78 E. CARLETON MACDOWELL 



the male mode still holds to +2 but the proportion of +3 flies 

 has grown to nearly equal the mode. The proportions of +4 

 and +5 flies have also increased although +5 is the limit. The 

 female mode is actually at +5 but the expected mode is easily 

 seen to be at +4. So far the progress and increase has been 

 steady and unquestionable. The following five generations do 

 not show any such advance. In generations seven, eight, nine, 

 and ten the male mode remains at +2 and the proportion of 

 +3 flies is very sUghtly less. In the eleventh generation the +2 

 and +3 males are equal, however the proportion of +1 flies 

 has greatly increased in this generation, so this does not mean 

 any general advance. The female curves have modes at +3 in 

 the 7th and 8th generations, with +3 nearly as high. In the 

 last 3 generations the female mode is at 4 and the curves are 

 in general of the same kind. 



To show what parents were used in the different generations 

 more clearly than can be done by the curves, their distributions 

 are shown in table 10. The averages of the parents are given 

 in two columns at the left and the averages of their children 

 are in two columns towards the right. This table gives a clear 

 summary of the progress of selection. The averages of the 

 children, males and females, show increase up to the sixth gener- 

 ation, when minor fluctuations appear seemingly without any 

 significant change. 



If the variability in the last five generations be amenable to 

 selection, a gradual decrease in the standard deviation would 

 be expected, even though some hypothetical physiological 

 limit prevented the means and modes from advancing as before. 



Fig. 5 Eleven generations of selection. Original parents indicated by a 

 square (father) and triangle (mother) above Fi broken lines, males; solid lines 

 females. The flies chosen as parents are arranged in small curves above the 

 curves of their children. In generations 8 and 9 the male parents were all grade 

 5 and are represented by a square the proper distance from the base line. A 

 finely dotted line means males and females together. The curves are plotted 

 in such a way as to include like areas, so the numbers of individuals included are 

 given at the side of each curve. The upper limits are in some cases drawn on the 

 base line to show that such extremes appeared, although in too small numbers 

 to form a whole unit in plotting. 



