LIGHT AND COLOUR 153 



darkness to prevent the appearance of colour or to 

 destroy a pre-established colour in the course of 

 generations. 



With this thought we may look out on the colours 

 of the animal world and observe the broad general 

 agreement between the arrangement of these tints 

 and the green colour of plants. The two sides of an 

 animal differ as do the two surfaces of a leaf ; the upper 

 one, being the more exposed to the sun, is the darker ; 

 the lower, more shaded surface, is the lighter one. 

 Animals, such as worms, that keep one end of their 

 body root-like buried in the ground become at that 

 extremity bleached, and those that are altogether 

 excluded from the light, like a tuber, are totally 

 colourless. A plant grown in the dark becomes pale 

 and chlorotic ; an animal, though more slowly, becomes 

 no less drastically etiolated. Transported from dark- 

 ness to light, the leaves of a cellar-grown or shaded 

 shrub reassume their green colour, and with no less 

 certainty the colourless troglodyte becomes dusky 

 when illuminated. Nor is the full light of the sun 

 necessary for the development and preservation of the 

 colours of animals and plants. In a dim light vast 

 numbers of jungle plants, forests of brown and red 

 seaweeds, multitudes of crepuscular, creeping things, 

 and deep-sea fish work out their life with as full a 

 colour as their relatives and brethren of the sun. 

 The mole and bat are no less deeply coloured than the 

 sparrow or skylark ; the pygmies of the dark Congo 

 forest are brown as the tribes of open country ; the moss 



