Chap. VIII.] SEXUAL SELECTION. 2i1 



alone of certain moths and of some crustaceans (e. g., Ta- 

 nais) have imperfect, closed mouths, and cannot feed. 

 The complemental males of certain Cirripedes live like 

 epiphytic plants either on the female or hermaphi'odite 

 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 important organs, which the other mem- 

 bers of the same group possess. In otlier cases it is the 

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

 glowworm 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 (Curculionidce) there is a great 

 difference between the male and female in the lensrth of 

 the rostrum or snout ; ^ but the meaning of this, and 

 of many analogous differences, is not at all understood. 

 Differences of structure between the two sexes in relation 

 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 differences 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 procuring it. A male ani- 

 mal which lived for a very short time might without det- 

 riment 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, or walking, if she gradually acquired habits 

 which rendered such powers useless. 



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

 Fritz Midler. 



* Kirby and Spence, ' Introduction to Entomology,' voL iii. 1820, p. 809. 



