Let me add that this doctrine of the absolute nature of species was not fully elaborated till our own day, when the Swiss (afterwards American) naturalist, Louis Agassiz, went so far as to maintain that not only the highest and the lowest categories, but all those coming between them, were categories established and sharply separated by Nature herself. But in spite of much ingenuity and his wide and comprehensive outlook he exerted himself in vain to find satisfactory and really characteristic definitions of what was to be considered a class, an order, a family, or a genus. He did not succeed in finding a rational definition of these systematic concepts, and his endeavour may be regarded as the last important attempt to prop up an interpretation of nature already doomed to fall. But in referring to Louis Agassiz I have anticipated the historical course of scientific development, and must therefore go back to the last quarter of the eighteenth century. The first unmistakable pioneer of the theory of descent, which now emerged for the first time as a scientific doctrine, was our great poet Goethe. He has indeed been often named as the founder of the theory, but that seems to me saying too much. It is true, however, that the inquiring mind of the poet certainly recognized in the structure of 'related' animals the marvellous general resemblances amid all the differences in detail, and he probed for the reason of these form-relations. Through the science of 'comparative anatomy,' as it was taught at the close of the century by Kielmeyer, Cuvier's teacher, and later by Cuvier himself, Blumenbach, and others, numerous facts had become known, which paved the way for such questions. It had, for instance, been recognized that the arm of man, the wing of the bird, the paddle of the seal, and even the foreleg of the horse, contain essentially the same chain of bones, and Goethe had already expressed these relations in his well-known verse, 'Alle Gestalten sind ähnlich, doch keine gleichet der andern, Und so deutet der Chor auf ein geheimes Gesetz.' As to what this law was he did not at that time pronounce an opinion, though he may even then have thought of the transformation of species. At first he contented himself with seeking for an ideal archetype or 'Urtypus' which was supposed to lie at the foundation of a larger or smaller group. He discovered the archetypal plant or 'Urpflanze,' when he rightly recognized that the parts of the flower are nothing more than modified leaves. He spoke plainly of the 'metamorphosis of plants,' meaning by that the transformation of his 'archetype' into the endless diversity of actual plant forms. But at first he certainly thought of this transformation only in the ideal sense, and not as a factual evolutionary process.