CYTOPLASMIC HEREDITY 421 



protoplasmic system, this including its interaction with the environment. 

 If we define heredity as "the occurrence of related but not necessarily 

 identical conditions, events, or characters in successive generations of 

 organisms as a consequence of their protoplasmic organization" (p. 285), 

 we must regard the entire protoplasmic system as the "physical basis of 

 heredity." The nuclear theory of heredity, properly conceived, does not 

 attribute the existence of a given character wholly to nuclear action, 

 although it rightly explains many of the differences between related 

 individuals (probably all of such differences in many cases) as results of 

 diversities in nuclear constitution. It is such differences which have been 

 studied most in modern cytogenetics, but the striking success attending 

 these studies should not obscure the fact that any complete explanation 

 of the phenomena of heredity must include much that lies outside of the 

 nucleus. 



It should now be more plainly evident why we have stated (p. 284) 

 that the problem of development cannot be entirely divorced from that of 

 heredity. Much has been learned about the transmission of hereditary 

 elements through successive generations, but the manner in which these 

 elements function in the actual development of the characters remains to 

 be discovered. This is one of the major biological problems of the future, 

 and not before it has been solved can we be satisfied with our conceptions 

 of the mechanism of either development or heredity. 



