The Descent of Man

sex. Nevertheless some few apparent exceptions
occur; thus, as I hear from Dr. Baird, the
males of certain Entozoa, or internal parasitic
worms, differ slightly in colour from the females;
but we have no reason to suppose that such
differences have been augmented through
sexual selection. Contrivances by which the
male holds the female, and which are indispensable
for the propagation of the species, are
independent of sexual selection, and have been
acquired through ordinary selection.
Many of the lower animals, whether hermaphrodites
or with separate sexes, are ornamented
with the most brilliant tints, or are shaded and
striped in an elegant manner; for instance, many
corals and sea-anemones (Actiniae), some
jelly-fish (Medusae, Porpita, etc.), some Planariae,
many star-fishes, Echini, Ascidians, etc.;
but we may conclude from the reasons already
indicated, namely, the union of the two sexes in
some of these animals, the permanently affixed