326 SEXUAL SELECTION. Part II. 



of Hectocotyle. But this marvellous structure may be 

 classed as a primary rather than as a secondary sexual 

 character. 



Although with the Mollusca sexual selection does not 

 seem to have come into play ; yet many univalve and 

 bivalve shells, such as volutes, cones, scallops, &c, are 

 beautifully coloured and shaped. The colours do not 

 appear in most cases to be of any use as a protection ; 

 they are probably the direct result, as in the lowest 

 classes, of the nature of the tissues ; the patterns and 

 the sculpture of the shell depending on its manner of 

 growth. The amount of light seems to a certain extent 

 to be influential ; for although, as repeatedly stated by 

 Mr. G-wyn Jeffreys, the shells of some species living at a 

 profound depth are brightly coloured, yet we generally 

 see the lower surfaces and the parts covered by the 

 mantle less highly coloured than the upper and exposed 

 surfaces. 3 In some cases, as with shells livino- ainono-st 

 corals or brightly-tinted sea-weeds, the bright colours 

 may serve as a protection. But many of the nudibranch 

 mollusca, or sea-slugs, are as beautifully coloured as 

 any shells, as may be seen in Messrs. Alder and Han- 

 cock's magnificent work ; and from information kindly 

 given me by Mr. Hancock, it is extremely doubtful 

 whether these colours usually serve as a protection. 

 With some species this may be the case, as with one 

 which lives on the green leaves of algae, and is itself 

 bright-green. But many brightly-coloured, white or 

 otherwise conspicuous species, do not seek concealment; 

 whilst again some equally conspicuous species, as well 

 as other dull-coloured kinds, live under stones and in 



3 I have given ('Geolog. Observations on Volcanic Islands,' 1844, 

 p. 53) a curious instance of the influence of light on the colours of 

 a frondescent incrustation, deposited by the surf on the coast-rocks of 

 Ascension, and formed by the solution of triturated sea-shells. 



