COLOURS OF ANIMALS 



the female those bright colours which are normally produced 

 in both sexes by general laws ; and I have attempted to 

 explain many of the more difficult cases on this principle 

 ("A Theory of Birds' Nests," chap. vi. ante). As I have 

 since given much thought to this subject, and have arrived 

 at some views which appear to me to be of considerable im- 

 portance, it will be well to sketch briefly the theory I now hold, 

 and afterwards show its application to some of the detailed 

 cases adduced in Mr. Darwin's work. 



The very frequent superiority of the male bird or insect 

 in brightness or intensity of colour, even when the general 

 coloration is the same in both sexes, now seems to me to be, 

 in great part, due to the greater vigour and activity and the 

 higher vitality of the male. The colours of an animal usually 

 fade during disease or weakness, while robust health and 

 vigour adds to their intensity. This is a most important and 

 suggestive fact, and one that appears to hold universally. In 

 all quadrupeds a " dull coat " is indicative of ill-health or low 

 condition, while a glossy coat and sparkling eye are the 

 invariable accompaniments of health and vital activity. The 

 same rule applies to the feathers of birds, whose colours are 

 only seen in their purity during perfect health ; and a similar 

 phenomenon occurs even among insects, for the bright hues 

 of caterpillars begin to fade as soon as they become inactive 

 preparatory to undergoing their transformation, or if attacked 

 by disease. Even in the vegetable kingdom we see tke same 

 thing, for the tints of foliage are deepest, and the colours of 

 flowers and fruits richest, on those plants which are in the 

 most healthy and vigorous condition. 



This intensity of coloration becomes most developed in the 

 male during the breeding season, when the vitality is at a 

 maximum. It is also very general in those cases in which 

 the male is smaller than the female, as in the hawks and in 

 most butterflies and moths. The same phenomena occur, 

 though in a less marked degree, among mammalia. When- 

 ever there is a difference of colour between the sexes the 

 male is the darker or more strongly marked, and the differ- 

 ence of intensity is most visible during the breeding season 

 (Descent of Man, p. 533). Numerous cases among domestic 

 animals also prove that there is an inherent tendency in the 



