374 THE DESCENT OF MAN. 



wonderful horns, many with stridulating organs, and others 

 ornamented with splendid metallic tints. Hence it seems 

 probable that all these characters have been gained through 

 the same means, namely, sexual selection. With butterflies 

 we have the best evidence, as the males sometimes take 

 pains to display their beautiful colors; and we cannot be- 

 lieve that they would act thus, unless the display was of 

 use to them in their courtship. 



When we treat of birds we shall see that they present in 

 their secondary sexual characters the closest analogy with 

 insects. Thus many male birds are highly pugnacious, and 

 some are furnished with special weapons for fighting with 

 their rivals. They possess organs which are used during 

 the breeding-season for producing vocal and instrumental 

 music. They are frequently ornamented with combs, 

 horns, wattles and plumes of the most diversified kinds, 

 and are decorated with beautiful colors, all evidently for 

 the sake of display. We shall find that, as with insects, 

 both sexes in certain groups are equally beautiful, and are 

 equally provided with ornaments which are usually confined 

 to the male sex. In other groups both sexes are equally 

 plain-colored and unornamented. Lastly, in some few 

 anomalous cases the females are more beautiful than the 

 males. We shall often find, in the same group of birds, 

 every gradation from no difference between the sexes to an 

 extreme difference. We shall see that female birds, like 

 female insects, often possess more or less plain traces or 

 rudiments of characters which properly belong to the males 

 and are of use only to them. The analogy, indeed, in all 

 these respects between birds and insects is curiously close. 

 Whatever explanation applies to the one class probably 

 applies to the other; and this explanation, as we shall here- 

 after attempt to show in further detail, is sexual selection. 



