266 



The Descent of Man. 



Part II. 



were best able to hold her, have left the greatest number of 

 progeny to inherit their respective advantages.' 

 In some of the lower crustaceans, the right anterior antenna 

 of the male differs greatly in structure 

 from the left, the latter resembling in 

 its simple tapering joints the antennae 

 of the female. In the male the 

 modified antenna is either swollen in 

 the middle or angularly bent, or 

 converted (fig. 4) into an elegant, 

 and sometimes wonderfully complex, 

 prehensile organ.' It serves, as I hear 

 from Sir J. Lubbock, to hold the 

 female, and for this same purpose one 

 of the two posterior legs (b) on the 

 same side of the body is converted 

 into a forceps. In another family the 

 inferior or posterior antennEe are 

 " curiously zigzagged " in the males 

 alone. 



In the higher crustaceans the an- 

 terior legs are developed into chelae 

 or pincers ; and these are generally 

 larger in the male than in the female, 

 — so much so that the market value of 

 the male edible crab {Cancer pagtirus), 

 according to Mr. C. Spence Bate, is 

 five times as great as that of the fe- 

 male. In many species the chelse are 

 of unequal size on the opposite side of 

 the body, the right-hand one being, as 

 I am informed by Mr. Bate, generally, 

 though not invariably, the largest. This inequality is also often 

 much greater in the male than in the female. The two chelse 

 of the male often differ in structure (figs. 5, 6, and 7), the 

 smaller one resembling that of the female. What advantage is 

 gained by their inequality in size on the opposite sides of the 



Fig. 4. Labidocera Darwinli 

 (from Lubbock). 



a. Part of rigbt anterior an- 

 tenna of male, forming a 

 prehensile organ. 



&. Posterior pair of thoracic legs 

 of male. 



c. Ditto of iemale. 



* * Facts and ArgumeDts for 

 Darwin,' English translat. 1869, p. 

 20. See the previous discussion on 

 the olfactory threads. Sars has 

 described a somewhat analogous 

 case (as quoted in * Nature,' 1870, 

 p. 45.5) in a Norwegian crustacean, 

 the P'lntop'iretit afjitii^, 



° See Sir J. Lubbock in 'Annals 



and Mag. of Nat, Hist.' vol. xi. 

 1853, pi. i. and a.; and vol. xii. 

 (1853) pi. Tii. See also Lubbock in 

 ' Transact. Ent. Soc' vol. iv. new 

 series, 1856-1858, p. 8. With re- 

 spect to the zig-zagged antennae 

 mentioned below, see Fritz llnller, 

 ' Facts .-md Arguments foi- Darwiu,' 

 18u9, p. 40, fuot-uote. 



