PROTECTIVE COLOURING IN ANIMALS 309 



ming over a light-coloured river-bed, yet when looked at 

 by other fishes or by otters in the water, the effect of the 

 light from above on this disposition of dark and light 

 tints on the fish's body must be the same as that demon- 

 strated by Mr. Thayer's " disappearing duck," and must 

 often render the fish absolutely invisible, even at close 

 quarters. 



Mr. Thayer has pursued this subject during the past 

 seven years, and last autumn he gave some interesting 

 demonstrations in the Zoological Gardens in London. 

 He showed a model of a white egret, which was but little 

 noticeable when standing up clear against a bright, white- 

 clouded sky. The long plumes on the wings, developed 

 in the breeding season, were shown (by putting them on 

 and taking them off) to assist in causing invisibility, 

 since they made the side of the body flat and concealed 

 the shadow on its rounded underside. A similar bird- 

 model marked with strong black on the neck and legs 

 the rest being white refused (so to speak) to shape itself 

 as a bird at all, and looked at a distance of twenty yards 

 like a bit of rock or stump of wood with a twig and dead 

 leaf attached. The effect of different tones of brown 

 cardboard cut into the form of a butterfly, when seen on 

 different backgrounds, was shown ; but the most inter- 

 esting experiment was made with a black-green piece 

 of cloth cut to the shape of a butterfly and fastened on 

 to a sheet of dead-black cloth in the open air, in the 

 presence of white cloud light of moderate brilliancy. At 

 five yards one could see the outline of the dark-green 

 butterfly-shaped piece ; at fifteen yards one could just dis- 

 tinguish the edge separating the dark-green piece from 

 the black cloth. Now Mr. Thayer stuck in the middle of 

 the dark-green butterfly-wing a small circle of pure white 

 (about one-third of an inch across). The effect was 

 entirely to obliterate the previously visible edge; one 



