xiv PREFACE 



in which I more than once imagined myself successful, but 

 all of which broke down when further tested by facts, I 

 finally became convinced that an epigenetic development is 

 an impossibility. Moreover, I found an actual proof of the 

 reality of evolution, which will be explained in the chapter 

 on the structure of the germ-plasm. It is so simple and 

 obvious that I can scarcely understand how it was possible 

 that it should have escaped my notice so long. 



It is gratifying to me to find myself at one with the great 

 English naturalist Darw in, -^^ as w ell as with,4ie_Vries and 

 Wiesner, — at all events in the main point at issue ; and this 

 agreement seems to me to point to the possibility of solving i 

 in the end the problem of heredity, which might seem to b j 

 open only to the wildest speculations : we may now perhap, 

 hope to succeed in recognising the probable explanations 

 among the m.2ccvy possible ones, and in finally selecting from 

 among these the real solution of the problem. This will 

 assuredly be the work of time, and our approach to the truth 

 will be a very gradual one. But our path is marked out ; 

 reasoning supported by obseif>ation will lead us to the goal. 

 We are led by the observation of facts to form an opinion 

 as to their bearing on each other. This gives rise to further 

 problems and fresh investigations, which in their turn lead 

 to a new interpretation. In this way light has before now 

 been thrown on many a problem that seemed to baffle 

 explanation. I need only mention the insight that we have 

 now gained into the phenomenon of sexual reproduction. 



taught that there is no preformation of parts in the germ, but that the 

 fully-formed organism is produced by a gradual process of differentia- '. 

 tion. It will be seen that the word ' evolution,' as here used, has no 

 connection with the doctrine of descent \vith which it is usually con- \ 

 nected. — VV. N. R 



