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.