The Descent of Man

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