This certainly marked considerable progress, for it meant the beginning of a science of embryology, that is, the science of the form-development of the animal or plant from the ovum. The result was not so important in its theoretical aspect, for though the knowledge had been gained that the young animal goes through a long series of different stages, it had not been discovered how nature works this wonder and causes an animal of complex structure to arise from the apparently simple substance of the ovum. A solution of the difficulty was found by attributing to the ovum a formative power, afterwards called by Blumenbach the nisus formativus, which possessed the capacity of developing a complex animal from the simple 'slime,' or, as we should say, the simple protoplasm.
If we contrast the strictly theoretical part of the two theories, we find that Bonnet regarded the ovum as something only apparently simple, but in reality almost as complex as the animal which developed from it, and that he thought of the latter, not as being formed anew, but as being unfolded or evolved. That is to say, he thought that rudiments present from the outset in the ovum gradually revealed themselves and became visible. Wolff, on the other hand, regarded the ovum as being what it seemed, something quite simple, out of which only the nisus formativus could, by a series of transformations and new formations, build up a new organism of the relevant species.
Wolff's Epigenesis routed Bonnet's theory so completely from the field that, until quite recently, epigenesis was regarded as the only scientifically justifiable theory, and a return to the 'evolutionist' position would have been looked upon as a retrograde step, as a reversion to a period of fancy which had been happily passed. I myself have been repeatedly told, with regard to my own 'evolutionistic' theory, that the correctness of epigenesis was indisputably established, that is, was a fact, verifiable at any time by actual observation!
But what are the facts? Surely only that there is a succession of numerous developmental stages, which we know very precisely in the case of a great many animals, and that the miniature model which Bonnet assumed to be in the egg does not exist. Both these facts are now no longer called in question. But that does not furnish us with a theory of development, for theory is not the observation of phenomena or of a series of phenomena, it is the interpretation of them. Epigenesis, as formulated first by Aristotle and again by Harvey, Wolff, and Blumenbach, certainly offered an interpretation of development, not, however, by referring only to what was observable, but by going far beyond it; on the one hand taking the appearance of a homogeneous germ-substance for reality, and, on the other, assuming a special power, which caused a heterogeneous organism to arise from a homogeneous germ.
We cannot now accept either of these assumptions, for we know that the germ-substance is not homogeneous, and indeed is not merely a substance but a living cell of complex structure; and we no longer believe in a special vital force, and therefore not in a special 'power of development,' which could only be a modification of the former. We are thus as little able to accept the old epigenesis as the old evolution, and we must establish a theory of Development and Heredity on a new basis.
What this basis must be is in a general way beyond doubt. Since it is the endeavour of the whole of modern biology to interpret life more and more through the interactions of the physical and chemical forces bound up with matter, development, too, comes within this aim, for development is an expression of life. We seek to understand the mechanism of life, and, as a part of that, the mechanism of development and of heredity which is closely associated with it.
If we wished to attack the problem of heredity at its roots we should first of all have to try to understand the process of life itself as a series of physico-chemical sequences. Perhaps this will be achieved up to a certain point in the future, but if we were to wait for this we should in the meantime have to abandon all attempts at a theoretical interpretation of the phenomena of development and heredity, and might indeed have to postpone them to the Greek Kalends. That would be as though, in the practice and theory of medicine, all investigation into and speculation regarding disease had to wait until the normal, healthy processes of life were thoroughly understood. In that case we should now know nothing of bacteria diseases and the hundred other acquisitions of pathological science: physiology too would have remained far behind its present level if it had lacked the fruitful influence of experience in cases of disease, and the ideas and theories, true and false, which have been based thereon. In the same way we require a theory of development and heredity if we are to penetrate deeper into these phenomena, and must have it in spite of the fact that we are still very far from having a complete causal knowledge of the processes of life. For the raw material of observation, which is to some extent fortuitous, will never bring us any further on; observation must be guided by an idea, and thus directed towards a particular goal.
It is, however, quite possible to leave aside for the present all attempts at an explanation of life, and simply to take the elements of life for granted, and on this basis to build up a theory of heredity. We have already taken a step in that direction by establishing that the whole substance of the fertilized ovum does not take part in heredity in the same degree, but that only a small part, the chromatin of the nucleus, is to be looked upon as the bearer of the hereditary qualities, and by deducing, further, that this chromatin is made up of a varying number of small but still visible units, the ids, each of which virtually represents the whole organism, or, as I have already expressed it, each of which contains within itself, as primary constituents, all the parts of a perfect animal.
It was these 'primary constituents' which led us to the digression in regard to Bonnet's theory of 'Evolutio' and Wolff's 'Epigenesis.'