v COLOURS OF ANIMALS 365 
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 the 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 ata 
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 
