SEXUAL SELECTION 849 



mantle, less higUy colored than the upper and , exposed 

 surfaces.* In some cases, as with shells living among corals 

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

 a protection.' But that many of the nudibranch moUusca, 

 or sea-slugs, are as beautifully colored as any shells, may 

 be seen in Messrs. Alder and Hancock's magnificent work; 

 and, from information kindly given me by Mr. Hancock, 

 it seems extremely doubtful whether these colors usually 

 serve as a protection. With some species this may be the 

 case, as with one kind which lives on the green leaves of 

 algse, and is itself bright-green. But many brightly colored, 

 white, or otherwise conspicuous species do not seek conceal- 

 ment; while again some equally conspicuous species, as well 

 as other dull-colored kinds, live under stones and in dark 

 recesses. So that with these nudibranch mollusks, color 

 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 which have extremely 

 pretty shells. It is conceivable that two hermaphrodites, 

 attracted by each other's greater beaUty, might unite and 

 Jeave offspring which would inherit their parents' greater 

 beauty. But with such lowly organized 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 of the less 

 beautiful, so as to increase in number, unless indeed vigor 

 and beauty generally coincided. We have not here the case 

 of a number of males becoming mature before the females, 

 with the more beautiful males selected by the more vigor- 

 ous females. If, indeed, brilliant colors were beneficial to 



* I have given ("Geolog. Observations on Volcanic Islands," 1844, p. 53) 

 a curious instance of the Influence of light on the colors of a frondesoent 

 incrustation, deposited by the surf, on the coast-rocks of Ascension, and formed 

 by the solution of triturated sea-shells. 



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

 Coloration of MoUusca. "Proceedings of the Boston Society of Natural His- 

 tory," vol. xiv., April, 18U. 



