922 



SCIENCE 



[N. S. Vol. XLI. No. 1069 



Finally, mention must be made of Weis- 

 mann's theory of evolution which was a 

 direct outgrowth of his theory of heredity. 

 He maintained that evolution must depend 

 upon an evolution of the germ plasm and 

 that this was brought about chiefly, if not 

 entirely, by the mixture of different kinds 

 of germ-plasms (amphimixis) in the union 

 of the sex cells. There is no doubt that 

 many variations are produced by amphi- 

 mixis, but in general these combinations of 

 germ-plasms are not actual fusions; new 

 combinations of inheritance units are pro- 

 duced, but not new units, and usually these 

 new combinations split up in subsequent 

 generations according to Mendelian rules, 

 so that such temporary combinations of 

 different germ-plasms do not usually lead 

 to permanent modification, or to evolution, 

 of the germ-plasm. On the other hand, it 

 is probable that Weismann underestimated 

 the possible influence of environment in 

 producing changes in the germ-plasm and 

 hence its influence on evolution ; at least it 

 does not seem possible at present to explain 

 the origin of many inherited mutations 

 except by the influence of changed environ- 

 ment upon the developing germ cells. 



In his belief in natural selection Weis- 

 mann out-Darwined Darwin or any of the 

 Darwinians. Darwin dealt only with the 

 survival of individuals or races in the strug- 

 gle for existence and was always inclined 

 to assign a good deal of weight to the influ- 

 ence of environment in producing new 

 races. Weismann would not admit the 

 existence of any other factor of evolution 

 than selection and he extended this prin- 

 ciple from individuals or persons ("per- 

 sonal selection") to organs and tissues 

 ("histonal selection") and even to germ- 

 inal units such as determinants and bio- 

 phores ("germinal selection"). By means 

 of an assumed struggle for nutriment be- 

 tween different 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 per- 

 sonal selection. And by a similar struggle 

 between different biophores he showed that 

 the quality of a determinant would be 

 changed. By means of this highly ingeni- 

 ous but purely formal and hypothetical 

 system he was able to explain the degenera- 

 tion and disappearance of useless parts of 

 an organism and the concordant modifica- 

 tion of many different parts in the course 

 of evolution. 



Of all his theories those which grew out 

 of his belief in the "Omnipotence of Selec- 

 tion ' ' have found least confirmation in sub- 

 sequent work. The mutation theory of 

 de Vries has come in to modify in certain 

 important respects the theory of Darwin, 

 and the work of Jphannsen, 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 accounting 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 ques- 

 tions 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 ad- 

 vances of organization, and finally the general 

 course of development of organisms on our earth, 

 for everything in the living world rests on adapta- 

 tion. 



Clear thinking is necessary in the ad- 

 vance of science as well as fine technique, 

 and Weismann has demonstrated to a more 

 or less scornful world the importance of 



