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 
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