14 THE COLOURS OF ANIMALS 



patterns, their varying tints, and their relation to the 

 different parts of the body, have all been determined 

 by natural selection through innumerable generations. 

 Because the origin of all pigments is to be found in 

 the incidental result of the chemical and physical 

 nature of organic compounds, it by no means follows 

 that incidental or non-significant colours would have 

 appeared at all on the surface of most animals. 

 And we find as a matter of fact that such colours 

 tend to disappear altogether, directly they cease to be 

 useful, as in cave-dwelling animals. On the other 

 hand, the non-significant colour of blood or of fat 

 would persist undiminished in such forms. 



Colours may be destroyed by natural selection 



Just as natural selection may develop an appear- 

 ance which harmonises with the surroundings, out of 

 the material provided by non-significant colour, the 

 same agency may lead to the disappearance of the 

 latter when it impedes the success of an animal in the 

 struggle for existence. Thus the red colour of blood 

 has disappeared in certain transparent fishes, which 

 are thereby concealed from their enemies. Among the 

 manifold possible variations of nature is that of a 

 fish with colourless blood, which can, nevertheless, 

 efficiently perform all the duties of this fluid. While 

 such a variation would be no advantage to the great 

 majority of vertebrates, it would be very beneficial to 



