CHAPTER XIII 



The inheritance of acquired characters and the mnemic theory of heredity. 



No biological question during the last fifty years has given rise 

 to more acute and vigorous controversy than that of the inherit- 

 ance or non-inheritance of " acquired " characters. The following 

 paragraph may be quoted to show the manner in which Weismann 

 stated the case and endeavoured to give precision to the 

 terminology employed : 



" By acquired characters I mean those which are not preformed 

 in the germ, but which arise only through special influences 

 affecting the body or individual parts of it. They are due to the 

 reaction of these parts to any external influences apart from the 

 necessary conditions for development. I have called them ' somato- 

 genic ' characters, because they are produced by the reaction of 

 the body or soma, and I contrast them with the ' blastogenic ' 

 characters of an individual, or those which originate solely in 

 the primary constituents of the germ (' Keimesanlagen '). It is an 

 inevitable consequence of the theory of the germ-plasm, and of 

 its present elaboration and extension so as to include the doctrine 

 of determinants, that somatogenic variations are not transmissible, 

 and that consequently every permanent variation proceeds from 

 the germ, in which it must be represented by a modification of 

 the primary constituents." l 



As already pointed out, the view here expressed is directly 

 opposed to that of Lamarck and Darwin, who believed that 

 characters acquired in the life-time of the individual, either as a 

 result of the use or disuse of organs or of the direct action of the 

 environment, might be handed on by heredity from parent to 

 offspring, and Darwin's theory of pangenesis was essentially an 

 endeavour to imagine some mechanism by which such trans- 

 ference of acquired characters might be effected. 



We may at once agree with Weismann that blastogenic 

 characters alone are transmitted from parent to offspring, but the 

 real question is Can a somatogenic character be converted into, 



1 " The Germ-Plasm : A Theory of Heredity," by August Weismann (Con- 

 temporary Science Series, 1893), p. 31)2. 



