COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX Sex colours in the mollusca and crustacea--In insects--In butterflies and moths--Probable causes of these colours--Sexual selection as a supposed cause--Sexual coloration of birds--Cause of dull colours of female birds--Relation of sex colour to nesting habits--Sexual colours of other vertebrates--Sexual selection by the struggles of males--Sexual characters due to natural selection--Decorative plumage of males and its effect on the females--Display of decorative plumage by the males--A theory of animal coloration--The origin of accessory plumes--Development of accessory plumes and their display--The effect of female preference will be neutralised by natural selection--General laws of animal coloration--Concluding remarks.