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 haemoglobin 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. 



