350 THE DESCENT OF MAIf 



a hermaphrodite animal in relation to its general habits 

 of life, the more brightly tinted individuals would succeed 

 best and would increase in number; but this would be a 

 case of natural and not of sexual selection. 



Sub-kingdom of the Vermes: Class, Annelida {or Sea- 

 worms). — In this class, although the sexes, when separate, 

 sometimes differ from each other in characters of such im- 

 portance that they have been placed under distinct genera 

 or even families, yet the differencqp do not seem of the Mnd 

 which can be safely attributed to sexual selection. These 

 animals are often beautifully colored, but as the sexes do 

 not differ in this respect, we are but little concerned with 

 them. Even the Nemertians, though so lowly organized, 

 "vie in beauty and variety of coloring with any other 

 group in the invertebrate series"; yet Dr. Mcintosh" cannot 

 discover that these colors are of any service. The seden- 

 tary annelids become duller-colored, according to M. Qua- 

 trefages,' after the period of reproduction; and this I pre- 

 sume may be attributed to their less vigorous condition at 

 that time. All these worm-like animals apparently stand 

 too low in the scale for the individuals of either sex to 

 exert any cboice in selecting a partner, or for the individ- 

 uals of the same sex to struggle together in rivalry, 



Suh-hingdom of the Anthropoda: Class, Crustacea. — In this 

 great class we first meet with undoubted secondary sexual 

 characters, often developed in a remarkable manner. Un- 

 fortunately the habits of crustaceans are very imperfectly 

 known, and we cannot explain the uses of many structures 

 peculiar to one sex. With the lower parasitic species the 

 males are of small size, and they alone are furnished with 

 perfect swimming-leg§, antennae, and sense-organs; the fe- 

 males being destitute of these organs, with their bodies 



s See his beautiful monograph on "Britiah Annelids," part i., 1873, p. 3. 

 i See M. Perrier, "I'Origine de I'Homme d'apres Darwin," "Eevue Scien- 

 tiflque," Feb. 1813, p. 866. 



