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Show 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 1\fr. 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 Hancock'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 algre, 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 n. frondescent incrustation, deposited by the surf on the coast-rocks of Ascension, and formed by the solution of tritumted sea-shells. •CJIAr. IX. MOLLUSCS AND ANNELIDS. 327 dark recesses. So that with these nudibranch molluscs colour apparently does not stand in any close relatio~ to the nature of the places which they inhabit. ~hese naked sea-slugs are ~ermaphrodites, yet they pa1r together, as do land-snails, many of which have -extremely pretty shells. It is conceivable that two h~rmaphr?dites, attracted by each others' greater beauty, might umte and leave offspring which would inherit their .parents' greater. beauty. But with such lowly'? r~amsed creat~res this is extremely improbable. Nor IS It at all obvwus how the offspring from the more beautiful pairs o~ hermap~rodites would have any advantage, so as to ~ncrease m ~umbers, over the offspring of the less beautiful, unless mdeed vigour and beauty generally coincided. We have not here a number of males becoming mature before the females and the more beautif~l ones se~e~ted by the mor~ vigorous females. If, mdeed, brilliant colours were beneficial to ~n her~aphrodite animal in relation to its general habits of hfe, 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. Su~-kingdom of the Vermes or Annulosa : Class, Annebda (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 ca~ ~e- safely a_ttributed to s~xual selection. These animals, hl..c ~hose m the precedmg classes, apparently stand too low m the scale, for the individuals of either sex to -e~ert any choice in selecting a partner, or for the indiVIduals of the same sex to struggle together in rivalry. |