II THE PIGMENTS OF ORGANISMS 27 



apparently little or no change in composition. A 

 more remarkable case still is found in the bones of 

 certain fishes {Belone, Protopterus, Lepidosiren), of an 

 amphibian, and of a lizard. In these the bones are 

 coloured a bright green and have been found to 

 contain vivianite, a mineral consisting of a phosphate 

 of iron. This mineral has also been found in con- 

 nection with fossil bones, but is there probably the 

 result of a chemical change during the process of 

 fossilisation. 



In the vast majority of cases, however, the pigments 

 of plants and animals are definite organic compounds, 

 produced by the organism in which they occur. 

 Such pigments are of very varied characters and 

 composition, some being substances of great com- 

 plexity, while others are relatively simple. Just as 

 in the laboratory of the chemist, a coloured substance 

 capable of being employed as a pigment may be 

 produced at many stages in a series of chemical 

 transformations, so in the laboratory of nature 

 pigments may occur at many stages in metabolism. 

 This at once suggests an interesting question — is 

 there any relation between the colour and the 

 chemical composition of pigments ? That is, are the 

 pigments of simple chemical composition usually 

 certain particular colours, and those of complex 

 structure other colours? Such statements have 

 under various forms been made repeatedly, and later 

 we must consider them in detail, but there is as yet 

 no evidence to show that the colour of pigments 

 can be employed as a direct means of classification. 



The classification of the pigments of plants and 

 animals is indeed a matter of profound difficulty. 



