Darwin's Theory of Sexual Selection 177 



species of Squilla, and a species of Gelasimus. In the latter 

 case Darwin thinks that the difference is probably due to 

 sexual selection. In addition to these cases, recorded by 

 Darwin, there may be added the two remarkable cases, 

 shown in our Figure 2 A, B, of Calocalanus pavo, the female of 



Fig. 2. — A male of the copepod, Calocalanus plumulosus. B and C, a male and 

 a4bmaje of Calocalanus pavo. (After Giesbrecht.) 



which has a gorgeous tail worthy of a peacock, and of Calo- 

 calanus plumulosus, in which one of the setae of the tail is 

 drawn out into a long featherlike structure. In the former, 

 the male is much more modestly adorned, as shown in Fig- 

 ure 2 C ; in the latter species the male is unknown. 



In spiders, where as a rule the sexes do not differ much 



