326 
SEXUAL SELECTION. 
Pakt II- 
of Hectocotyle. But this marvellous structure may he 
classed as a primary rather than as a secondary sexual 
character. 
Although with the Mollusea sexual selection does not 
seem to have come into play ; yet many univalve and 
bivalve shells, such as volutes, cones, scallops, &c., ate 
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 o» 
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 th e 
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 colon 1 ' 3 
may serve as a protection. But many of the nudibrand 1 
mollusea, or sea-slugs, are as beautifully coloured llS 
any shells, as may be seen in Messrs. Aider and Ha"' 
cock’s magnificent work ; and from information kindly 
given me by Mr. Hancock, it is extremely doubt !' 1 
whether these colours usually serve as a protection 
With some species this may be the case, as with ° D ® 
which lives on the green leaves of algae, and is its e ' 
bright-green. But many brightly-coloured, white t,r 
otherwise conspicuous species, do not seek concealment 
whilst again some equally conspicuous species, as 
we'd 
as other dull-coloured kinds, live under stones and 1,1 
3 I have given (‘Geolog. Observations on Volcanic Islands,’ 18 
p. 53) a curious instance of the influence of light on the colours ^ 
a frondescent incrustation, deposited by the surf on the const-rocl s 
Ascension, and formed by the solution of triturated sea-she-lls. 
