COLORATION 109 



to try to escape observation by remaining motionless. 

 There are, however, plenty of instances to the contrary. A 

 good example, among closely related species, is the case of 

 the desert chat compared with the black and white chat. 

 Both birds are found along the edge of the Egyptian desert. 

 The black and white chat is a boldly colored bird, with 

 actions which correspond; it is alert, wary, active; its col- 

 oration advertises it at a long distance, and it makes no 

 attempt to hide, dodging round rocks and through crev- 

 ices, and trusting to its agility and watchfulness for safety. 

 The desert chat is colored above much like the desert itself; 

 when it crouches it offers an excellent example of concealing 

 coloration (non-countershaded), and it makes use of this 

 fact in a way that tends to show that the coloration is, in 

 this case, a factor of survival value; for its main reliance, 

 when danger threatens, is in crouching motionless to avoid 

 observation. As regards the black and white chat, it is 

 self-evident that neither natural selection nor any other 

 tendency toward the production of a concealing coloration 

 has been at work; evolution has brought about a highly 

 advertising coloration of a kind which can hardly serve any 

 purpose of utility, and habit is practically the sole factor 

 in securing the bird's safety from its foes. (Mr. Thayer's 

 theory that their coloration has any effect, as regards any 

 of these insect-eating birds, in helping them secure their 

 prey need not be taken seriously.) As regards the desert 

 chat, the coloration may have been developed by natural 

 selection for purposes of concealment; but as it is of sub- 

 stantially the same type as the coloration of many other 

 desert animals which do not trust to, and are not benefited 

 by, concealment, it seems more likely that it is due to 



