Chap. VIII. SEXUAL SELECTION. 255 



crustaceans (e.g. Tanais) have imperfect, closed mouths, 

 and cannot feed. The Complemental males of certain 

 cirripedes live like epiphytic plants either on the female 

 or hermaphrodite form, and are destitute of a mouth 

 and prehensile limbs. In these cases it is the male 

 which has been modified and has lost certain import- 

 ant organs, which the females and other members of the 

 same group possess. In other cases it is the female 

 which has lost such parts ; for instance, the female glow- 

 worm is destitute of wings, as are many female moths, 

 some of which never leave their cocoons. Many female 

 parasitic crustaceans have lost their natatory legs. In 

 some weevil-beetles (Curculionidse) there is a great 

 difference between the male and female in the length 

 of the rostrum or snout ; 2 but the meaning of this and 

 of many analogous differences, is not at all understood. 

 Differences of structure between the two sexes in rela- 

 tion to different habits of life are generally confined to 

 the lower animals ; but with some few birds the beak 

 of the male differs from that of the female. No doubt 

 in most, but apparently not in all these cases, the dif- 

 ferences are indirectly connected with the propagation 

 of the species : thus a female which has to nourish a 

 multitude of ova will require more food than the male, 

 and consequently will require special means for procur- 

 ing it. A male animal which lived for a very short 

 time might without detriment lose through disuse its 

 organs for procuring food ; but he would retain his 

 locomotive organs in a perfect state, so that he might 

 reach the female. The female, on the other hand, 

 might safely lose her organs for -flying, swimming, 



regard to the statement about Tanais, mentioned below, I am indebted 

 to Fritz Muller. 



2 Kirby and Spence, 'Introduction to Entomology,' vol. iii. 1826, 

 p. 309. 



