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 , Protoptevus , 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. 
