BODILY CHARACTERISTICS INCONSPICUOUSNESS 283 



to produce an optical illusion, whereby the effect of a solid on 

 the eye is simulated, and this is done by shading darkly the side 

 opposite to the one from which the light is supposed to come. 

 The exact opposite to this is brought about in nature as a pro- 

 tective device, and a solid object is made to appear more or 

 less flat by dark colouring above shading into light colora- 

 tion below, a very common state of things which has already 

 been briefly referred to (p. 271). A very ingenious model, of 

 which a copy exists in the Cambridge Museum of Zoology, has 

 been devised to demonstrate this. The back of a rectangular 

 box, of which the front and sides are glass, is marked with a 

 tangle of blotches and streaks to represent the confused mixture 

 of light and shade which may be seen in such a natural back- 

 ground as that presented by the sedges and other plants which 

 line the margin of a pond. A little in front of this a rod is 

 fixed from side to side, and to this are attached a couple of clay 

 models representing conventionally the bodies of two birds, with 

 the usual curved outlines. The upper parts of these are tinted 

 like the background and blotched in a similar manner, and are 

 gradually shaded off at the sides so as to pass insensibly into 

 the unshaded whitish tint of the under parts of the models. It 

 should further be stated that the rod can be rotated on its long 

 axis by means of a handle so as to move the diagrammatic 

 birds. Standing at a little distance from the model, before being 

 acquainted with its purpose, an average observer will almost cer- 

 tainly fail to see the dummy birds, but they at once flash, as it 

 were, into view when moved by means of the handle. The 

 device clearly illustrates why in so many animals the upper 

 surface is dark and the under surface light, and also shows that 

 a protected form must remain motionless if it is to derive much 

 benefit from its resemblance to surroundings. The principle is 

 illustrated in nature by such Mammals as Hares and Rabbits, by 

 almost any common sort of small Bird, by Lizards, Snakes, Frogs, 

 and innumerable Fishes. The last case is of particular interest, 

 for in such a form as, say, a Whiting, the distribution of colour 

 results in inconspicuousness as seen in side view, while the 

 darker upper side of the body is liable to be confounded (at 

 least in shallow water) with the background presented by the 

 sea-floor, and the pale under side is not particularly obvious when 

 looked at from below. It is clear that floating or swimming 



