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DARWINISM 
CHAP. 
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 arc 
absorbed. Interference colours are produced either by thin 
films or by very fine stria; on the surfaces of bodies, which 
cause rays of certain wave-lengths to neutralise each other, 
leaving the remainder to produce the effects of colour. Such 
