96 E. CARLETON MACDOWELL 



CONCLUSIONS 



From a pair .of wild flies a race of Drosophila ampelophila 

 has been established which has regularly more than the normal 

 four thoracic bristles. 



By selecting high grade parents and inbreeding brother to 

 sister, the number of extra bristles was gradually increased for 

 six generations. 



From the seventh to the eleventh generations fluctuations 

 were found showing no further increase. 



The maintenance of the high grades of extra bristles does not 

 depend upon selection as, low grade parents from mass cultures 

 started from the fifth and sixth generations that have run eight 

 months, when raised in single pairs, give as high grade offspring 

 as inbred and selected parents mated at the same time. 



A Mendelian factor is involved in the inheritance of extra 

 bristles, and as normal dominates extra, this may be regarded as 

 a dominant factor that restricts the number of bristles to four. 



This factor is not sex linked, although males are apt to have 

 fewer extra bristles than females. 



The extracted extra bristled flies have a lower distribution 

 than that of the inbred flies of the corresponding generation, 

 although the high extremes of the inbred race are also found 

 among the extracted extras. 



There is a greater difference between the inbred and extracted 

 distributions when the cross is made after eight selections than 

 when made after only one selection. 



Environment influences the number of extra bristles, and since 

 small flies are not apt to have as many extra bristles as large 

 ones, it appears that the amount of food eaten is an important 

 factor. 



From the above statement an explanation may be found for 

 the fact that an apparently normal fly may be genetically homo- 

 zygous for extra bristles. 



The hypothesis of accessory factors will explain all the facts, 

 and that of modification of a Mendelian factor may be employed 

 to interpret most of them. 



