Chap. IX. SEXUAL SELECTION. 321 



CHAPTER 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 

 strup-o-le 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 or sub-kingdoms, such as the 

 Protozoa, Coelenterata, Echinodermata, Scolecida, true 

 secondarv sexual characters do not occur ; and this fact 

 agrees 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 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. 



VOL. I. Y 



