84 THE PROBLEMS OF EVOLUTION 



if cytoplasm has specific value in heredity, since 

 it comes chiefly or entirely from the mother, but 

 they are usually the same. The exceptions, such 

 as the mule and hinny, are an interesting problem, 

 but they are sufficiently rare to present no general 

 contradiction of his statements. One other opinion 

 of interest here is Lillie and Just's conclusion: 

 **The materials of the cytoplasm are . . . being 

 constantly consumed in the metabolism, and the 

 process of renewal and increase of such materials 

 involves interaction of nucleus and cytoplasm." ^^ 

 Whatever the cytoplasm brings to the cell, then, 

 falls immediately under the control of the nucleus 

 with which it is associated and soon becomes an 

 expression of the powers of that nucleus. 



But until it is definitely established that the 

 sovereignty of the genes is complete, that they are 

 not merely a controlling center but also a produc- 

 ing center, some function in heredity must be rec- 

 ognized in the cytoplasm. It is a reaction product; 

 the quality of the ancestral cytoplasm has a part 

 in its origin. Granting even that it will be main- 

 tained in a constant state in spite of enormous 

 fluctuations in the conditions governing its pro- 

 duction, it is still impossible to avoid the chance 

 that it may vary even in the same type of cell, 

 governed by the same complex of chromosomes. 

 Nor can we believe that the same determiners 



" Cowdry's General Cytology, p. 471. 



