COLOUR DUE TO PLANTS 155 



is formed and oxygen provided. The green colouring 

 matter of plants is, consequently, concerned in the 

 processes of replacing the matter of living substance. 

 Its fundamental importance and its simple require- 

 ments explain the ubiquity and abundance of green 

 colouring in plant-life. 



Amongst animals there is no such fundamental, 

 widespread, and nutritious pigment. That mysterious 

 and seemingly simple hold upon earth, air, and water 

 by which plants make their living, if it exists in animals, 

 is no prominent feature of their life. Their food is no 

 cunning mixture of gas and water. In spite of fable, 

 the chameleon does not live on air, but on flies ; nor 

 does the possession of a green colour by many frogs, 

 snakes, birds, insects, or worms confer upon these 

 animals (with one remarkable exception) the ability to 

 subsist without solid food, even where the green matter 

 has proved to be identical with that of plants. 



A plant living in an animal, as plants (Flagellates) 

 do (p. 106) in Protozoa, anemones, and some worms, 

 is only capable of keeping its host from starvation by 

 submitting to be eaten up piecemeal, and should 

 further solid food be lacking the menagerie dissolves 

 partnership. Whatever value we can attribute to 

 animal colouration, a nutritive one would seem to be 

 the least probable, and we turn to the other side of 

 life's balance-sheet, the income of energy, to see if 

 there is any animal pigment which has the property 

 of allying itself with the oxygen of air and of purveying 

 that oxygen to the tissues of the body. 



