COLOURS OF ANIMALS. 



187 



and composition of their internal tissues ; and this 

 accords with the uniformity of the tints of blood, 

 muscle, nerve, and bone throughout extensive groups, as 

 compared with the great diversity of colour of their 

 external organs. It seems a fair conclusion that colour 

 per se may be considered to be normal, and to need no 

 special accounting for ; while the absence of colour (that 

 is, either white or black), or the prevalence of certain 

 colours to the constant exclusion of others, must be 

 traced, like other modifications in the economy of living 

 things, to the needs of the species. Or, looking at it in 

 another aspect, we may say, that amid the constant 

 variations of animals and plants colour is ever tending 

 to vary and to appear where it is absent; and that natural 

 selection is constantly eliminating such tints as are 

 injurious to the species, or preserving and intensifying 

 such as are useful. 



This view is in accordance with the well-known fact, of 

 colours which rarely or never appear in the species in a 

 state of nature, continually occurring among domesticated 

 animals and cultivated plants ; showing us that the 

 capacity to develop colour is ever present, so that almost 

 any required tint can be produced, which may, under 

 changed conditions, be useful, in however small a degree. 



Let us now see how these principles will enable us to 

 understand and explain the varied phenomena of colour 

 in nature, taking them in the order of our functional 

 classification of colours. 



Theory of Protective Colours. — ^We have seen that 

 obscure or protective tints in their infinitely varied 

 degrees are present in every part of the animal kingdom, 

 whole families or genera being often thus coloured. 



