GENETICS AND CYTOLOGY OF SACCHAROMYCES 265 



amount, the other organelles cannot obtain the supply of molecules necessary 

 for maintenance and increase until the amount of the deficient organelle has 

 increased. 



The chromosomes differ from the other permanent organelles in their high 

 degree of linear heterogeneity. It is this characteristic which has given them 

 the spurious appearance of "controlling" other cellular activities. Mutations 

 with which we are familiar in the laboratory constitute defects or deletions 

 in the extraordinarily heterogeneous chromosomes. The deficiency in the 

 organism caused by the defect — the deletion of the contribution ordinarily 

 made by the intact region of the chromosome — becomes apparent only be- 

 cause the rest of the chromosome produces sufficient materials to enable the 

 defective cell to continue to grow in its absence, although in a manner differ- 

 ent from that which was previously characteristic. 



Any transmissible defect in a homogeneous structure like the cell wall, 

 the cell membrane, the nuclear membrane, the centrosome, or the centro- 

 chromatin would result in total failure of the organism to survive and bring 

 all vital activity to a halt. The survival of the defective mutants in their 

 altered condition due to the defect in the chromosome (which has been called 

 a mutant gene) has led to the view that genes are different from other cellu- 

 lar components since they can reproduce variations in themselves. This is an 

 incorrect point of view. It is more proper to say that when a defect or dele- 

 tion occurs in a small segment of a chromosome, the rest of the organism can 

 carry on, albeit in a changed condition due to the absence of the contribution 

 previously made by that region, now called the gene. This denies the im- 

 portance of the ordinary mutations encountered in the laboratory as factors 

 for progressive evolution, and implies that progress in evolution must occur 

 in some other way. 



It may be that progressive evolution occurs more frequently as the result 

 of changes in the chromosomes than of other organelles. But the present hy- 

 pothesis does not exclude the possibility that advances in evolution can 

 occur by ''progressive" changes in the composition of any one of the eternal 

 organelles such as the nuclear membrane or the centrosome. The condition 

 for the perpetuation of any change would be that the mutated organelle 

 could be provided with the materials necessary for its continuance by the 

 cell as a whole in its surrounding environment at the time of its occurrence. 

 On this hypothesis, progressive changes in evolution are not confined to any 

 single cellular component, but constitute a potential of every component of 

 the cell. Although progressive changes of the different substances compris- 

 ing the chromosome may not occur significantly more frequently than 

 changes in the substances making up the other organelles, more changes may 

 occur in the chromosomes in toto because a change in each individual com- 

 ponent of the extraordinarily heterogeneous chromosome registers as a sepa- 

 rate change. 



