10 CALIFORNIA ACADEMY OP SCIENCES. 



ited in their powers of reproduction and ultimately wear 

 out. To this latter rule Weismann makes one important 

 exception, viz: the germ cells. In the case of some in- 

 sects and other invertebrates it has been demonstated 

 that almost at the commencement of embryonic devel- 

 opment the reproductive cells are set apart. In the case 

 of these organisms it is evident that there is an immor- 

 tal chain of reproductive cells from one generation to 

 another. Observation has proved, however, that it is 

 only in rarely exceptional cases that the reproductive 

 cells are thus set apart, and that in ordinary metazoa 

 they appear after the embryo is well advanced toward 

 maturity. Weismann argues very reasonably that it is 

 at variance with the laws of development to assume, as 

 Nsegeli does, the existence of a nucleoplasm which first 

 develops into the more complex body cells and then be- 

 comes simplified into reproductive cells; and he accord- 

 ingly substitutes an hypothesis of his own. He assumes 

 the existence of germ-plasm intermingled with the body 

 plasm and capable of producing the latter, although 

 body plasm cannot be converted into germ-plasm. 

 There is, then, in the higher metazoa, not an immortal 

 chain of reproductive cells, but an immortal chain of 

 germ-plasms. 



From this standpoint the heredity of acquired char- 

 acters is obviously impossible. The germ-plasm cannot 

 be influenced by the body plasm. The hypothesis, from 

 its very nature places an effectual barrier against the in- 

 heritance of acquired characters, and in case it could be 

 demonstrated as true, it would be necessary to explain 

 all such supposed cases of transmission in some other 

 way. This is what Weismann and his followers have 

 done, but before considering the evidence for and against 

 such transmission of acquired characters, it will be ad- 

 visable to consider how well founded this theory is. 



