Selection, however, will of course feel that this color-law is its work; and 

 since it is so almost universally in use, and accounts, apparently, so almost 

 exhaustively, for all the attributes of graded animal coloring, I believe it will 

 ultimately be recognized as the most wonderful form of Darwin's great law." 



The foregoing extracts together fully state the newly-revealed principle, 

 which in its various elaborations is the foremost subject of the present book. 

 But it may be well before going further to dwell at greater length on the sim- 

 plest aspect of this fundamental principle. 



No one who has studied animals in nature can have failed to notice either 

 their frequent wonderful inconspicuousness, or the fact that ninety-nine per 

 cent of them are dark colored on the back and light colored on the underside. 

 On the other hand, even school children are daily taught that the only way 

 to draw a representation of a ball or cylinder is to shade it from a bright 

 central point or middle line to dark borders — or, if the object is to be shown 

 in side view, under a top light, to shade it from very bright above to deeply 

 dark below. Yet the obvious conclusion that the contrary gradation of 

 shades, as it exists on the rotund bodies of animals, is the cause of their wonder- 

 fully unsubstantial appearance, has never been drawn till now, and even now 

 is but slowly accepted by most people. This is because few people recognize 

 the vast part played in the visible world by light-and-shade. As has already 

 been said, the known fact oj solidity suffices, to many minds, without any 

 inquiry into the means by which that solidity is manifest to their sight. Light- 

 and-shade, color, and line, are the three great factors of visibility. Line 

 perspective enables the eye to judge to a large degree of the forms of objects, 

 and the various distances of their different parts, especially in the case of large 

 ones of elaborate shape, such as buildings; but the visibility of line is de- 

 pendent on color, and still more on light-and-shade. I here use 'line' to 

 mean the visibility of the boundaries of material surfaces and their parts. 

 It is obvious that this is dependent on color, since if a monochrome flat sur- 

 face is so placed relative to the eye of an observer that one of its boundaries 

 is against another flat surface of precisely the same color, and similarly lighted, 



16 



