CHAPTER XIII 



BIRDS, ETC. THE INHERENT OBLITERATIVE POWER OF MARKINGS. 

 'RUPTIVE' AND 'secant' PATTERNS, ETC. 



UP to this point we have considered markings and patterns almost solely 

 as adjuncts and dependents of obliterative shading. As far as ground- 

 haunting species are concerned, this function of markings seems by far the 

 most important, but they have yet a separate and inherent significance, which 

 among non-terrestrial species is often the dominant feature of disguise. As 

 we have seen, perfect uniformity of coloration makes a thing conspicuous, 

 allowing every part to assume exactly the aspect dictated by its own form, 

 without exaggeration or omission. Markings, on the other hand, of whatever 

 sort, tend to obliterate, — to cancel, by their separate and conflicting pattern, 

 the visibility of the details and boundaries of form. The main solidity, and 

 its details, are shown by graduated light-and-shade — the outline, the external 

 contours, by relieving either light or dark or differently colored against the 

 background. To all this markings are unfriendly, both on objects actually 

 monochrome and therefore visually not so, and on objects which present, with 

 the aid of counter shading, a perfectly monochrome appearance. Rapid 

 and manifold are the vicissitudes of illumination and relation to background 

 of a moving bird, or butterfly among trees and open spaces. Now it is dark 

 against a sky vista, or against brightly-lighted foliage, and the next instant, 

 by some slight change in its position, or in that of the beholder, it shows light 

 against dark shadow-spaces. (See Figs. 56-57.) Delicate picture-patterns 

 cannot avail against these grosser 'visibilities,' but strong 'secant' and 

 'ruptive' patterns can. If the bird's or butterfly's costume consists of 

 sharply contrasted bold patterns of light and dark, in about equal propor- 



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