112 SEX INHERITANCE 



testes in the male is presumably the essential 

 condition that brings about the development of 

 horns in that sex when only one gene for horns is 

 present. We have already seen that the castrated 

 Merino ram fails to develop horns. In this case, 

 then, some substance is produced by the testes 

 that causes horns to develop. In another race, in 

 which both sexes are horned, but the horns of the 

 male are larger than those of the female, castration 

 limits the development of the horns to the condition 



Fig. 36D. — Euschistus. To left, E variolarius male; to right, 

 E servus male. (After Foot and Strobell.) 



shown by the females. In this race the testes cause 

 the horns to develop to a higher stage than that 

 shown by the female. 



The second case is that of the two species of 

 bugs (Fig. 36 D) viz. Euchistus variolarius and E. 

 servus. The former has, in the male, a black spot 

 on the ventral surface of the end of the abdomen, 

 which is lacking in the male of the other species. 

 Among the females of both species the spot is 

 lacking. Foot and Strobell have shown that a 



