32 COLOUR IN NATURE chap. 



globin in these cases is functional in a degree at 

 all comparable to that seen in Vertebrates. If it 

 were, we should expect that when once acquired- 

 by the members of a group it would be retained by 

 all their descendants, however widely they might 

 diverge in other respects ; and the irregular distribu- 

 tion of haemoglobin among the Invertebrate groups is 

 contrary to this supposition. 



If, however, we accept Macallum's theory that 

 haemoglobin is a modification of nuclear chromatin, 

 then it may quite well be that it has a primary 

 meaning in metabolism which accounts for its 

 presence in Invertebrates and in the muscles of 

 Vertebrates, while under certain conditions, as in the 

 case of the haemoglobin of the blood of almost all 

 Vertebrates, it may acquire supreme importance as 

 a respiratory pigment. Such a suggestion would 

 explain many peculiarities of distribution which are 

 at present exceedingly puzzling. 



Haemoglobin is of course a chemical compound 

 of great complexity. Just as it is itself probably 

 formed by the modification of a still more complex 

 substance, so it in its turn undergoes a process of 

 breaking down, during which it gives origin to a 

 number of simpler substances. Some of these form 

 pigments which will have to be considered under 

 Group 2. 



Chlorophyll. — Chlorophyll is, like haemoglobin, a 

 compound of considerable complexity, but owing to 

 its great instability its exact composition is unknown. 

 We have, further, no exact knowledge as to the part 

 it plays in the metabolism of the plant. It is of 

 course a familiar fact of experience that green plants 



