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The Descent of Man and Selection in Relation to Sex

Secondary Sexual Characters In The Lower Classes Of
The Animal Kingdom
These characters absent in the lowest classes--Brilliant colours--Mollusca --Annelids--
Crustacea, secondary sexual characters strongly developed; dimorphism; colour;
characters not acquired before maturity--Spiders, sexual colours of; stridulation by the
males--Myriapoda.
With animals belonging to the lower classes, the two sexes are not rarely united in the
same individual, and therefore secondary sexual characters cannot be developed. In many
cases where the sexes are separate, both are permanently attached to some support, and
the one cannot search or struggle for the other. Moreover it is almost certain that these
animals have too imperfect senses and much too low mental powers to appreciate each
other's beauty or other attractions, or to feel rivalry.
Hence in these classes or sub-kingdoms, such as the Protozoa, Coelenterata,
Echinodermata, Scolecida, secondary sexual characters, of the kind which we have to
consider, do not occur: and this fact agrees with the belief that such characters in the
higher classes have been acquired through sexual selection, which depends on the will,
desire, and choice of either 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 condition of others, and the low mental powers
of all, that such colours do not serve as a sexual attraction, and have not been acquired
through sexual selection. It should be borne in mind that in no case have we sufficient
evidence that colours have been thus acquired, except where one sex is much more
brilliantly or conspicuously coloured than the other, and where there is no difference in
habits between the sexes sufficient to account for their different colours. But the evidence
is rendered as complete as it can ever be, only when the more ornamented individuals,
almost always the males, voluntarily display their attractions before the other sex; for we
cannot believe that such display is useless, and if it be advantageous, sexual selection will
almost inevitably follow. We may, however, extend this conclusion to both sexes, when
coloured alike, if their colours are plainly analogous to those of one sex alone in certain
other species of the same group.
How, then, are we to account for the beautiful or even gorgeous colours of many animals
in the lowest classes? It appears doubtful whether such colours often serve as a
protection; but that we may easily err on this head, will be admitted by every one who
reads Mr. Wallace's excellent essay on this subject. It would not, for instance, at first
 
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