objects, that the eye is made aware of their existence, their main form, their 

 position, and all their minor modelings. Now, since this is the case, it follows 

 that animals, however colored, would always be more or less conspicuous in 

 their natural environment, and all the details of their form would be dis- 

 tinctly visible, unless their surfaces bore such an arrangement of light and 

 dark shading of the colors as could counteract the shading which the de- 

 scending daylight applies to their solid bodies. This counter gradation of 

 shades, from dark mid-backs to white mid-bellies, is, as we have seen, pre- 

 cisely the system of coloration ('Mimicry' — vide Chapter II — aside) of 

 almost all protectively colored animals. 



The ghostly elusiveness of a counter-shaded creature's appearance is at 

 its best under a diffused sky-light, such as that in the forest, or the open fields 

 on a cloudy day, because no color gradation can adequately cope with the 

 full and concentrated light of the sun itself, which produces sharply contrasted 

 areas of light and shadow, rather than a graduated shading. Even in full 

 sunlight, however, the light from the wide expanse of sky is still the principal 

 factor. To understand this, the reader should compare the difference between 

 a sunlit patch of ground and a neighboring one which is cut off from the direct 

 sunlight, with the difference between the latter and the mouth of a deep hole 

 which is cut off from both sun- and sky-light. The one is the slight difference 

 between a sunny and a shady spot, the other is the vast difference between 

 night and day. A patch of bright sky no bigger than the sun is far less brilliant, 

 but the vast sum of such patches which the entire expanse of sky contains, 

 yield a far greater light than the sun itself. (This is analogous to the principle 

 of sound, which makes the sum of the concurrent echoes of a clap of thunder 

 far louder than the initial sharp electrical report itself.) An animal's 

 counter shading, then, is effective even on open ground on a sunny day, 

 although the superadded direct sunlight interferes with the perfection of 

 its working. 



On this basis of the obliteration of the light-and-shade aspects of a solid 

 creature, the most exquisite color resemblances to the creature's background 



i8 



