68 The Unity of the Organism 



the following: "After all this, there can be no doubt that the 

 germ cells may reach tlieir differentiation and separation 

 from somatic cells only when the germ-layers have long since 

 been formed, and it is impossible to accept as a general law 

 the view of Nusshauin that sex cells are 'absolutely indepen- 

 dent of tlie germ layers.' So far as we can now see, the sex- 

 cells always arise in the hydroids from elements of one of the 

 germ-layers and tliey are not merely inclusions in a germ- 

 layer but are derivatives, arc division products of it."^- 

 Stri})ped of all sophistry, how is it possible to avoid seeing 

 that we liave before us a clear case in \\4iich Weismann can 

 defend his doctrine of heredity at one of its most critical 

 points only by making purely speculative considerations 

 sup})lant observational evidence which he himself produced 

 at an earlier period in his career? 



The conception of an "hereditary substance" distinct 

 from a non-hereditary substance, by whatever name called, 

 and continuous from parent to offspring is contrary to the 

 observed facts of sexual reproduction in the hydromedusa' 

 as established by Weismann himself and by other and later 

 biologists of unquestioned com})etency and trustworthiness. 

 To this conclusion we are forced by an examination of the 

 available knowledge of the sex-cells in their relation to the 

 germ-layers in this group of organisms. 



The Strongly Organisnial I niplications of Goette's Conclu- 

 sions on the Origin and Migration of Germ- 

 Cells in Hydroids 



With this conclusion we return to the examination of the 

 constructive as contrasted with the destructive results of 

 Goette's researcli. We have shown the most specific and im- 

 mediate of these as viewed from the standpoint of this sec- 

 tion on the organism and its germ-layers, that, namclvj 



