538 THE WONDER OF LIFE 



be primarily any virtue in their colour, we hasten to point 

 out that they are often of very great physiological value, 

 and that their colour, as well as their chemical composition, 

 may be of vital importance. Speaking metaphorically, 

 we may say that this has been one of the methods of evolu- 

 tion to catch up some quality which is present for some 

 deep constitutional reason, and give it a novel secondary 

 value often life-saving, (a) The whole world of life 

 depends on the green pigment, chlorophyll, which is 

 characteristic of plants, for it is a condition of the photo- 

 synthesis or upbuilding of sugar and other organic com- 

 pounds in the leaves that the sunlight should reach the 

 living matter through the screen of chlorophyll. (6) The 

 red pigment, haemoglobin, which made its first appearance 

 (as far as we can judge) in some Eibbon- worms or Nemer- 

 teans, was also a physiological discovery of the highest 

 importance, for its capacity of entering into a loose union 

 with oxygen, and thus becoming an oxygen- carrier, must 

 have greatly facilitated and improved the function of res- 

 piration. Along with haemoglobin, which occurs in all Verte- 

 brates and in some Invertebrates (such as some Nemerteans 

 and Annelids), there have to be ranked a number of other 

 respiratory pigments. One of the commonest of these 

 among Invertebrate animals is haemocyanin, of a faint 

 bluish colour. In addition to transporting oxygen, some 

 pigments are of great value in storing it within the body, 

 e.g. in the muscles, (c) Another use of pigment is in 

 connexion with vision, for the dark pigments of the retina 

 are continually undergoing chemical change, and they 

 often show remarkable alterations in position. In the 

 peculiar condition known as night-blindness there appears 

 to be a lack of the normal * visual purple ' in the retina. 



