I THE COEOURS 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. 



