264 Th^ Descent of Man. Part 11. 



living amongst corals or brightly-tinted sea-weeds, the bright 

 colours may serve as a protection.' But that many of the midi- 

 branoh moUusca, or sea-slugs, are as beautifully coloured 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 colours usually serve 

 as a protection. With some species this may be the case, as with 

 one kind which hves on the green leaves of algse, 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 dark recesses. So that with these nudi- 

 branch 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 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 oifspring from the 

 more beautiful pairs of hermaphrodites would have any ad- 

 vantage over the offspring of the less beautiful, so as to increase 

 in number, unless indeed vigour 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 vigorous females. If, indeed, brilliant 

 colours were beneficial to a hermaphrodite animal in relation 

 to its general habits of life, 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. 



8ub-hingdom of the Vermes: ClasB, Annelida (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 can be safely at- 

 tributed to sexual selection. These animals are often beauti- 

 fully coloured, but as the sexes do not differ in this respect, we 

 are but little concerned with them. Even the Nemertians, 

 though so lowly organised, "vie in beauty and variety of 

 " colouring with any other group in the invertebrate series;" yet 



^ Dr. Morse has lately discussed ' Proc. Boston See. of Nat. Hist, 

 tl^is subject in his paper on the vol. xiv., April, 1871. 

 Adaptivf Colciation of MoUusca, 



