RESULTS OF CROSSING. 329 



back-cross. We used, however, the same method as for the 

 longer and more complicated coils of servus, i. e., photographing 

 each intromittent organ at a magnification of 20 diameters, and 

 measuring the photographs with a small pair of architect's 

 dividers. (For details see Plates I.-III., p. 337.) 



TABLE III. 



Lengths of intromittent organs from 71 specimens of offspring from back-cross 

 i. e.,i 9 (from E. variolarius 9 X E. iclericus d" ) by E. ictericus c? 



X 20 diams. 

 60 mm 2 



62 mm 2 



62.5 mm i 



63.5 mm i 



64 mm 4 



66 mm 8 



66.5 mm 3 



67 mm I 



68 mm 7 



68.5 mm I 



69 mm 7 



70 mm 10 



70.5 mm 2 



71 mm 3 



72 mm 7 



72.5 mm 2 



73 mm i 



73.5 mm i 



74 mm. - 3 



74.5 mm. i 



75 mm i 



76 mm 2 



76.5 mm i 



4,897.5 mm TI_ 



Mean length = 68.97 mm. 



Genital Spot. A study of the inheritance of the genital spot 

 in the offspring from the back-cross of variolarius-ictericus is 

 of interest merely as furnishing additional data in support of 

 the results given by the crosses of variolaritts by servus. These 

 results, however, are by no means as definite as those from 

 the intromittent organ, for the dimensions of the genital spot 

 can be only relatively stated, they cannot be accurately measured 

 as in the case of the length of the intromittent organ. The in- 



