298 WILLIAM LAWRENCE TOWER. 



From this cross there came larvae which in the F^ generation 

 were all of the female parental type, and from these emerged a 

 single class of adults intermediate between the two parents, a 

 mid type. These adults, very uniform in character, when inbred, 

 gave in the Fg generation abundant progeny. The larvae of the 

 F2 generation were in the first stage all alike ; in the second stage 

 they were clearly divisible into two classes in the proportion of 

 white 245 : yellow 190; while in the third stage the yellow larvae 

 further divided into larvae which were yellow with black spots 

 on the back (YIS), and yellow without black spots on the back 

 (Yls). The white larvae similarly divided into two classes, white 

 with black spots (WhS), and white without black sopts (Whs), 

 The census of larvae in the third stage gave for Pair C : 



Each of these classes of larvae was somewhat variable, and it 

 was often difficult to decide in the case of a white larva whether 

 it belonged in the class with black spots or in the class without 

 black spots, because there is an almost continuous gradation of 

 color from zero to a full development of the dorsal color pattern. 

 In making the division, the practice was followed of examining 

 the larvae with a lens magnifying ten diameters, and if the slight- 

 est trace of pigment was detected in the centres in which pigment 

 develops, they were then classed as larvae with spots, even though 

 the spots were not visible to the unaided eye. 



Each of these four classes of larvae gave rise to three classes of 

 adults, that is, each one split into forms like the female type, 

 like the male type, and an intermediate type. In every case 

 each of the four classes has given a typical Mendelian splitting 

 in the second hybrid generation, but the ratios are in most cases 

 somewhat variable, owing to the necessity of breeding these F^ 

 hybrids under rather variable conditions. The behavior in this 

 culture is shown in Plate V. The census of the adults derived 

 from Pair C was as follows: 



