AUTHOR'S PREFACE TO THE TRANSLATION. 



The promulgation of the principle of unit-characters is 

 the main theme of this work, as is emphasized in the first 

 sentence of the Introduction. At the time of the publication 

 of the first part of the German edition (1900) this principle 

 was new and was also in evident opposition to the current 

 belief in the slow and gradual evolution of the organic world. 

 During the years that have since elapsed, it has gained almost 

 universal acceptance, though there are still some authors, 

 especially among zoologists, who are opposed to it. 



The evidence which supported this view was derived 

 from three main sources. 



First a clearer understanding of the processes of selec- 

 tion in agricultural plant-breeding. This conception has 

 since been corroborated in the most convincing manner by 

 the work of Nilsson and of Korschinsky^ and it points to 

 the elementary species as the real material for artificial and 

 natural selection. 



Secondly, the experimental evidence afforded by the 

 evening primroses and some other groups of plants ; espe- 

 cially the observed origin of Oenothera gigcts, which ap- 

 peared suddenly in my cultures in the year 1895 and pos- 

 sessed, at its first origin, all the attributes of a new species, 

 including constancy and even a double number of chromo- 

 somes in its nuclei. 



Thirdly, the new light, thrown by the principle of the 

 unit-characters on the work of Mendel, neglected up to 





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