The modes in which colour is of use to different classes of organisms is very varied, and have probably not yet been all discovered; while the infinite variety and marvellous beauty of some of its developments are such as to render it hopeless to arrive at a complete and satisfactory explanation of every individual case.

So much, however, has been achieved, so many curious facts have been explained, and so much light has been thrown on some of the most obscure phenomena of nature, that the subject deserves a prominent place in any account of the Darwinian theory.
_The Problem to be Solved._ Before dealing with the various modifications of colour in the animal world it is necessary to say a few words on colour in general, on its prevalence in nature, and how it is that the colours of animals and plants require any special explanation.

What we term colour is a subjective phenomenon, due to the constitution of our mind and nervous system; while, objectively, it consists of light-vibrations of different wave-lengths emitted by, or reflected from, various objects.

Every visible object must be coloured, because to be visible it must send rays of light to our eye.

The kind of light it sends is modified by the molecular constitution or the surface texture of the object.