74 EVOLUTION AND ANIMAL LIFE 



The characters may also be of the type called reciprocal, 

 that is, organs which exist in functional condition in one sex, but 

 in the other appear in rudimentary and often nonfunctional 

 forms, as the reduced horns of female antelopes and goats, the 

 undeveloped stridulating organs of female crickets and katydids, 

 small spurs on the female pheasant, reduced mammae of male 

 mammals, undeveloped mimicry of male butterflies, etc. 



FIG. 44. Male (A) and female (B) of the fly, Calotarsa insignia Aid., showing secondary 

 sexual characteristics on the feet of the male. (After Aldrich.) 



Finally the characters may be indifferent, that is, without 

 any apparent utility; as the reduced wings of numerous female 

 insects, the rudimentary alimentary canal of the male Rota- 

 toria, absence of antlers of female deer, loss of wings in insect 

 females, small differences in size and markings between males 

 and females, slight differences in wing form in hummingbirds, 

 dragon flies, and butterflies, differences in number of tarsal 

 and antennal segments in insects, etc. 



The explanation of these various differences between males 

 and females plainly cannot be a single one. The extreme vari- 

 ety of the secondary sexual differences of itself makes it neces- 

 sary to find more than one explanation for their existence. To 

 take the most obvious case, it is apparent that the useful 

 characters, such as the fighting antlers of the male deer, can be 

 explained probably by natural selection. At least these char- 

 acters fall readily into line with precisely that type of useful 

 specialization for whose explanation we rely on natural selec- 

 tion. So practically all those secondary sexual characters of 

 our first category, namely, those obviously useful to the pos- 

 sessor or to its young, such as organs of offense and defense, 

 brood pouches, food-producing or gathering organs, special 



