260 THE DESCENT OP MAN. 



was weakly, into a small and ill-provided garden. After a short 

 time the strong and healthy individual disappeared, and was 

 traced by its track of slime over a wall into an adjoining well- 

 stocked garden. Mr. Lonsdale concluded that it had deserted its 

 sickly mate; but after an absence of twenty-four hours it re- 

 turned, and apparently communicated the result of its successful 

 exploration, for both then started along the same track and dis- 

 appeared over the wall. 



Even in the highest class of the Mollusca, the Cephalopoda or 

 cuttlefishes, 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 animals pos- 

 sess highly-developed sense-organs and have considerable mental 

 powers, as will be admitted by every one who has watched their 

 artful endeavors to escape from an enemy.' Certain Cephalopoda, 

 however, are characterized by one extraordinary sexual character, 

 namely, that the male element collects within one of the arms 

 or tentacles, which is then cast off, and clinging by its sucking- 

 discs to the female, lives for a time an independent life. So com- 

 pletely does the cast-off arm resemble a separate animal, that it 

 was described by Cuvier as a parasitic worm under the name of 

 Hectocctyle. But this marvelous structure may be classed as a 

 primary rather than as a secondary sexual character. 



Although with the Mollusca r,cxual selection does not seem to 

 have come into play; yet many univalve and bivalve shells, such 

 as volutes, cones, scallops, &c., are beautifully colored and shaped. 

 The colors 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 sculp- 

 ture of the shell depending on its manner of growth. The amount 

 of light seems to be influential to a certain extent; for although, 

 as repeatedly stated by Mr. Gwyn Jeffreys, the shells of some spe- 

 cies living at a profound depth are brightly colored, yet we gen- 

 erally see the lower surfaces, as well as the parts covered by 

 the mantle, less highly-colored than the upper and exposed sur- 

 faces.* In some cases, as with shells living amongst corals or 

 brightly-tinted sea-weeds, the bright colors may serve as a pro- 

 tection." But that many of the nudi-branch mollusca, or sea,-slugs, 



» See, for instance, the account which I have given in my 'Journal 

 of Eesearches,' 1845, p. 7. 



•> I have given ('Geolog. Observations on Volcanic Islands,' 1844, p. 53) 

 a curious Instance of the Influence of light on the colors of a frondes- 

 cent incrustation, deposited by the surf on the coast-rocks of Ascen- 

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



^ Dr. Morse has lately discussed this subject in his paper on the 

 Adaptive Coloration of Mollusca, 'Proc. Boston Soc. of Nat. Hist.,' 

 vol. xlv., April, 1871. 



