Page:Popular Science Monthly Volume 75.djvu/560

554 background, and, as now proves to be the case with practically the whole animal kingdom, wear its colors (or some of them).

Bark moths, terrestrial animals in general, terrestrial habited birds, etc., all such species, when thus in actual contact with their backgrounds, share its varying illumination from minute to minute. The patch of sunlight that falls on the squatting woodcock illumines also

 shows the revealing-effect of being seen against a contrasting background, and illustrates the fact that no aerial creature, can go about at all without constantly passing across both revealing and concealing backgrounds.

the surrounding ground, so that the bird continues to seem a part of it; and the same is of course true of the rest of this great class of animal. But a very different fate attends the life of aerial species destined constantly to appear against more or less distant backgrounds which do not share their particular momentary illumination, and which constantly show, now light, when the flying bird or butterfly is dark, and the next instant dark when he is light. This fate causes all aerial species, in a very vital sense, to be conspicuous. Fig. 5 illustrates this. On the white of the sky a white butterfly has been pasted, which, of course, does not show. In the same way, a black one has been placed upon the darkest part of the tree's shadow, and a ground-colored one on the ground. Both of these, like the white one, are, of course, practically invisible, and, could they be always seen against these same backgrounds, they might be classed as cryptic. But their habits preclude the possibility of this, their own changes of position, not to speak of those of the spectator, bringing them, as they fly about, across, often in a single second, the whole gamut of backgrounds, from brightest sky to deepest tree-shadow, and back. And against every one, except the single one which they match, they are clearly visible. The black one shows against the sky, the ground, etc., the white one against the various