Page:Popular Science Monthly Volume 75.djvu/561

Rh darks, and so forth. Nor is it only their varying background that dooms them to visibility. Their flight carries them with equal speed through a series of metamorphoses of their own aspect. Now, for an instant of their passage, they are themselves practically black, because of being in deep shadow, and are perhaps seen against a bright sky-space. The next has brought them out into full sunlight, and they blaze bright against a new background of perhaps inky darkness. The principle of the inevitable visibility produced by this swift succession of visible moments, though alternating with repeated vanishings, is well illustrated by the complete visibility of landscape through the cracks in

 In this figure the two inconspicuous butterflies in the middle show the effacing-power of pattern when it repeats the background. At the left a butterfly of the same costume is represented passing through a moment of illumination too great to admit of its patterns' still cutting it apart into notes of the background. At the right a butterfly of the same pattern Is going through the reverse experience, being for an instant too much in shadow for its pattern to save It from appearing as one single dark form against the light space beyond. This illustration reminds one how perpetually such vicissitudes must succeed each other in the life of such species.

a board fence, to the eyes of any one passing swiftly by. The view recurs again and again to the retina, in time to keep up the image. This is why the average observer thinks he sees these butterflies through all their course. This plate only goes so far as to show how fatal to invisibility it is to have the wrong background. Fig. 6 illustrates the above explanation of that perpetual disharmony between flying species and their background which plays fast and loose, part of every second, with all their patterns' power to cut their forms into deceptive shapes, by making them, so constantly, first so bright against dark that all parts, even the blackest, fall into one light silhouette, and then so dark in shadow, against bright light, that even their white parts join the rest in one dark silhouette. These two climaxes of visibility, first one and then the other, occur in the flight of a bird or butterfly often at the rate of several a second, while, during the rest of the second, the creature is effaced by passing a background that his costume matches, and by