352 THE EVOLUTION THEORY 



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 apparent ly 

 simple, but in realit}^ 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 in- 

 disputably 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 precise^ 

 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 



