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. Gwyn 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 living amongst 
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 algse, 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 tbe 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. 
