C >UP. IX. 
SEXUAL SELECTION. 
321 
CHAPTER IX. 
Secondary Sexual Characters in the Lower Classes of 
the Animal Kingdom. 
Hose characters absent in the lowest classes i'rillifiut colours 
Mollusca — Annelids — Crustacea, secondary sexual characters 
strongly developed ; dimorphism ; colour; chaiacters not acquired 
before maturity — Spiders, sexual colours of; stridulation by the 
males — Myriapoda. 
^ the lowest classes the two sexes are not rarely united 
in the same individual, and therefore secondary sexual 
characters cannot he developed. In many cases in which 
die 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 
hiuch too low mental powers to feel mutual rivalry, 
°r to appreciate each other’s beauty or other attrac- 
tions. 
Hence in these classes or sub-bin gdoms, such as the 
•Protozoa, Ocelenterata, Echinodermata, Scolecida, true 
secondary sexual characters do not occur ; and this fact 
a grees with the belief that such characters in the 
higher classes have been acquired through sexual selec- 
tion, which depends on the will, desires, and choice ot 
either sex. Nevertheless some few apparent exceptions 
°ccur; thus, as I bear from Dr. Baird, the males of 
certain Eutozoa, or internal parasitic worms, differ 
slightly in colour from the females ; hut we have no 
r eason to suppose that such differences have been 
augmented through sexual selection. 
VOL. i. 
Y 
