514 University of California Publications in Zoology [Vol. i;; 



The ratios 3 to 1, 15 to ], and 63 to 1, point to a Mendelian in- 

 heritance, but the ratios 5 to 1, 7 to 1, and others are at present un- 

 explainable. They may have some significance as such or they may 

 merely be intermediates in a continuous series. The latter seems im- 

 probable, however, on account of the closeness of many ratios to 

 jMendelian expectations. Perhaps some explanation will be found to 

 make the other ratios understandable. 



Conclusions 



1. We may be dealing with a case of Mendelian inheritance in 

 which one, two, and three ecjuivalent factors are concerned. How- 

 ever, there are various facts which this interpretation does not explain 

 and for the explanation of other ratios obtained it seems necessary to 

 assume also, in certain cases, fluctuating variation or alternating dom- 

 inance. Other hypotheses might also, of course, be formulated to 

 account for the results, but it does not appear worth while to discuss 

 these in the present state of the problem. Data showing the linkages 

 of the factor or factors for extra bristles with other factors appear 

 to aiford the most probable basis for a solution of the question. 



2. There seems to be a partial inheritance or intermediate condi- 

 tion, as shown by the occasional occurrence of thickened hairs. 



3. In accord with the results of MacDowell (1915), the inheritance 

 of extra bristles is found not to be sex-linked. 



4. Selection does not increase the number of bristles in extra- 

 bristled individuals, nor does it give a pvire line. 



5. There are more extra-bristled flies appearing in crosses between 

 extra X extra than between extra X normal. The ratio 63 :1 was 

 never obtained in extra X extra, and the ratio 3 :1 never occurred in 

 normal X extra. 



6. The amount of food does not appear to influence the number 

 of extra bristles, as they appear all through the hatchings. 



7. Extras appear with equal frequency in low-grade and high- 

 grade cultures when extra is crossed with normal. 



8. Temperature plays no important part in the development of 

 extra bristles. 



Transmitted August 31, 1916. 



