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.—Luschistus. To left, E variolarius male; to right, 
Ie 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 
