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 may 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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