LECTURE XI 



SEXUAL SELECTION 



Decorative colouring of male butterflies and bii-ds— Wallace's interpretation— 

 Preponderance of males— Choice of the females — Sense hy sight in butterflies- 

 Attractive odours— Scent-scales— Fragrance of the females— The limits of natural 

 and sexual selection not clearly defined — Odours of particular species — Odours of 

 other animals at the breeding season — Song of the Cicadas, and of birds — Diversity 

 of decoration successively acquired — Humming-birds — Substitution of other aids to 

 wooing in place of personal decoration — Smelling organs of male insects and ci-abs — 

 Contrivances for seizing and holding the female — Small size of certain males— Weapons 

 of males used in struggle for the females — Turban eyes of Ephemerids — Hoods that 

 can he inflated on the head of birds — Absence of secondary sexual characters in lower 

 animals — Transference of male characters to the females — Lyca?na — Parrots— Fashion 

 operative in the phyletic modifications of colour — Pattern of markings on the upper 

 surface of a butterfiy's wing simpler than on the under side — Conclusion. 



We found in the process of Natural Selection an explanation 

 of numerous effective adaptations in plants and animals, as regards 

 form, colouring, and metabolism, of the most diverse weapons and 

 protective devices, of the existence of those forms of blossoms which 

 we call flowers, of instincts, and so on. The origin of the most 

 characteristic parts of whole orders of insects can only be understood 

 as adaptations to the environment brought about by means of natural 

 selection. Impressed by this, we have now to ask whether all the 

 transformations of organisms may not be referred to adaptation 

 to the continually changing conditions of life? We shall return to 

 this question later, but in the meantime we are far from being 

 able to answer it in the affirmative, for there are undoubtedly 

 a great many characters, at least in animals, which cannot have 

 owed their origin to natural selection in the form iu which we have 

 studied it so far, 



How could the sjjlendid plumage of the humming-birds, of the 

 pheasants, of the parrots, the wonderful colour-patterns of so many 

 diurnal buttei-flies, be referred to the process of natural selection, 

 since all these characters can have no significance for their possessors 

 in the struggle for existence? Or of what use in the struggle for 

 existence could the possession of its gorgeous dress of feathers be to 

 the bird of Paradise ; or of what service is the azure blue iridescence 

 of the Morpho of Brazil, which makes it conspicuous from a distance 



