ings discernible upon close scrutiny of the forest ground, together with the 

 larger pattern formed by groups of slightly more distant leaves and twigs, etc., 

 with their high-lights, their middle tones, and their dark shadows, — all these 

 things, variously reduced by perspective, are clearly suggested to an appreci- 

 ative observer by the marvelous patterns of the forest Caprimulgidce. The 

 fact that none of these detail-picturings is so patently realistic as to be appreci- 

 able to everyone when the bird is seen away from its natural environment, is 

 part of the very marvel of the thing. Thanks to some process * which in its 

 visible results has amounted to something like an averaging of all the normal 

 backgrounds, against which, from aboriginal times, the animals have been 

 seen, they bear a pattern precisely similar to none, yet amply fitting all. This 

 effect of perfect averaging or compounding is one of the most beautiful and es- 

 sential parts of the obliterative principle. (In certain cases, which will be 

 considered later on, an animal's background is subject to so little variation 

 that a more simple and single imitation of absolute details is possible.) 



Though fully developed, the obliterative shading underlying this pattern- 

 system of the goatsuckers is slight in range, conformably to the diffuseness 

 of the top-light in deeply shaded woods, which these birds inhabit during 

 the day. True obliterative coloration perhaps makes its nearest approach 

 to mimicry among animals bearing this form of pattern. For while the coun- 

 ter shading as well as the character of the markings proves the case to be one 

 of obliteration, or merging with the background, yet the apparent extreme 

 nearness of some of the pictured details, which in certain views will even ' co- 

 alesce' perfectly with the markings of the very object on which the animal is 

 sitting, such as a stone or mossy log, gives the phenomenon, in part, close 

 kinship with the exact mimicry of surface-detail on an animal whose protec- 

 tion is the simulation, with full appearance of solidity, of a single inanimate 

 object. It is furthermore undeniable that a finely-patterned bird such as we 

 have been describing does occasionally pass for an excrescence of the log or 

 rock on which it sits. This may be the case, for instance, when it is seen in 



* We ourselves attribute all such work to natural selection, pure and simple and omnipotent. 



36 



