Variation in an Aphis (Myzus ribis, Linn.) 37 



Table 2 shows the mean ratios of the successive generations in 

 four lines of descent, including the red and green controls. The 

 extinction of the green control line was unfortunate, and in future 

 experiments it will be very desirable to obtain a pure green line. 

 At present the explanation that suggests itself of the variation of 

 the RgGrg. . . line is that the influence of red feeding persists for at 

 least two, and probably three generations after removal to different 

 food, and this is somewhat confirmed by the R4G1. . . etc. line. 



Tables 3, 3a, 4, 4a and 5, 5a, give the effect of transference 

 upon the mean ratios of the first, second, and third generations 

 respectively, and below each is an analysis of the ratio of each 

 segment, indicating its increase or decrease over previous genera- 

 tions and the controls. 



Examination of the figures seems to show that the ratios of the 

 first generation after transference vary irrespectively of the 

 parental ratio. 



In transference to Red, the ratio of Seg. v. increases over that 

 of the parental ratio, but in Seg. vi. it decreases (Table 3). In 

 transference to Green, the results for both segments are quite 

 inconclusive as regards the parental ratio (Table 3a). In the 

 second generation after transference to Red, the results are like- 

 wise inconclusive for both segments (Table 4). After transference 

 to Green, the ratio of Seg. v. shows a tendency to rise above, 

 and Seg. vi. a tendency to fall below, the parental and grand- 

 parental ratios (Table 4a). 



In the third generation after transference to Red, the ratio of 

 Seg. v. rises above the ancestral ratios, and that of Seg. vi. falls 

 (Table 5). After transference to Green, the ratio of Seg. v. rises 

 above those of the ancestral generations, and that of Seg. vi. rises 

 in one case and falls in the other (Table 5a). 



These results are inconclusive, but examination of the control 

 ratios shows that, with occasional exceptions, the ratio of a genera- 

 tion with a mixed ancestry tends to rise above that of the Red 

 control, but remains below that of the Green. Many more experi- 

 ments in transference are required, and a much larger number of 

 individuals must be examined before any conclusion can be 

 reached; but at present the evidence suggests that the antennae 

 of Myzus ribis are modified according to the food supplied, and 

 that the effect induced by feeding in one generation is discernible 

 in the succeeding three or four generations. It is difficult otherwise 

 to explain the difference between the ratios of Rg and R2G4, and 

 between Rg and R4G3, which, translated into the terms of human 

 relationship, would be third cousins, and first cousins once removed, 

 respectively, for all were produced by parthenogenesis, and, 

 except for the food, reared side by side under identical environ- 

 mental conditions. 



