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COLOURS OF ANIMALSthe female those bright colours which are normally producedin both sexes by general laws and I;have attempted toexplain many of the more difficult cases on this principle("A Theory of Birds' Nests," chap. vi. ante). As I havesince given much thought to this subject, and have arrivedat some views which appear to me to be of considerable importance,it will be well to sketch briefly the theory I now hold,and afterwards show its application to some of the detailedcases adduced in Mr. Darwin's work.The very frequent superiority of the male bird or insectin brightness or intensity of colour, even when the generalcoloration is the same in both sexes, now seems to me to be,in great part, due to the greater vigour and activity and thehigher vitality of the male. The colours of an animal usuallyfade during disease or weakness, while robust health andvigour adds to their intensity. This is a most important andsuggestive fact, and one that appears to hold universally. Inall quadrupeds a " dull coat " is indicative of ill-health or lowcondition, while a glossy coat and sparkling eye are theinvariable accompaniments of health and vital activity. Thesame rule applies to the feathers of birds, whose colours areonly seen in their purity during perfect health and a;similarphenomenon occurs even among insects, for the bright huesof caterpillars begin to fade as soon as they become inactivepreparatory to undergoing their transformation, or if attackedby disease. Even in the vegetable kingdom we see tke samething, for the tints of foliage are deepest, and the colours offlowers and fruits richest, on those plants which are in themost healthy and vigorous condition.This intensity of coloration becomes most developed in themale during the breeding season, when the vitality is at amaximum. It is also very general in those cases in whichthe male is smaller than the female, as in the hawks and inmost butterflies and moths. The same phenomena occur,though in a less marked degree, among mammalia. Wheneverthere is a difference of colour between the sexes themale is the darker or more strongly marked, and the differenceof intensity is most visible during the breeding season(Descent of Man, p. 533). Numerous cases among domesticanimals also prove that there is an inherent tendency in the