312 SEXUAL SELECTION [Tart II. 



CHAPTER IX. 



SECONDARY SEXUAL CHARACTERS IN THE LOWER CLASSES 



OF THE ANIMAL KINGDOM. 



These Characters absent in the Lowest Classes. — Brilliant Colors. — Mol- 

 lusca. — Annelids.— Crustacea, Secondary Sexual Characters strongly 

 developed; Dimorphism; Color; Characters not acquired before 

 Maturity. — Spiders, Sexual Colors 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 

 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 feel mutual rivalry, or to appreciate each 

 other's beauty or other attractions. 



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. Nevertheless some 

 few apparent exceptions occur; thus, as I hear from Dr. 

 Baird, the males of certain Entozoa, or internal parasitic 

 worms, diifer slightly in color from the females ; but we 



