12 PHYSIOLOGICAL GENETICS 



example illustrating our point we reproduce a representation of 



Hersh's data for different lines of Wild, Bar, and Ultrabar at 

 different temperatures, plotting; temperature against the loga- 

 rithms of facet number (Fig. 2). The point of interest for the 

 descriptions in this chapter is the phenocopic effeet in both 

 directions between Bar and Ultrabar: at 15°, genetic Ultrabar 

 resembles genetic Bar at 25°; etc. Just as the Bar series of 

 multiple allelomorphs controls different numbers of ommatidia,so 

 temperature acting upon the larvae shifts the phenotypes within 

 the series of existing or possible allelomorphic phenotypes. 



Still more interesting in regard to the analysis of gene action is 

 the work on temperature action upon the wing of the mutant ves- 

 tigial in Drosophila, inaugurated by Roberts (1918) and continued 

 by Harnly (1930-1935), Nadler (1926),Riedel (1935), and Stanley 

 (1928-1935). Roberts was the first to show that an increase in 

 temperature during the larval stage of vestigial flies increased the 

 length of the wing through all the intermediate stages up to a 

 normal wing. Like all his successors he expressed this change in 

 terms of wing length or growth. It is, in fact, something very 

 different, as will be shown later, but for the sake of description and 

 measurement these terms may pass for the present. The point of 

 importance at this juncture is that the stages of "lengthening" of 

 the wing reproduce the phenotypes of the series of allelomorphs of 

 vestigial. More exact data were obtained by Harnly (1930). 

 The following table gives his results in terms of wing length; it has 



Table 2 



to be added that, with length, breadth also increases and that the 

 general shape of the wing (the amount of scalloping) assumes the 



