292 JENNINGS: CHANGES IN HEREDITAEY CHARACTERS 



is to modify eosin in the direction of a darker color: this factor 

 he calls "dark." None of these factors has any effect save on 

 eosin-eyed flies. 



As you see, these things add tremendously to our gradations 

 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 grada- 

 tions of eye color in Drosophila therefore takes now the following 

 form: 



Heritable grades of eye color, Variations that give modifica- 



due to diverse variations of a tions of the intensity of eosin, 



single unit located in Chromo- but are located in other chro- 



some I. mosomes. 



1. White 



2. Tinged 



3. Buff 



4. Eosin . . 



5. Cherrv 



6. Blood 



7. Red 



1. Whiting 



2. Cream b 



3. Cream a 



4. Fourth diluter 



5. Fifth diluter 



6. Sixth diluter 



7. Dark 



Let us hasten to add that these seven new grades are not lo- 

 cated in the same unit factor as are the seven primary ones; 

 their loci are in other chromosomes (or possibly in other parts of 

 the same chromosome). 



Here again then we have minutely differing conditions of a 

 single shade of color, brought about by seven modifying factors. 

 Bridges makes the following remark concerning them: 



A remarkably close imitation of such a multiple factor case as that 

 of Castle's hooded rats could be concocted with the chief gene eos'n for 

 reduced color, and these six diluters which by themselves produce no 

 effect, but which carry the color of eosin through every dilution stage 

 from the dark yellowish- pink of the eosin female to a pure white. 9 



Now this is an extremely interesting statement, one that must 

 arouse the keen interest of the student of the method of evolu- 

 tion. In Drosophila we could get the same sort of graded results 

 that Castle does with his rats, only in Drosophila this is by 



9 Bridges, 1916, p. 149. (See Bibliography.) 



