260 ANIMAL COLORATION. 



Where the two sexes resemble each other in coloration the 

 tints may be brilliant (e.g., the toucans) or dull (e.g., the robin). 



It is among the humming birds (fig. 30), birds of paradise, 

 and many gallinaceous birds (fig. 31), that the most marked 

 differences of colour occur, associated very frequently with the 

 presence of accessory plumes or specially elongated feathers. 

 Compare, for instance, in the accompanying figure (fig. 32), 

 the male of the paradise bird (Cincinnurus regius), possessing 

 the extraordinary tail feathers, with the very ordinary-looking 

 female. 



Sexual Dimorphism of Colour most marked in Birds and Butterflies. 

 It is a very suggestive fact that sexual dimorphism in colour 

 is most marked in birds and butterflies ; in both of these 

 groups, particularly in birds, the colours are largely " struc- 

 tural" i.e., they are chiefly determined by the actual minute 

 structure of the part coloured though, as I have already 

 mentioned, a background of pigment is required to show off 

 the colours. Furthermore, in both these groups, as Mr. Wallace 

 has pointed out, the extent of the coloured surface is much 

 larger in proportion to the body than in many others. Hence 

 sexual differences in colour are much exaggerated, as a neces- 

 sary consequence : three or four bits of glass coloured in a very 

 slightly different way from three or four other bits, will, when 

 multiplied and arranged by a kaleidoscope, produce an effect 

 greatly diversified in the two cases. It is perfectly clear that 

 sexual differences of colour may exist in animals, where 

 nothing like sexual selection can come into play ; there is 

 therefore a groundwork for the production of the complicated 

 patterns we see in the feathers of birds. We presume that 

 birds have been evolved out of a lizard-like ancestor. Now, 

 lizards present us, occasionally, with sexual differences in 



