Chap. IX. 
SEXUAL SELECTION. 
321 
CHAPTEB IX. 
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. 
In the lowest classes the two sexes are not rarely united 
in the same individual, and therefore secondary sexual 
characters cannot be developed. In many cases in which 
the two sexes are separate, both are permanently at- 
tached 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 feel mutual rivalry, 
or to appreciate each other’s beauty or other attrac- 
tions. 
Hence in these classes, such as the Protozoa, Coelen- 
terata, Echinodermata, Scolecida, true secondary sexual 
characters 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, desires, and choice of either sex. Never- 
theless 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. 
VOL. i. 
Y 
