46 
COLOUR IN NATURE 
CHAP. 
Hickson to perform the same functions as chlorophyll. 
It is apparently not chlorophyll (see Chap. IV.). 
Yellow is an exceedingly common and widely 
distributed colour, occurring most frequently, however, 
in animals in which other pigments are also present. 
It not infrequently occurs in the form of spots or 
stripes upon a dark ground, although it is common 
in flowers as a ground colour. In most cases yellow 
is due to the presence of yellow lipochromes, which 
are perhaps the most universally distributed of all 
pigments. In view of their exceedingly wide dis¬ 
tribution, it is perhaps remarkable that yellow animals 
should not be more common than they are. Apart 
from the yellow lipochromes we have as important 
yellow pigments lepidotic acid, the waste product 
which occurs in the wings of some butterflies, and 
the yellow pigments of the eggs of birds. The 
yellow or tawny colour of the hair in many mammals 
is not due to a special yellow pigment, but to the 
uniform distribution of a small amount of dark 
pigment. 
A red colour is perhaps always due to pigment, 
and red pigments are fairly numerous. Large 
numbers of red animals are coloured with lipochrome 
pigment; the red lipochromes indeed begin at the 
Protozoa and extend upwards to birds. Their 
colour varies from deep orange to pure rose-red, but 
all have an indescribable fatty appearance which 
makes them readily recognised by any one accus¬ 
tomed to working with lipochromes. Red is very 
common among marine animals, especially among 
Crustacea, Echinoderma, some Mollusca, Protozoa, 
and Coelentera. In Crustacea it occurs among some 
