■Chap. IX. 
MOLLUSCS AND ANNELIDS. 
327 
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 which have 
extremely pretty shells. It is conceivable that two 
hermaphrodites, attracted by each others’ 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 ad- 
vantage, so as to increase in numbers, over the offspring 
of the less beautiful, unless indeed vigour and beauty 
generally coincided. We have not here a number of 
males becoming mature before the females, and the 
more beautiful ones selected by the more vigorous 
females. If, indeed, brilliant colours were beneficial 
io an 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. 
Sub-kingdom of the Vermes or Annulosa: Class, 
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 wdiick can 
be safely attributed to sexual selection. These animals, 
like those in the preceding classes, apparently stand too 
low in the scale, for the individuals of either sex to 
exert any choice in selecting a partner, or for the indi- 
viduals of the same sex to struggle together in rivalry. 
