88 



H. E. EWING. 



The results obtained for these last 4 generations of subline II 

 are further illustrated in Fig. 9 and Table IX. Older plants, 

 therefore, cause a remarkable diminution in the size of Aphis 

 avence individuals. Perhaps it is because these aphids are not 

 able to extract as much food from the harder tissues of older 

 plants, but it may be due to a change in the nature of the juices 

 sucked from the food plant, or due to both of these causes. 



The Effects of Selection for 44 

 Generations on a Fluctuating 



Variation. 



The results of selection in the 

 whole subline n are shown 

 graphically in Fig. 10. Here we 

 have plotted the exact lengths 

 of each individual selected, scale 

 X5O, and also the lengths re- 

 presenting each fraternal mean, 

 plotted to the same scale. The 

 barred vertical columns repre- 

 sent the lengths of the individuals 

 selected, and the solid vertical 

 columns represent the lengths 

 expressed by the fraternal means. 

 These two kinds of columns are 

 arranged in the diagram Fig. 10 

 so that the two belonging to 

 the same generation come in 

 pairs, the number of the genera- 

 tion being given at the top of the 

 dotted vertical guide line. The 



curved arrows, each of which passes from the top of the column 

 representing the length of the individual selected, to the top of 

 the column representing the length of the mean of its offspring 

 in the next generation, shows the amount of regression, or di- 

 gression, as the case may be, for the fraternal mean in question. 

 If we examine this figure we find that with but a single excep- 

 tion there is a dropping of the fraternal mean downward. The 



FIG. 9. Plotted results of selection 

 in the last four generations of isola- 

 tion No. ii. Curves plotted on the 

 same plan as in Figs. 7 and 8. 



