THE DOCTRINE OF IMMORTALITY 57 



simply "gemmules." He assumed that they pass over 

 from the parents to the descendants, and usually develop 

 themselves in the first generation. Darwin's pangenesis 

 explains heredity. It is the hypothesis of a master, and as a 

 succinct and comprehensive explanation of the facts of heredity 

 must always command admiration. Since Darwin's time 

 many modifications of the doctrine of pangenesis have been 

 proposed. These modifications, however, possess for us 

 merely historical interest, for with the progress of science they 

 have become superfluous. 



The new doctrine of heredity is due to Professor Moritz 

 Nussbaum, who laid special stress upon the discovery of 

 the germinal paths in animals, for he recognized in these an 

 arrangement to separate special germinal cells from the 

 somatic cells. He concluded that a portion of the germ-plasm 

 is withheld from the developing ovum, kept comparatively 

 unaltered, and employed for the formation of sexual elements, 

 so as to become directly the germ-plasm of a new generation. 

 It is clearly superfluous to still employ the expression germ- 

 plasm which corresponds to speculative needs, and which we 

 may now leave out of consideration. It is simpler to speak 

 merely of living substance. Nussbaum's theory has in the 

 course of time become, strictly speaking, the only theory of 

 heredity which we value. 



If the time at our disposal permitted, it would be interesting 

 to analyze carefully some of the theories of heredity which 

 have arisen in association with Nussbaum's doctrine. The 

 majority of these theories search for a special germ-plasm, to 

 use Weissmann's expression. Nageli speaks of idioplasm. 

 Some authorities have sought to bring heredity into relation 

 with visible parts of the protoplasm or of the nucleus. Oskar 

 Hertwig was the first to interpret the nucleus as the organ of 



