CHAPTER XVIII 



THEORIES OF EVOLUTION CONTINUED: 

 WEISMANN, DE VRIES 



Weismann's views have passed through various stages of 

 remodeling since his first public championship of the Theory 

 of Descent on assuming, in 1867, the position of professor of 

 zoology in the University of Freiburg. Some time after that 

 date he originated his now famous theory of heredity, which 

 has been retouched, from time to time, as the result of 

 aggressive criticism from others, and the expansion of his 

 own mental horizon. As he said in 1904, regarding his 

 lectures on evolution which have been delivered almost reg- 

 ularly every year since 1880, they "were gradually modified 

 in accordance with the state of my knowledge at the time, 

 so that they have been, I may say, a mirror of my own intel- 

 lectual evolution." 



Passing over his book, The Germ Plasm, published in 

 English in 1893, we ma y fairly take his last book, The 

 Evolution Theory, 1904, as the best exposition of his con- 

 clusions. The theoretical views of Weismann have been 

 the field of so much strenuous controversy that it will be well 

 perhaps to take note of the spirit in which they have been 

 presented. In the preface of his book just mentioned, he 

 says: "I make this attempt to sum up and present as a har- 

 monious whole the theories which for forty years I have been 

 gradually building up on the basis of the legacy of the great 

 workers of the past, and on the results of my own investiga- 



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