THEORIES OF MIMICRY 



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darks, and so forth. jSTor 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 vanishing s, is well 

 illustrated by the complete visibility of landscape through the cracks in 



A B 



Fig. 6. 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 



