AUGUST WEISMANN. xi 



even to germinal units such as determinants and biophores ("ger- 

 minal selection"). By means of an assumed struggle for nutri- 

 ment between dififerent determinants he believed that the weaker 

 ones would tend to grow still weaker and to disappear while the 

 stronger ones would increase in strength until they reached such 

 importance that they were checked, or increased, by personal selec- 

 tion. And by a similar struggle between different biophores he 

 showed that the quality of a determinant would be changed. By 

 means of this highly ingenious but purely formal and hypothetical 

 system he was able to explain the degeneration and disappearance 

 of useless parts of an organism and the concordant modification 

 of many different parts in the course of evolution. 



Of all his theories those which grew out of his belief in the 

 " Omnipotence of Selection " have found least confirmation in sub- 

 sequent work. The Mutation Theory of deVries has come in to 

 modify in certain important respects the theory of Darwin, and the 

 work of Johannsen, Jennings, Pearl and others has shown that even 

 " personal selection " has little or no influence in creating new types. 

 And yet we have not seen the end of the selection doctrine. The 

 elimination of the unfit is still the only natural means of account- 

 ing for fitness in organisms and we may well ponder these words of 

 Weismann in the preface of his last book: 



" Although I may have erred in many single questions which the future 

 will have to determine, in the foundation of my ideas I have certainly not 

 erred. The selection principle controls in fact all categories of life units. 

 It does not create the primary variations but it does determine the paths 

 of development which these follow from beginning to end, and therewith all 

 differentiations, all advances of organization and finally the general course 

 of development of organisms on our earth, for everything in the living world 

 rests on adaptation." 



Clear thinking is necessary in the advance of science as well as 

 fine technique and Weismann has demonstrated to a more or less 

 scornful world the importance of brains as well as of hands and 

 eyes in the discovery of truth. It does not fall to the lot of any 

 man to make no mistakes, and in this respect Weismann was only 

 human. But it has fallen to the lot of few men to do so much 

 work of lasting value and to have so profound an influence on his 



