BRISTLE INHERITANCE IN DROSOPHILA 117 



Standard deviations 



If the extremes in a frequency distribution are stationary, 

 the movement of the mean will tend to modify the standard 

 deviation. If the mean and the mode of a curve are close to 

 one end of the distribution, their movement towards the middle 

 of the range will have the tendency to raise the standard devia- 

 tions. There is a fairly close approximation to these conditions 

 in the frequencies of the different generations. The early gen- 

 erations show means and modes close to the lower end of the 

 scale of grades; in later generations the means rise toward the 

 middle of the scale, but the extremes are not greatly changed. 

 This leads one to expect the increase in the standard deviations 

 that actually has been found to follow the increase in the means. 

 Moreover the standard deviations fall when the means fall. 



In figure 5 A and B the standard deviations for the sons and 

 daughters are separately compared with their respective means. 

 Attention must be called to the fact that the means and the 

 standard deviations are plotted in these curves on different 

 vertical scales, and that the coincidence of the lines does not in- 

 dicate fluctuations of equal magnitude, but rather, fluctuations 

 in like directions. The close parallelism between these two sets 

 of constants disappears soon after the 32d generation. This is 

 apparently due to the fact that, as explained, the complete yield 

 from each bottle was not recorded. This incomplete sampling 

 slighted the lower part of the distributions so that the means 

 were raised, but the standard deviations were reduced and 

 rendered irregular. Besides rising and falling with the means 

 in each sex, the standard deviations of the sons are lower than 

 those of the daughters, in the same way as the means of the 

 sons are lower. 



The general conclusions to be drawn from the study of the 

 standard deviations may be stated as follows: The standard 

 deviations of the frequencies with lower means are lower than 

 the standard deviations of the frequencies with higher means. 

 The generations between the 32d and the 49th of course form 

 an exception to this statement, but in accord with the expla- 



