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106.) These animals appear also susceptible of some degree of permanent attachment: an accurate observer, Mr.

Lonsdale, informs me that he placed browser gratis download a pair of land-snails, (Helix pomatia), one of which was weakly, into a small and ill-provided garden. After a short time the strong and browser gratis download healthy individual disappeared, and was traced by browser gratis download its track of slime over a wall into an adjoining well-stocked garden. Lonsdale concluded that it browser gratis download had deserted its sickly mate; but after an absence of twenty-four hours it returned, and apparently browser gratis download communicated the result of its successful exploration, for browser gratis download both then started along the same track browser gratis download and disappeared over the wall. Even in the highest class of the Mollusca, the Cephalopoda or cuttle- fishes, in which the sexes are separate, secondary sexual characters of the present kind do not, as far as I can discover, occur. This is a surprising circumstance, as these browser gratis download animals possess highly-developed sense-organs and have considerable mental powers, as will be admitted by every one who has watched their artful endeavours to escape browser gratis download from an enemy. See, for instance, the account browser gratis download which I have given in my 'Journal of browser gratis download Researches,' 1845, p. 7.) Certain Cephalopoda, however, are characterised by one extraordinary sexual character, namely that the male element collects within one of the arms or tentacles, which is then cast browser gratis download off, and clinging by its sucking-discs to the female, lives for a time an independent life. So completely does the cast-off arm resemble a separate animal, that it was described by Cuvier as a parasitic worm under the name browser gratis download of Hectocotyle. But this marvellous structure may be classed as a primary rather than as a browser gratis download 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, etc., are beautifully coloured and shaped. The colours do not tetris download appear in most cases to be of any use download free web browser browser gratis download 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 word 2003 download browser gratis download manner of growth. The amount of light seems browser gratis download to be influential to a certain extent; for browser gratis download 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, as well as the parts covered by the mantle, less highly-coloured than the upper and exposed surfaces. I have given ('Geological Observations on Volcanic Islands,' 1844, p. 53) a curious instance of the influence of browser gratis download light on the colours of a frondescent incrustation, deposited by the surf on the coast-rocks of Ascension and formed by the solution of browser gratis download triturated sea-shells.) In some cases, as with shells browser gratis download living amongst corals or brightly-tinted seaweeds, the bright browser gratis download colours may serve as a protection.

Morse has browser gratis download lately discussed this subject in his paper on browser gratis download the 'Adaptive Coloration of Mollusca,' 'Proc. April 1871.) But that many of the nudibranch Mollusca, browser gratis download or sea-slugs, are as beautifully coloured as any shells, may be seen in Messrs. Alder and browser gratis download Hancock's magnificent work; and from information kindly given me by Mr. Hancock, it seems extremely doubtful whether these colours usually serve as browser gratis download a protection. With some species this may be browser gratis download the case, as with one kind which lives browser gratis download on the green leaves of algae, and is browser gratis download itself bright-green. But many brightly-coloured, white, or otherwise browser gratis download conspicuous species, do not seek concealment; whilst again some equally conspicuous species, as well as other dull-coloured kinds live under stones and in 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 browser gratis download which have extremely pretty shells. It is conceivable that two hermaphrodites, attracted by each other's 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 advantage over the offspring browser gratis download of the less beautiful, so as to increase browser gratis download in number, unless indeed vigour and beauty generally browser gratis download coincided. We have not here the case of browser gratis download a number of males becoming mature before the browser gratis download females, with the more beautiful males selected by the more vigorous females. If, indeed, brilliant colours were beneficial to a hermaphrodite animal in relation to its general habits of life, the browser gratis download 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.