48 GENETICS AND EUGENICS 



heredity. It is coming to be recognized that the truth Hes 

 somewhere between these extreme views. 



What in general were Weismann's views and how did he 

 arrive at them ? 



Weismann's Method 



Weismann's method of constructing an hypothesis to 

 account for heredity differed fundamentally from Darwin's. 

 Darwin reasoned inductively, Weismann deductively. Dar- 

 win tried first to ascertain what characteristics are inherited 

 and then to imagine a mechanism which might explain their 

 inheritance. The result was " pangenesis." Weismann, on 

 the other hand, first inquired what is the mechanism of in- 

 heritance and, having answered this to his own satisfaction, 

 proceeded to the conclusion that only such characters are 

 inherited as have their basis in this mechanism. The result 

 was the chromosome theory of inheritance. It has this fea- 

 ture in common with " pangenesis," the inherited character- 

 istics are supposed to be determined in advance and to be 

 represented in the germ-cell by material bodies. These are 

 the " gemmules " of Darwin, the " determiners " of Weis- 

 mann. Darwin supposed that the " gemmules " migrate 

 from all parts of the body into the germ-cells and so make it 

 inevitable that the organism which develops out of the germ- 

 cell shall have the same parts and properties as the parent. 

 As regards the origin of variations, pangenesis might be 

 called a centripetal theory, since determiners are supposed 

 by it to migrate centrally toward the germ-cells. 



Weismann's theory, on the other hand, is centrifugal; he 

 supposes that the " determiners " originate solely in the 

 germ-plasm and migrate thence out into the various parts of 

 the developing body and that thus differentiation is pro- 

 duced. There is on his view no centripetal movement of 

 determiners whatever; they never pass from soma to germ- 

 cells, but only in the reverse direction. 



