28 
COLOUR IN NATURE 
CHAP. 
The chemistry of most is so imperfectly known that 
the composition cannot be employed as a basis, while 
the suggestions as to physiological function, with 
which the literature of the subject is full, are for the 
most part merely guesses. But while we admit that 
it is at present impossible to draw up a logical 
classification, it may be well to mention certain 
categories into which some of the better known 
pigments are said to fall. These are as follows :— 
i. Pigments of direct physiological importance, as 
in respiration, etc. 
Native J 2. Derivatives of such pigments. 
Pigments. 3. Waste products or modifications of such. 
4. Reserve products or pigments associated with 
, reserves. 
5. Introduced pigments. 
The pigments falling under these heads we shall 
consider in some detail. 
1. Pigments of Direct Physiological 
Importance 
The pigments in this group have always attracted 
much attention, and have been attentively studied in 
many cases. As types we may take haemoglobin 
and chlorophyll, the one so characteristic of higher 
animals, the other of plants. 
Hcemoglobin .—As is well known, haemoglobin, the 
red colouring-matter of the blood of Vertebrates, is 
a compound of a pigment containing iron (haematin) 
and a proteid, which is one of the few as yet 
obtained in crystalline condition. Its great im - 
portance is due to the fact that, owing probably to 
