Decorative colouring of male butterflies and birds--Wallace's interpretation--Preponderance of males--Choice of the females--Sense by 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 crabs--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 be inflated on the head of birds--Absence of secondary sexual characters in lower animals--Transference of male characters to the females--Lycæna--Parrots--Fashion operative in the phyletic modifications of colour--Pattern of markings on the upper surface of a butterfly'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 in which we have studied it so far.