Page:Auk Volume 13-1896.djvu/175

﻿ The reason of her visibility is that I have artificially extended her top colors down her sides, thereby destroying her counter-gradation and forcing her solidity to manifest itself.

The reader, I think, must try these experiments for himself before he can believe that in Fig. 3 and Fig. 9 I tinted the under surfaces exactly as dark as the upper, and no darker. But I beg him to look at any horizontal branch in the woods which is either on the level of his eye or below it. He will see that although it has exactly the color of its surroundings, it is not in the least concealed, because, being of uniform color above and below, like the birds after I had painted their under sides, it wears that universal attribute of a solid, namely, a gradation of shading from its light side to its dark side.

I leave to the reader the pleasure of discovering for himself that this principle of gradation in color is almost universal in the animal kingdom In certain classes of birds and of flying insects, however, the principle gives place, more or less, to the device pointed out by Bates ; namely, the employment of strong arbitrary patterns of color which tend to conceal the wearer by destroying his apparent continuity of surface. This makes, for instance, the Mallard's dark green head tend to detach itself from his body, and to join the dark green of the shady sedge: or the ruby of the Hummingbird to desert him and to appear to belong to the glistening flower which he is searching. Vet many other cases of color applied apparently at random conform essentially to the law stated above. The dark patches are on top, the light ones beneath. The dark breast-mark, so widely used by nature on birds, usually has the effect of putting out a conspicuous and shining rotundity of some bright or light color, as in the Meadowlark and the Flicker; because it comes just where the breast, in its usual position, rounds upward and faces the sky. The dark collars of the males of most species of Duck are absolute counter-shading to the light from the sky, when the birds sit in their characteristic positions. For most female Ducks