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 ^ 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 Ccelentera. In Crustacea it occurs among some 



