188 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 consjiicuous a character as 

 colour, which so often constitutes the most obvious distinction 

 of sjDecies 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 coloiu" 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 eveiy 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 Avorld it is necessary to say a few Avords 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 Aasible 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 oliject. 

 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 stride 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 



