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 
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Fic. 2,— A male of the copepod, Calocalanus plumulosus. B and C,a male and 
a female of Calocalanus pavo. (After Giesbrecht.) 
which has a gorgeous tail worthy of a peacock, and of Cado- 
calanus plumulosus, in which one of the sete 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 
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