80 . E. CARLETON MACDOWELL 



But the standard deviations show a gradual increase while 

 selection is effective according to the curves and the means, 

 and when selection has seemingly become ineffective, the stand- 

 ard deviations fluctuate, but do not show a gradual decrease. 



b. Single lines 



The most immediate facts are presented in the form of the his- 

 tory of five inbred lines through single matings, as shown by curves 

 in figure 6, A, B, C, D, E. The grades of the parents selected 

 from each generation are indicated below the curve of their own 

 fraternity and above the curves representing their offspring. 

 Squares are the males; triangles are the females. Four of the 

 lines start from the same pair of first generation flies. The first 

 two generations which are common ancestors of all four lines, are 

 represented in only one set of curves. The parents of families 

 Nos. 87, 71 and 88 are all sibs in family No. 56. The parents 

 of family No. 116 come from family No. 88. The Une headed 

 by family No. 17 starts with a different pair of first generation 

 flies. As the history told by these curves is clear it does not 

 seem necessary to discuss each set of curves in detail as has 

 been done for the total generations. In Table 11, A, B, C, D, E, 

 the means for these curves are given. These curves give the 

 clearest picture of the basic facts. Clear progress is shown in 

 the first five or six generations. In each line are differences 

 counter-balanced in the total generations, yet in no case do these 

 differences modify the general conclusion drawn from the study 

 of total generations. 



3. SUMMARY 



In brief, selection appears to have made advances for six 

 generations. AVhen large enough numbers are observed to afford 

 a counter-balancing of the fluctuations of individual families, 

 the advance is steady. That this increase is really genetic is 

 shown by the increase in the distribution of extras extracted 

 from a cross of long selected flies, over the distribution of extras 

 extracted from a cross of unselected flies. In, later generations 



