ours of animals are
useful, some hurtful to them; and he believed that many of the most
brilliant colours were developed by sexual choice; while his great
general principle, that all the fixed characters of organic beings have
been developed under the action of the law of utility, led to the
inevitable conclusion that so remarkable and conspicuous a character as
colour, which so often constitutes the most obvious distinction of
species from species, or group from group, must also have arisen from
survival of the fittest, and must, therefore, in most cases have some
relation to the wellbeing of its possessors. Continuous observation and
research, carried on by multitudes of observers during the last thirty
years, have shown this to be the case; but the problem is found to be
far more complex than was at first supposed. 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. 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. Interference
colours are produced either by thin films or by very fine striae on the
surfaces of bodies, which cause rays of certain wave-lengths