Inheritance of Small Variations 



211 



tions in eye color. We had already been furnished seven 

 grades, from white to red; now we have seven secondary 

 grades within a single one of these seven primary grades. 

 Our list of gradations of eye color in Drosophila therefore 

 takes now the following form: 



Heritable grades of eye color, 

 due to diverse variations of a 

 single unit located in Chromo- 

 some I. 



1. White 



2. Tinged 



3. Buff 



4. Eosin 



5. Cherry 



6. Blood 



7. Red 



Variations that give modifica- 

 tions of the intensity of eosin, but 

 are located in other chromosomes. 



1. Whiting 



2. Cream b 



3. Cream a 



4. Fourth diluter 



5. Fifth diluter 



6. Sixth diluter 



7. Dark 



Here again then we have minutely differing conditions of 

 a single shade of color, brought about by seven modifying 

 factors. 



But what are these modifying factors? And here we 

 come to the essential point. These modifying factors are 

 themselves alterations in the hereditary constitution. Bridges 

 leaves no doubt upon this poinb. He lists and describes them 

 specifically as mutations ; as actual changes in the hereditary 

 material. 



What then is the difference in principle between such cases 

 and the theory of gradual alterations in a single unit fac- 

 tor? The difference is that in the case of the multiple modi- 

 fying factors the minute changes occur, not all in one factor 

 in one locus of the chromosome but in a number of di- 

 verse parts of the germinal material; this appears to have 

 been clearly demonstrated. But this is a matter of detail; 

 it does not touch the fundamental question. 



This fundamental question is as to the occurrence of these 

 minute changes in the hereditary constitution, and as to the 



