The Biological Problem of To-day: Preformation Or Epigenesis?. Oscar Hertwig

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.