II 
THE PIGMENTS OF ORGANISMS 
35 
2. Derived Pigments 
The second group of pigments includes those which 
are formed by the decomposition or breaking down 
of the pigments of the first group. These are prob¬ 
ably always useless so far as any direct function is 
concerned, and it might be thought that they should 
be simply classed as waste products. It seems, how¬ 
ever, better at present to reserve the term waste 
products, at least in animals, for those results of 
nitrogenous metabolism to which the term is usually 
applied by physiologists. We have already mentioned 
that hemoglobin undergoes a series of retrogressive 
changes, resulting in the production of various 
pigments. Most of these, in Vertebrates at any rate, 
are of little importance in producing coloration, are 
speedily eliminated from the body, and in our own 
species are chiefly of interest to the practical physician. 
There are, however, some exceptions. Melanin, the 
dark pigment which colours the hair or skin of most 
mammals, and which, as we have seen, is indirectly 
of importance in producing many of the gorgeous 
tints of birds, is thought by some authorities to be 
a derivative of haemoglobin. Again, the frequently 
exquisite colours of birds’ eggs are almost certainly 
due to pigments derived from the blood. It is quite 
possible that this group of pigments may ultimately 
turn out to be a large one, but as yet haemoglobin 
is one of the few pigments whose metamorphoses 
have been fully worked out. 
