BRISTLE INHERITANCE IN DROSOPHILA 145 



As stated, this paper has not attempted to deal with the 

 problem of absolute stability of germ plasm; it has questioned 

 the theoretical possibility of a solution for that problem; but it 

 has attempted to show that none of the various phenomena 

 herein described require the assumption of any internal, or 

 spontaneous, change taking place during the course of these 

 experiments. 



SUMMARY 



I. A race of flies with extra bristles has been selected for 49 

 generations for the production of high numbers of extra bristles. 

 From the study of this race the following points have been 

 determined : 



1. In any generation after the early ones the distribution of a 

 single family is similar to that of the distribution of al the 

 families taken together in that generation. 



2. For about 8 generations the means rose; following this were 

 two periods not comparable with each other, within neither of 

 which was any evidence of further advance to be found. 



3. Continued selection did not produce any high extremes that 

 were not obtainable near the beginning of the experiment. 

 The range of variation changed only very slightly; the low 

 Umits being most frequently at or 1, the high limit at 9 for the 

 females, at 7 for the males. 



4. The standard deviations rose and fell together with the 

 means; as the means of the females are higher than those of the 

 males, so the standard deviations of the females are higher than 

 those of the males. These relationships do not hold true when 

 the complete yields of the bottles are not included (generations 

 33-49). 



5. Changes in the means of the parents are not accompanied 

 by changes in the means of their offspring, except at the begin- 

 ning of the experiment. 



II. By selecting low grade parents from the second generation 

 of the extra bristled race a race of flies was established which 

 had markedly lower means than the high selected race. 



III. By selecting low grade flies from the 15th generation of 

 the high race and continuing to select for low grades, it was 



