GENETICS 77 



ing this much, there are still important matters 

 to be dealt with in the association of these units 

 with the process of evolution. 



Genes have never been seen, but since their 

 existence is established beyond reasonable doubt, 

 we may judge them by the things which we know 

 of the chromosomes containing them. Their 

 functions in heredity are recognized; they have 

 even been regarded as independent of external 

 conditions in their hereditary capacities. Neverthe- 

 less the succession of generations is always ac- 

 complished by com.plete cells, not by nuclear 

 units or nuclei alone; chromosomes have never yet 

 been shown capable of action independent of the 

 surrounding nuclear and cytoplasmic materials. 

 For this reason the part played by the cytoplasm, 

 as well as the known functions of the chromosomes, 

 is a matter for serious consideration. Opinions of 

 its importance vary widely. Some scientists have 

 concluded that it is responsible for a transmission 

 of characters no less definite and orderly than that 

 carried on by the chromosomes, and have sought 

 in cytoplasmic inclusions the material basis for 

 this transmission. Others have denied that any 

 orderly inheritance proceeds through the cyto- 

 plasm. From the point of view of this study it is 

 necessary to seek the relationship existing between 

 it and the nucleus, rather than to defend any ex- 

 isting view. 



