolded germs, the standard of appreciation must not be the present range of our knowledge. They must be viewed historically, in the light of the knowledge of these days.

Nowadays it is not so much pure reason as a wider empirical knowledge of nature, with its consequent transformation of ideas, that makes the doctrine of enfoldment difficult. Abstract thought sets no limit to smallness or greatness; for mathematics deals with the infinitely small and with the infinitely great. So long as actual observation had not determined the limits of minuteness in the cases in question, there were no logical difficulties in the doctrine of enfolded germs. The biology of earlier centuries had not our empirical standard. What appeared then to be a simple organic material we have resolved into millions of cells, themselves consisting of different chemical materials. The chemical materials have been analysed into their elements, and chemistry and physics have determined the dimensions of the ultimate molecules of these. It is only because the minute constitution of matter is no longer a secret to us that the theory of germ within germ now touches the absurd.

It was very different in earlier days; the acutest biologists and philosophers were evolutionists, and an epigenetic conception of the process of development could find no foothold alongside the apparent logical consistency of the theory of preformation.

Wolff's Theoria Generationis (1759) failed to convince his contemporaries, because he could bring against the closed system of the evolutionists only isolated observations, and these doubtful of interpretation; and because, in his time, on account of the rudimentary state of the methods of research in biology, men attached more importance to abstract reasoning than to observation. His effort was the more praiseworthy in that it was observation bearing witness against abstract and dogmatic conceptions. By means of actual observation he tried to expose the fallacy in preformation, to show that the organism was not fully formed in the germ, but that all development proceeded by new formation, or epigenesis; that the germ consisted of unorganised organic material, which became formed or organised only little by little in the course of its development, and that Nature really was able to produce an organism from an unorganised material simply by her inherent forces.

It is interesting to display the essential contrast between preformation and epigenesis in the poetical words of Wolff himself. 'You must remember,' so run his words in the second argument against the probability of preformation, 'that an evolution would be a phenomenon formed in its real essence by God at the Creation, but created in condition invisible, and so as to remain invisible for long before it would become visible. See, then, that a phenomenon of enfolding is a miracle, differing from ordinary miracles only in these: first, it was at the creation of the world that God produced it; second, it remained invisible for long before it became visible. In truth, therefore, all organic bodies would be miracles. Would not this change for us the presence of Nature? Would it not spoil her of her beauty? Hitherto we had a living Nature, displaying endless changes by her own forces. Now it would be a fabric displaying change in seeming only, in truth and essence remaining unchanged and as it was constructed, save that it gradually becomes more and more used up. Formerly it was a Nature destroying herself and creating herself anew, only that endless changes might become visible and new sides be brought to light. Now it would be a lifeless mass shedding off piece after piece until the stock should come to an end.'

None the less, who seeks in Wolff's 'Theoria Generationis' an account of the means or forces by which Nature builds up organic forms will seek in vain. The vis essentialis (inherent force) with which Wolff endowed his plastic organic material, or the nisus formativus (formative force), afterwards suggested to science by Blumenbach—what are they but empty words by which men seek to grasp in thought what has eluded them? Wolff's epigenesis was not a complete explanation—indeed, from its fundamental conception it could not possibly be such. For investigation of the natural forces by which development proceeds can advance only slowly and step by step, and for long will constitute the foremost task of biology. The prosecution of biological investigation will continuously endow the theory of epigenesis with a fuller and fuller meaning, but will never transform it into a solution final in the sense of the theory of preformation.

It seems to me that the significance of Wolff's doctrine lies in this: it rejected the purely formal theory of preformation because actual observations were against it. Thereby Wolff freed research from the straitened bonds of prejudice, and entered the only possible path by which science can advance—the path along which the biology of our century has made so great advances.

Biologists of to-day approach the problem of organic development equipped with incomparably greater knowledge and with more delicate methods of research. But in our thoughts to-day, as we discuss the essential nature of the process of organic development and the mutual causal relations between rudiments and their products, the same contradictory views are present, altered only as our methods of expression have altered.

In a striking fashion Roux[1] has contrasted the opposing ideas inherent in our modern conception of development, but yet identical with those which formerly found expression in the theories of preformation and epigenesis.

By the term "embryonic development," in its ordinary acceptation, we understand the appearance of visible complexity. But when we speak of the visibility of the resulting complexity, we use a subjective term, the value of which is relative to the human eye. Going further into the matter, we must break up the conception into two parts, and distinguish between the actual production of complexity and the mere transformation of complexity from a condition invisible to us into complexity visible to our senses.

'The two kinds of development I have indicated bear a relation to each other that recalls the old opposing doctrines of preformation and epigenesis, the alternatives of a time when it was a task—perhaps the only possible task—to record the completed results of the stages in development as they became complete—in fact, to record the externally visible changes of shape. In this descriptive investigation of the development of external form, epigenesis, the successive formation of new shapes, gained a complete victory over evolution, the mere becoming visible of pre-existing details of shape.

'The closer investigation of embryonic development that is necessary in a search for causes brings us once more against the old alternatives, and compels us to a closer scrutiny of them.

'In this, if we still retain the old terms, epigenesis would mean not merely the building up of complicated form through the agency of a substratum, apparently simple, but perhaps with an extraordinarily complicated, minute structure, but, in the strictest sense of the term, the new formation of complexity, an actual increase of complexity. Evolution, on the other hand, would imply the mere becoming visible of pre-existing latent differentiation. Clearly, according to these general definitions, occurrences which outwardly exhibit epigenesis may be in reality partial or complete evolution. In fact, the deepest consideration leads us again to the original question: Is embryonic development epigenesis or evolution? Is it the new formation of complexity, or is it the becoming visible of complexity previously invisible to us?'

Thus, in our own days, after the controversy has been at rest for long, biologists are assembled in opposing groups, one under the standard of epigenesis, another under that of preformation.

Weismann[2] leads the van for preformation; for the last ten years he has occupied himself with the theoretical discussion of the questions set forth above; and now, in a recent treatise, The Germplasm, he has combined his views, already many times modified, in a coherent theory. Now he explains candidly that he has been driven to the view that epigenetic development does not exist. 'In the first chapter of my book,' he remarks, 'will be found an actual proof of the reality of evolution, a proof so simple and obvious that I can scarcely understand to-day how it could have escaped my notice so long' (Germplasm, p. 14). Elsewhere he writes: 'I believe that I have established that ontogeny can be explained only by evolution, and not by epigenesis.'

A mental process, which consciously or unconsciously plays a great part with evolutionists, and helps to determine their conclusions, is characteristic of the direction of their inquiries. They set out from the fact that the characters of the parents, often to the smallest detail, are transmitted to children by means of the germ or rudiment; they conclude that the active causes of all the complexity that arises must be contained in the apparently homogeneous germ, embryological differentiation being a spontaneous process. It follows that the apparent homogeneity is, in reality, latent complexity which becomes patent during the progress of ontogeny. Latent complexity implies a material substratum, consisting of actual particles for which many different names have been found. As our senses can give us no experimental knowledge of these particles, which are so small as to be invisible, modern evolutionists attempt to picture them, in imagination, by reflecting all the visible characters of the perfected organism upon the undivided egg-cell, so peopling that globule of yolk with a system of minute particles corresponding in quality and in spacial arrangement with the larger parts of the adult.

Weismann has practised this art in the true spirit of a virtuoso, and has elaborated it into a novel mode of biological investigation. Take an example;—'It would be impossible,' he says in The Germplasm (p. 138), 'for any small portion of the human skin to undergo a hereditary and independent change from the germ onwards, unless a small vital element corresponding to this particular part of the skin existed in the germ substance, a variation in this element causing a corresponding variation in the part concerned. Were this not the case, birth-marks would not exist.'

Thus, in a slightly altered fashion, we come again to the position of the evolutionists of last century, for whom the germ was an extremely small miniature of the adult creature. The new evolution, as Weismann in especial has established it, seems to me to differ from the old doctrine only in two important points; and these must be placed to the credit of the greater scientific knowledge of our century. The first point concerns the relative positions of the parts in the patent and latent conditions. The older evolutionists assumed that these were identical, that the germ was a true miniature. It is true that Weismann regards his almost countless germinal particles as being held together in an architectural structure of almost inconceivable complexity. For him the germ is an exceedingly complicated living being, a microcosm in the truest sense, in which every independently variable part that ever appears throughout the whole life is represented by a living particle, and in which each of the living particles is endowed with a definite, inherited position, a constitution, and the power of rapid multiplication. It is upon the qualities of these ultimate particles that he makes depend the qualities of the corresponding parts of the adult, the parts that are cells as well as the parts built of many cells. As, however, during visible development the parts of the embryo undergo many changes of position and metamorphoses, Weismann is compelled to make the assumption that the germ, as a micro-organism, is not simply a miniature of the adult, but that its minute particles have an arrangement totally different from that of the corresponding parts in the adult organism.

The second point is the origin of each new generation. To explain the continuity of development, the old evolutionists held that the generations lay enfolded one within another. Weismann avoids this difficulty by endowing his germs with divisibility, but he gives us no proof that division could possibly take place in the case of structures composed of innumerable particles built up into a definite and most complicated architectural system.

Although the new evolution differs from the old in the points mentioned above, the two theories obviously agree very closely in the nature of their arguments and conclusions. When, to satisfy our craving for causality, biologists transform the visible complexity of the adult organism into a latent complexity of the germ, and try to express this by imaginary tokens, by minute and complicated particles cohering into a system, they are making a phantasmal image which, indeed, apparently may satisfy the craving for causality (to satisfy which it was invented), but which eludes the control of concrete thought, by dealing with a complexity that is latent, and perhaps only imaginary. Thus, craftily, they prepare for our craving after causality a slumbrous pillow, in the manner of the philosophers who would refer the creation of the world to a supernatural principle.

But their pillow of sleep is dangerous for biological research; he who builds such castles in the air easily mistakes his imaginary bricks, invented to explain the complexity, for real stones. He entangles himself in the cobwebs of his own thoughts, which seem to him so logical, that finally he trusts the labour of his mind more than Nature herself.

'Experiment,' says Weismann in The Germplasm, 'is not the only way to reach general views, nor is it always the safest means of discrimination, although at first it seems conclusive....[3] It seems to me that in this case we can draw more prudent conclusions from the general facts of inheritance than from the results of experiments that are neither quite clear nor undubious, although in themselves they are most valuable, and deserve the most careful consideration. If one remembers what was said in my section on the architecture of the germplasm as the basis of the theory of determinants, it will be agreed with me that ontogeny must find its explanation in evolution, and not in epigenesis.'[4]

I take up a more epigenetic position, and years ago I attacked evolutionary doctrines in many of their modifications.[5] Thus, in the Studien zur Blätter Theorie, published by Richard Hertwig and myself, I combated the supposed law that the germinal layers histologically were primitive organs. Next, in a pamphlet entitled The Problem of Fertilisation: a Theory of Heredity, I attempted to disprove the principle of His that there were organ-building foci in the germ. In my treatise On Ovogenesis and Spermatogenesis in the Nematodes, I declared against the suppositions involved in Weismann's doctrine of the germplasm, and sharply distinguished the theory, simultaneously propounded by Strasburger and myself, that the nucleus is the bearer of the hereditary material, from the evolutionistic interpretation given it by Weismann.

A paper on 'The Blastopore and Spina Bifida,' and an occasional lecture on 'Old and New Theories of Development,' gave me the opportunity of dealing with Roux's mosaic theory, although that not only shows learning, but apparently is the outcome of experiment. I advocated in its place the theory that 'the embryological development of an organism is no mosaic work. The parts of an organism develop in relation to each other, the development of a part depending upon the development of the whole.' The labours of Roux, as well as the valuable researches of Driesch, induced me to carry out a series of experiments with the object of getting a surer basis for my epigenetic conception of development. The results of these were published recently under the title, On the Value of the First Cleavage-cells in the Formation of the Organs of Embryos.

In the latter treatise I confined myself advisedly to the exposition and interpretation of the results of my investigations, having in view a subsequent discussion of the more theoretical bearings of my results. It is this that sees the light in the present book.

As for many years I have occupied myself with the problem of development, pursuing observation and framing theory, there is due to myself and to others an exposition of the position I have assumed in many of my treatises, but in a more connected and elaborated fashion than has been possible hitherto. This course is the more imperative, as in his recent magnum opus on the germplasm Weismann has propounded a theory of evolution wrought with the greatest care and acuteness, and totally irreconcilable with my conclusions. The chief differences between my views and those of Weismann have now become clearer and more tangible than ever. It is true that in my text-book, On the Structure and Function of Cells,[6] published in the autumn of 1892, I gave a short account of my theory of heredity in chapter ix., 'The Cell as the Material Beginning of the Organism.' But in that I could not deal with Weismann's work, which appeared simultaneously, and, moreover, in a text-book it was impossible to do more than sketch my views.

My present task is twofold; it has both a positive and a negative side. First, I have to examine the arguments recently alleged in favour of the theory of preformation, testing them to reveal their inherent weaknesses, and to controvert their fallacies. As Weismann unquestionably is the chief of those who have advocated preformation, and has made a closed system of it again, it is necessary for me to take special notice of his conception as it is set forth in The Germplasm. Although I am no friend of polemic, the case demands it. For the decision of a question so momentous as the relative scopes of evolution and epigenesis in embryology must have an important bearing on the future of biology, upon its aim and the method of research.

But criticism of Weismann's hypothesis is not to be an end in itself; I am more anxious to show the lines upon which, as I think, the real meaning of the process of organic development will come to be learned. In a second section, therefore, I shall explain my own views in greater detail, and, as I hope, place them on a firmer foundation than formerly was possible.