214 GENETICS AND EUGENICS 



strongly intergrading condition." This fact shows that minor 

 genetic changes have occurred subsequent to the original 

 mutation either in the same genetic locus, or loci, or in 1 

 other genetic loci. Banta has shown that the degree of in- ^ 

 tergradeness is considerably influenced by environmental 

 conditions, but that the facts are as stated, when all needed 

 control observations are made. This leads to the strong con- 

 viction that genetic changes probably occur with consider- 

 able frequency in the parthenogenesis of animals as well as 

 in bud- variation among plants. ^ There is, however, some 

 negative evidence on record. Ewing selected for forty -four 

 parthenogenetic generations a species of plant louse. Aphis 

 avenoB, which was observed to vary as to length of body, 

 length of antennae, and length of cornicles (honey-dew tubes) . 

 All the observed variations were apparently due to environ- 

 mental conditions, because no permanent modification of 

 the race was effected by selection. The variation curve went 

 up and down with change in environmental conditions 

 (temperature and the like) but returned to normal when 

 normal conditions were restored. Hence it appears that 

 genetic variations affecting size were not occurring with any 

 considerable frequency, if at all, in the particular characters 

 studied at the particular time they were studied. This is 

 not surprising when we consider what a specialized organism 

 a plant louse is, adapted and limited to a particular host 

 plant. But a single positive case, like that studied by Banta, 

 outweighs any number of negative cases so far as concerns 



^ Banta (1919) has also studied the effects of selection, in pure line cultures of 

 Simocephalus vetulus, upon the sensitiveness of this species to light stimulation, as 

 measured by its reaction time. The selection experiment was continued for 54 

 months, 181 parthenogenetic generations. In the first two-month period, no differ- 

 ence could be detected in the average reaction time of plus selected and minus 

 selected strains of the same pure line, but subsequently the two strains gradually 

 diverged in reaction time so that "in the final ten generations the strain selected 

 for greater reactiveness to light had a reaction time less than one-third as large as 

 that for the strain of the same line selected for reduced reactiveness to light." No 

 differences in general vigor between the selected lines could be detected. The 

 change was a specific one in relation to light reactiveness and had been attained 

 gradually. 



