42 COLOUR IN NATURE chap. 



plants and the simpler forms of animal life. Thus 

 the blue colours of many butterflies, of the feathers 

 and skin of birds, of the mandrill among mammals, 

 are all due to structural coloration, while the blue 

 of hyacinths, of some jelly-fish, and of the lobster 

 are pigmental colours. In flowering plants, as will be 

 seen in the chapter on plant pigments, a blue colour 

 is in some cases associated with an alkaline condition 

 of the cell-sap, but beyond this there is still much 

 uncertainty. In animals a blue colour occurs especi- 

 ally in surface forms, whatever their relations, e.g. 

 jelly-fish, molluscs as in lanthina, and Tunicates. It 

 is also remarkable as one of the conspicuous colours 

 of coral-reefs where it occurs in corals, sea-anemones, 

 Turbellarian worms and starfishes ; the blue colours 

 of the cuttles and fishes of the same situation are no 

 doubt optical. In general, therefore, in animals blue 

 pigmental colours occur in organisms of simple struc- 

 ture exposed to strong light. A comparative in- 

 vestigation of the blue pigments of surface animals 

 would be of great interest, but has yet to be made. 

 The two main hypotheses as to origin are on the one 

 side that of the protective value of the colour, and 

 on the other of the direct action of light as the 

 important factor ; it is not improbable that it may 

 turn out that it is abundance of free oxygen which 

 is the really important point. In general the blue 

 pigments of animals are characterised by their usual 

 solubility in water, their instability, and the fact that 

 they frequently give either the litmus reaction or some 

 modification of it on acidification. To what extent 

 this indicates affinity among them is at present quite 

 unknown ; apparently most are readily reduced. 



