276 THE DESCENT OF MAN 



wonn is destitute of wings, as also are many female moths, 

 some of which never leave their cocoons. Many female 

 parasitic crustaceans have lost their natatory legs. In some 

 weevil-beetles (Curoulionidse) there is a great difference 

 between the male and female in the length 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. In the Huia of New Zealand the difference is won- 

 derfully great, and we hear from Dr. BuUer' that the male 

 uses his strong beak in chiselling the larvre of insects out 

 of decayed wood, while the female probes the softer parts 

 with her far longer, much curved, and pliant beak; and 

 thus they mutually aid each other. In most cases, differ- 

 ences of structure between the sexes are more or less directly 

 connected with the propagation of the species: thus a female, 

 which has to nourish a multitude of ova, requires more food 

 than the male, and consequently requires special means for 

 procuring it. A male animal which lives for a very short 

 time might lose its organs for procuring food through disuse, 

 without detriment; 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. 

 jC. We are, however, here concerned only with sexual selec- 

 tion. This depends on the advantage which certaiu-^indi- 

 viduals have over others of the same sex and species solely 

 in respect of reproductionX When, as in the cases above 

 mentioned, the two sexes differ in structure in relation to 

 different habits of life, they have no doubt been modified 

 through natural selection, and by inheritance limited to one 



' See Kirby and Spence's work, "Introduction to Entomology," voltune ML, 

 1826, page 309. 



» '^Birds of New Zealand," 1812, p. 66 



