Danger Signals 



choose their mates, and to select the most beauti- 

 ful and ornamental ones, hence the greater 

 showiness of these in most sexually dimorphic 

 species. Wallace does not accept this theory. 

 He thinks it unnecessary. He looks upon the 

 brilliant colouring of the males as due to their 

 superior vigour ; moreover, he says that it is the 

 hen that sits upon the eggs, and so requires a 

 greater degree of protection than the male, and 

 therefore natural selection has not permitted her 

 to develop all the ornaments displayed by the 

 cock. With the phenomenon of sexual dimor- 

 phism we shall deal at length in the next 

 chapter. 



Dr Wallace recognizes yet another exception 

 to the rule that animals are cryptically coloured. 

 Many creatures possess on the body markings 

 which tend to render them conspicuous rather 

 than difficult to see. Where such markings 

 occur on gregarious animals, Wallace believes 

 that they have been evolved by natural selection, 

 either to enable their possessors to recognize one 

 another, or to act as a danger signal to their 

 fellows. The white tail of the rabbit is believed 

 by Wallace to serve as a danger signal. The 

 first member of the company to espy the approach- 

 ing foe takes to his heels, and, as he moves, his 

 white tail catches the eye of his neighbour, who 

 at once follows him, so that, in less time than it 

 takes to tell, the whole company of rabbits is 



183 



