COLOUR AND PATTERN IN ANIMALS 77 



animal fades into a ghostly elusiveness against its natural back- 

 ground, especially when the light is rather diffused, as on a cloudy 

 day, or where a creature is lurking in a shadowy corner for its prey, 

 or tips and fades into a covert when it is being pursued. A model 

 ^invented by Mr. Thayer and often exhibited in museums shows the 

 real invisibiUty conferred by counter -shading. Two bird-shaped 

 bodies are fixed on a rod that can be revolved, and are placed in 

 a case with a glass roof lighted from above, with the side next the 

 spectator open and the background and two other sides painted 

 with neutral grey. The body of one bird is painted all over as nearly 

 as possible the same shade as the background ; the body of the 

 other is darkened above and made hghter than the background 

 below. At a Httle distance, the self -tinted model stands out clearly 

 from the background as the soUd body of a bird, white above where 

 it is illuminated, dark below where it is in shadow ; but the other 

 model is almost invisible, for the counter-shading neutralises the 

 effect of the illumination. If the rod on which the models are 

 fixed be rotated, the neutrally tinted body remains visible, whilst 

 the counter-shaded model, as its shading is now disposed so as to 

 agree with the natural lighting, becomes visible at once, being even 

 more conspicuous than the other. 



So also if a white rabbit or hare, or a ptarmigan in its white 

 winter plumage, be placed in a similarly constructed case with a 

 white background, we shall see that the resemblance in colour to 

 the background does not protect it. On the other hand, if it be 

 seen in bright sunlight on a glistening surface of snow, the light 

 reflected up from the snow on the under surface of its body to a 

 certain extent counteracts the natural effect of light and shadow 

 and brings about a fair degree of invisibility. The polar bear 

 stalking seals at the edge of the ice probably enjoys a similar 

 softening of outline from reflected light. Desert animals may also 

 gain something from the Hght reflected upwards from the sand, and 

 in their cases the contrast between the upper and under surfaces 

 is slight. So also it is shght from another reason in animals that 

 haunt the interior of shady forests ; such hght as filters through 

 the trees is diffused and obscure, and the upper surface is only shghtly 

 darker than the under surface. The contrast is greatest in those 

 animals that hve on dark ground under the open sky, as in many 

 of the rodents, kangaroos and deer that hve on open plains. 



Counter -shading is a character that may be found in various 

 degrees of strength combined with many different kinds of colora- 



