66 CHILDHOOD OF ANIMALS 



the pattern, although inevitable and associated with structure 

 that no doubt is useful, is not in itself useful. We do not frame 

 explanations of its meaning and purpose when it is concealed 

 within the body and made visible only by dissection and the micro- 

 scope, but if it crop out on the surface and so is visible, then we 

 are disposed to imagine that it must have some special fitness for 

 the conditions in which the animal lives, and to speculate as to 

 how the conditions could have called into existence the pattern 

 that fitted them. I do not doubt that such inevitable growth- 

 patterns sometimes confer an advantage on an animal, and have 

 been maintained by the operation of natural selection, but it appears 

 to me that it is their absence and not their presence that requires 

 explanation, and that natural selection has been more effective in 

 smoothing out and obliterating the inevitable growth-patterns than 

 in preserving them, or being the agent in their formation. 



All visible things must have colour, and so also it is inevitable 

 that animals must have colour. The colour may be due to one of 

 several causes or to a combination of causes. Many hues, especially 

 those with metallic sheen, depend on the structure of the surface 

 on which the light falls, the white light being broken up in the 

 process of reflection. When a piece of transparent glass or ice is 

 powdered it becomes white like snow, and this appearance is due 

 to the total reflection of the light from the mixture of little solid 

 particles and intervening bubbles of air. The white of animal 

 tissues is produced in this way. The fur and feathers of arctic 

 mammals and birds, white patches on the skin and so forth come 

 about because there are little bubbles of air or of some other gas 

 entangled in the structure of the tissue. The blues and greens of 

 many birds and insects which do not change in colour according 

 to the angle at which light is reflected from them, and the still 

 more vivid metallic iridescent colours which change as they are 

 moved about, and which are conspicuous in the eyes of the peacock's 

 tail and in the bright tints of birds-of -paradise, are due to a combina- 

 tion of structure and pigment. Frequently there is a dark pigment 

 underlying a transparent layer, forming a kind of mirror, and the 

 play of colours comes from the varying incidence of light and the 

 varied sculpturing or thickness of the transparent layer. 



Other colours may be due to the presence of pigments that is 

 to say, actually coloured substances. Blues and greens occasion- 

 ally, reds, yellows, blacks and browns almost invariably are 

 pigmentary. The brilliant crimson of the feathers of the turacos is 



