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 
