420 EDWIN CARLETON MacDOWELL 



A summary of these results has already been presented (Mac- 

 Dowell, '17 a). Due to the lapse of time since the appearance 

 of the second report, a brief resume of the general results seems 

 needed to introduce the new calculations. 



PREVIOUS RESULTS 



Extra dorsocentral bristles, in a certain race of Drosophila 

 melanogaster, were found to act as a simple Mendelian char- 

 acter when crossed to normal wild flies. The number of these 

 extra bristles varied, thus affording material for the employ- 

 ment of artificial selection. Starting from one pair of flies with 

 extra bristles, and making brother by sister matings throughout, 

 selections for increased bristle numbers were made for forty- 

 nine generations. After the early generations, this selection 

 did not modify the means of the race, atlhough the high limit 

 of variation was far from being reached. Selections for de- 

 creased numbers of extra bristles at the beginning established a 

 low race; similar selections from the later generations of the 

 high-selected race were unsuccessful. Yet after a cross with 

 normal these same generations of the high-selected race became 

 immediately as amenable to low selection as were the unselected 

 flies at the beginning. These general results were interpreted 

 as being due to genetic differences among the germ cells of the 

 original extra-bristled flies. These differences were quite inde- 

 pendent of the single factor that controlled the appearance of 

 any extra bristles. Selection reduced the number of these dif- 

 ferences by reducing the amount of heterozygosis; crossing with 

 normals increased these differences by increasing the amount of 

 heterozygosis. The numbers of extra bristles that appear on a 

 fly are influenced by external conditions as well as by genetic 

 factors. The average number of extra bristles can be largely 

 controlled by the amount of food a brood has a chance to eat 

 before pupating; this amount depends upon the amount of food 

 present and upon its attractiveness. This external influence 

 naturally acts as a blind to the relationship between the grade 

 and the genetic constitution of an individual, but it is obvious 

 that this was not a complete blind, as there must have been 



