THE PURE LINE AND SELECTION 148 



a. Tower's Potato-Beetles 



As an illustration of the effect of selection within 

 pure lines may be mentioned Tower's exhaustive experi- 

 ments on the Colorado potato-beetle Leptinotarsa 

 decemlineata. These beetles had been inbred for such an 

 extended period that they were presumably homozygous 

 for the character of color. Among the numerous cul- 

 tures which were under control, a considerable varia- 

 tion in color, nevertheless, made its appearance. For 

 convenience in classification these variations were 

 graded into arbitrary classes or graduated variants 

 ranging from dark to light. 



When a male and a female from the extreme class at 

 the dark end of the series were allowed to breed together, 

 their progeny were not dark, but fluctuated in color 

 around the original average of the entire series. The 

 process of selecting each time an extreme pair of dark 

 parents was continued for twelve generations, as shown 

 in Figure 31, without in any way increasing the per- 

 centage of brunette potato-beetles in the progeny. 



Thus in a pure line formed by the breeding of two 

 individuals, alike with respect to color, the selection of 

 an extreme variant was quite without effect in modifying 

 the color of the progeny. 



b. Drosophila Bristles 



Among the "hairs" on the scutellum of Drosophila 

 melanogaster there are four larger hairs or bristles, 

 as shown in Figure 32. 



