i THE COLOURS OF ORGANISMS 9 
marked surface gloss is, of course, very obvious, and, 
though the size of the coloured particles is too small 
to make it easy to note the effect produced by injury 
to this surface, yet the disappearance of the colour 
when the surface is thoroughly wetted with water or 
oil can be observed very readily. 
Having now distinguished generally between the 
colours due to pigment and those which are the 
result of a special structure, we must proceed to 
consider these different methods of colour-production 
in detail. As much less is known of the structural 
colours than of the pigmental, we shall devote the 
remainder of this chapter to the former, leaving the 
latter for a further chapter. 
Characters of Structural Colours and 
their Classification 
i. White is probably the simplest and most readily 
understood of all structural colours, and, except in 
rare cases, is always structural. The colours of the 
lily, of white feathers, of Arctic mammals, are all due 
to the same cause ; namely, the total reflection of 
light produced by the intercalation of numerous 
bubbles of air, or some other gas, among colourless 
solid particles. Thus in the lily the colourless cells 
are separated by very numerous intercellular spaces 
containing air. This is one of the simplest forms of 
structural colour, not only because it is so readily 
explicable physically, but also because there is no com¬ 
plication arising from the presence of pigment in the 
tissues, and because the modification of “ structure ” 
necessary to produce it is of the simplest nature. 
