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. Pigments
absorb certain rays and reflect the remainder, and this reflected
portion has to our eyes a definite colour, according to the portion of
the rays constituting white light which are absorbed.
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