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 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 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.
SUB-KINGDOM OF THE VERMES: 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 which can be safely attributed to sexual selection. These
animals are often beautifully 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.
McIntosh (6. See his beautiful monograph on British Annelids, part i.
1873, p. 3.) cannot discover that these colours are of any service. The
sedentary annelids become duller-coloured, according to M. Quatrefages
(7. See M. Perrier: L Origine de l Homme d apres Darwin, Revue
Scientifique , Feb. 1873, p. 866.), after the period of reproduction;
and this I presume may be attributed to their less vigorous condition
at that time. All these worm-like animals apparently stand too low in
the scale for the individuals of either sex to exert any choice in
selecting a partner, or for the individuals of the same sex to struggle
together in rivalry.
295