116 THE MEANING OF EVOLUTION 
away from the light, is so shaded as to mark a dis- 
tinct boundary between the stone and the board. An- 
other cobblestone was colored on its upper side like 
the board, but the color faded into a lighter and 
lighter tint until the bottom of the stone was nearly 
white. This stone, placed upon the board, was at 
a short distance nearly invisible. In other words, al- 
though the pigment was actually lighter on the under 
side, it was so much less intensely illuminated, that 
the result was the same in tint as the other side un- 
der the clear sharp light of the sky. 
Many a person, looking down into the water from 
a bridge, sees nothing whatever of the fish in the wa- 
ter below, because their backs are exactly like the 
bottom of the stream. Suddenly one of the fish, by a 
quick movement, turns its lighter under side over in 
such a way that it is clearly illuminated from the sky. 
Immediately a flash as of silver strikes the eye of the 
onlooker and makes him aware of the presence of the 
fish which had previously been undetected. If ren- 
dered thus suspicious, the observer will carefully ex- 
amine the bottom of the water, he may quite likely 
find dozens of fish which had previously escaped his 
attention. 
Nature is very versatile. So many of her ap- 
parently chance ventures have proved successful that 
she has retained many devices by which her children 
