3 1 8 SEXUAL SELECTION. [Part II. 



creatures this is extremely improbable. Nor is it at all 

 obvious how the offspring from the more beautiful pfiirs 

 of hermaphrodites would have any advantage, so as to in- 

 crease in numbers, over the offspring of the less beautiful, 

 unless indeed vigor and beauty generally coincided. We 

 have not here a number of males becoming mature before 

 the females, and the more beautiful ones selected by the 

 more vigorous females. If, indeed, brilliant colors were 

 beneficial to an hermaphrodite animal in relation to its 

 general habits of life, the more brightly-tinted individuals 

 would succeed loest and would increase in number ; but 

 this would be a case of natural and not of sexual selection. 



Sub-kingdom of the Vermes or Annulosa : Class, An- 

 nelida (or Sea-worms). — In this class, although the sexes 

 (when separate) sometimes differ from each other in char- 

 acters of such importance that they have been placed un- 

 der distinct genera or even families, yet the differences do 

 not seem of the kind which can be safely attributed to 

 sexual selection. These animals, like those in the preced- 

 ing classes, apparently stand too low in the scale for the 

 individuals of either sex to exert any choice in selecting 

 a partner, or for the individuals of the same sex to strug- 

 gle together in rivalry. 



Sub-kingdom of the Arthropoda : Class, Crustacea. — 

 In this great class we first meet with undoubted second- 

 ary sexual characters, often developed in a remarkable 

 manner. Unfortunately, the habits of crustaceans are 

 very imperfectly known, and we cannot explain the uses 

 of many structures peculiar to one sex. With the lower 

 parasitic species the males are of small size, and they 

 alone are furnished with perfect swimming-legs, antennte 

 and sense-organs ; the females being; destitute of these or- 

 gans, with their bodies often consisting of a mere dis- 



