MOLLUSCS. 261 



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 bright- 

 ly-colored, white or otherwise conspicuous species, do not seek 

 concealment; whilst again some equally conspicuous species, as 

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

 recesses. So that with these nudi-branch molluscs, color appar- 

 ently 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 to- 

 gether, 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- 

 organized creatures this is extremely improbable. Nor is it at all 

 obvious how the offspring from the more beautiful pairs of her- 

 maphrodites would have any advantage over the offspring of the 

 less beautiful, so as to increase in number, unle-ss 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 vigorous females. If, 

 indeed, brilliant colors were beneficial to a hermaphrodite ani- 

 mal 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. 



Sub-Mngdom of the Vermes: Class, Annelida (or Sea-worms). 

 — In this class, although the sexes, when separate, sometimes dif- 

 fer 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 beautifully 

 colored, but as the sexes do not differ in this respect, we are but 

 little concerned with them. Even the Nemertians, though so 

 lowly organized, "vie in beauty and variety of coloring with any 

 "other group in the invertebrate series;" yet Dr. Mcintosh" can- 

 not discover that these colors are of any service. The sedentary 

 annelids become duller-colored, according to M. Quatrefages,' 

 after the period of reproduction; and this I presume may be at- 



' See his beautiful monograph on 'British Annelids, part i. 1873, p. 3. 

 ' See M. Perrier, 'I'Origine de rHomme d'apres Dajwin,' 'Revue 

 Scientifique,' Feb. 1873, p. 866. 

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