Chap. IX. MOLLUSCS AND ANNELIDS. 327 



dark recesses. So that with these nudibranch molluscs, 

 colour apparently does not stand in any close relation 

 to the nature of the places which they inhabit. 



These naked sea-slugs are hermaphrodites, yet they 

 pair together, as do land-snails, many of which have 

 extremely pretty shells. It is conceivable that two 

 hermaphrodites, attracted by each others' greater beauty, 

 might unite and leave offspring: which would inherit 

 their parents' greater beauty. But with such lowly- 

 organised creatures this is extremely improbable. Nor 

 is it at all obvious how the offspring from the more 

 beautiful pairs of hermaphrodites would have any ad- 

 vantage, so as to increase in numbers, over the offspring 

 of the less beautiful, unless indeed vigour 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 colours were beneficial 

 to an hermaphrodite animal in relation to its general 

 habits of life, the more brightly-tinted individuals would 

 succeed best and would increase in number ; but this 

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



Sub-kingdom of the Vermes : Class, Annelida {or Sea- 

 worms). — In this class, although the sexes (when separate) 

 sometimes differ from each other in characters of such 

 importance that they have been placed under 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 pre- 

 ceding 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 struggle together in rivalry. 



