SEXUAL SELECTION. 



193 



arrived at the conclusion that all or almost all the 

 colours of the higher animals (including among these 

 insects and all vertebrates) are due to voluntary or 

 conscious sexual selection ; and that diversity of colour 

 in the sexes is due, primarily, to the transmission of 

 colour-variations either to one sex only or to both sexes ; 

 the difference depending on some unknown law, and not 

 being due to natural selection. 



I have long held this portion of Mr. Darwin s theory 

 to be erroneous ; and have argued that the primary cause 

 of sexual diversity of colour was the need of protec- 

 tion, repressing in 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," in Contributions, 8jX., p. 231.) As I have since 

 given much thought to this subject, and have arrived 

 at some views which appear to me to be of consider- 

 able importance, 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, primarily, 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 







