156 THE MAIN CURRENTS OF ZOOLOGY 



stance is the germ-plasm. It is the living vital sub- 

 stance of organisms that takes part in the develop- 

 ment of new generations. 



Weismann (Fig. 32) points out that the many- 

 celled body was gradually produced by evolution, and 

 that in the transmission of life by the higher animals 

 the continuity is not between body-cells and their 

 like, but only between germinal elements, around 

 which in due course new body-cells are developed. 

 Thus he regards the body-cells as constituting a sort 

 of vehicle within which the germ-cells are carried. 

 The germinal elements contain the primordial sub- 

 stance around which the body-cells have developed, 

 and, since in all the long process of evolution the 

 germinal elements have been the only form of con- 

 nection between different generations, the substance 

 composing them has unbroken continuity. 



This conception of the continuity of the germ- 

 plasm is the foundation of Weismann's doctrine. 

 It is one of the most fruitful biological ideas devel- 

 oped during the nineteenth century, and it replaced 

 as a basis all earlier theories of heredity. Although 

 Weismann was not the originator of the idea of 

 germinal continuity, he is nevertheless the one who 

 has developed it the most extensively. 



