INTRODUCTION 5 



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 essenticdis (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 con- 



