150 ANIMAL LIFE 



struck by this influence of light on their distribution. 

 A dark back and a pale under-surface form the most 

 general of all schemes of colouring. Insects and 

 worms, shells and starfish, fish and frogs, birds and 

 most beasts, through a vast category of marine and 

 terrestrial forms, display the white breast. Squirrels, 

 the most vividly coloured of mammals, still keep the 

 rule ; whales and fish agree in the contrast of their 

 dark upper sides with their white nether surfaces. 

 Blue and brown butterflies are only so when seen from 

 above ; and if we turn up the palms of our sunburnt 

 hands, we find that there too pallor is more evident 

 than on the upper side. Colour and light-exposure, 

 pallor and light-starvation, are correlative terms. As 

 the green colour of plants fades in darkness and shade- 

 leaves, so the nether parts of animals are bleached by 

 the lack of light. 



To confirm this and give the mind living hold 

 upon its significance we must apply the test of experi- 

 ment. If light induces colour, and darkness prevents 

 its appearance, then we should, by inverting a young 

 animal, obtain some evidence as to the present-day 

 applicability of this principle. This has been done 

 both artificially and by that age-long trial that natural 

 experiments involve. Young plaice have been grown in 

 a tank, lighted from below and darkened above, so that 

 the under-surface of the fish, which is normally turned 

 away from the light, was exposed during the hours of 

 daylight to the light reflected from a mirror. After 

 an interval of a few weeks several of the batch so 



