THE FACTORS OF ORGANIC EVOLUTION. 



I0 9 



thousands of generations, and amid the most diverse 

 circumstances, bears strong testimony to the great sta- 

 bility of that living structure which is the basis of in- 

 heritance. On the other hand, all experience goes to 

 prove that the living substance of the body cells in gen- 

 eral is readily modified, and that in a surprisingly short 

 time. The fact of this great difference can not fail to 

 be recognised ; its cause is at present merely a matter of 

 conjecture. 



Weismann at one time supposed the cause of this to 

 be an absolutely stable, absolutely separate, and per- 

 petually continuous germ plasm. How- 

 Germinal proto- ever, there is the most convincing and 

 plasm relatively abundant evidence that although the 

 but not abso- . . , . . 



lutely stable. S erm P laSm 1S relatlvel y vei T stable and 



continuous, it does not possess those 

 divinely perfect characters ascribed to it. More re- 

 cently Weismann has practically abandoned each and 

 all of these characters,* and now, like a good Lamarck- 

 ian, finds " the cause of hereditary variation in the 

 direct effects of external influences on the biophores 

 and determinants." 



The outcome of the whole matter, then, is that we 

 find ourselves much in the same position as we were be- 

 fore Weismann denied the possibility of the inheritance 

 of acquired characters. All hereditary variations are 

 caused by the action of extrinsic forces on the germinal pro- 

 toplasm, producing changes in its structure. Strangely 

 enough, this proposition was admitted as a logical neces- 

 sity by one who undertook by rigorous logic to prove 

 the reverse. Since almost the only objection to this 

 position was the one raised by Weismann, it may now 

 be considered as definitely settled, and the only ques- 



* See Romanes's Examination of Weismannism, 1893. 



