THE GERM-PLASM THEORY 261 



theory of heredity have stood the test of time in an 

 admirable manner. 



Let us turn our attention, then, for a short space to 

 the Germ Plasm Theory of inheritance. On Weis- 

 mann's theory, as in most other theories of heredity 

 from the time of Darwin and Nageli downwards, the 

 separate parts of the living organism are supposed to 

 be represented by separate material particles in the 

 germ-cells. These representative particles are known 

 as determinants. A complete set of determinants in 

 which every part of the organism is thus represented 

 constitutes an id. So far Weismann's hypothesis is in 

 close agreement with the idea of representative particles 

 which we are driven to adopt by the facts of Mendelian 

 inheritance, except that, following de Vries, we should 

 speak of separate characters rather than parts as 

 being thus represented ; for there seems to be no doubt 

 that the same character-determinant can affect the 

 development of a number of different parts. But at 

 the next step the Mendelian parts company with 

 Weismann. The latter assumes that the cells of an 

 organism contain a large number of ids, or complete 

 sets of determinants, half of the total number being 

 derived from either parent, and that, although at the 

 reducing division which precedes the formation of the 

 gametes the total number of ids is reduced to half of 

 what it was in the somatic cells, still, several ids 

 derived from each parent are present in every germ- 

 cell. 



Thus the reduced number of chromosomes in the 

 germ-cells is regarded as containing all the primary 



