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 ( Habia ) 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- 
