HEREDITY 



419 



peculiarities which it transmits to the offspring. For the parent is 

 made up of two distinct parts, its own body and the reproductive sub- 

 stance contained within that bod}", and the two may not be identical 

 in character. 



The reproductive substance has been called by Weismann the germ- 

 plasm. He it was who first clearly recognized the fact that the germ- 

 plasm is distinct from the body which contains it, and that the in- 

 fluences which modify the character of the one do not of necessity 

 modify the character of the other. Thus he was able to show experi- 

 mentally that mutilations of the body, as loss of the tail in mice, are 

 not inherited, and to establish with a considerable degree of certainty 

 the principle that characters acquired by the body as a result of use, 

 disuse or other agencies are not inherited, because they have not 

 affected the constitution of the germ-plasm carried within the body. 



Weismann's two principles are of fundamental importance to a 

 right understanding of heredity. They are : ( 1 ) That the germ-plasm 

 is independent of the body containing it, or, as Weismann put it, that 

 the germ-plasm is continuous from generation to generation, whereas 

 the body dies, and (2) that acquired characters are not inherited. 



The hottest biological discussions of the last twenty years have been 

 waged over these two principles and the contest is by no means ended, 

 but year by year the correctness of Weismann's contentions is more 

 generally admitted. 



Common experiences support both principles. Thus the independ- 

 ence or continuity of the germ-plasm has been shown from time pre- 

 historic in the practise of castration upon the domesticated animals or 

 upon man. The germ-plasm is localized in particular organs of the 

 body, the reproductive glands. If these are removed reproduction be- 

 comes impossible, though all other functions of the individual persist. 

 Further, it is possible to show experimentally that the germ-plasm 



Fig. 1. A young black Guinea-pig, about three weeks old. The ovaries taken from 

 an animal like this were transplanted into the albino shown in Fig. 2. 



