40 FIFTY YEARS OF DARWINISM 



" I send by this post an essay by Hackel attacking 

 Pan. and substituting a molecular hypothesis. If I 

 understand his views rightly, he would say that with 

 a bird which strengthened its wings by use, the forma- 

 tive protoplasm of the strengthened parts became 

 changed, and its molecular vibrations consequently 

 changed, and that these vibrations are transmitted 

 throughout the whole frame of the bird, and affect 

 the sexual elements in such a manner that the wings 

 of the offspring are developed in a like strengthened 

 manner. . . . He lays much stress on inheritance 

 being a form of unconscious memory, but how far this 

 is a part of his molecular vibration, I do not under- 

 stand. His views make nothing clearer to me; but this 

 may be my fault." 



Should it hereafter be proved that acquired 

 characters are transmitted, I can not but think 

 that the interpretation will be on the lines of 

 Charles Darwin's hypothesis of Pangenesis. But 

 the probability that any such result will be estab- 

 lished, already shown to be extremely small, has 

 become even more remote in the light of the re- 

 cent investigations conducted by Mendelians and 

 mutationists. 



For the transmission of all inherent qualities, 

 including the successive stages of individual de- 

 velopment, Weismann's hypothesis of the con- 

 tinuity of the germ-plasm supplies a sufficient 

 mechanism. I remember, more than twenty 

 years ago, asking this distinguished discoverer 

 how it was that the hypothesis arose in his mind. 

 He replied that when he was working upon the 

 germ-cells of Hydrozoa he realized that he was 

 dealing with material which early and late in the 

 history of the individual was most carefully 



