20 HEREDITY [ch. 



adapted to the use to which it is normally put. Such 

 characters as these, arising in response to a stimulus, 

 and not appearing in its absence, are technically 

 called ' acquired characters,' a phrase which it will be 

 necessary to use rather frequently in the following 

 pages. As a rule, such 'acquired characters' are 

 adaptive, that is, they render the organism or 

 structure better fitted to its surroundings than if 

 they had not been developed. The older students of 

 heredity never doubted that these acquired characters 

 were inherited as strongly as the inborn characters 

 discussed above, but since the publication of 

 Weismann's theory of heredity (see Appendix I) with 

 the great body of evidence which he has collected 

 on the other side, opinion has turned increasingly 

 towards the belief that acquired variations are not 

 transmitted. Weismann regards the germ-cells as 

 essentially distinct from the rest of the body, so 

 that acquired modifications of the body cannot be 

 transmitted because the germ-cells are not affected. 

 The germ-cells collectively, or rather that part of 

 them which is concerned with the transmission of 

 hereditary characters, he calls ' germ-plasm,' the rest 

 of the body consisting of 'body-plasm/ and he regards 

 ' acquired ' modifications as affecting body-plasm only. 

 A developing germ-cell gives origin to both germ- 

 plasm and body-plasm of a new individual, and hence 



