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 Ianthina , 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. 
