28 COLOUR IN NATURE cha^ 



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 : — 



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. 



I. 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 



