246 COLOUR IN NATURE chap. 



crimination between pigmental and optical colours ; 

 but the facts are at least interesting, and it is quite 

 possible that the dull olive colours of many un- 

 specialised birds may be due to a mixture of lipo- 

 chrome and melanin, which, when separated, give 

 rise to vivid orange and black colours in the males. 

 We have already noticed facts of similar nature in 

 larval Amphibians as compared with the adults 

 (see p. 232). 



Distribution of Colour in Genera and 

 Colour Variation 



Closely associated with sexual colour differences 

 is the question of the distribution of colour in the 

 species of a genus, and in this connection a few 

 examples of the relation between yellow and red are 

 worth quoting from Mr. Keeler. That such a rela- 

 tion should exist at all is interesting, because it 

 presents some sort of parallelism to the relation 

 between red and yellow which exists in the 

 Lepidoptera. In that group there is some reason 

 to believe that the red pigment bears to the yellow 

 a direct chemical relation, but the reds and yellows 

 of birds are usually due to lipochromes, and the 

 relation between the red and yellow lipochromes is 

 still very obscure. 



Mr. Keeler lays down the general rule that 

 wherever red is present in a genus, yellow will also 

 be present. Thus in the grosbeaks {Habid) the 

 male of H. ludoviciana has a breast patch of bright 

 red, some of the wing-coverts being of the same 

 tint, in H. melanocephala these parts are lemon- 



