THE COLOURS OF DESERT ANIMALS 159 



pallidus) which breeds in the deserts and steppes of 

 Central Asia ; this race is desert-coloured, being a 

 very pale and dehcate blue-grey above, and paler 

 beneath than the western race. The western 

 Merlin, which is not desert-coloured, is well able 

 to find a Hving in the deserts in which it winters ; 

 on the supposition that the Merlin requires effacing 

 coloration, why is it necessary for the eastern race 

 to be coloured very pale, and particularly why should 

 the colour of its upper surface be paler than that of 

 the western race ? A good parallel is afforded by 

 the larger falcons. The Peregrine {Falco peregrinus), 

 which is not a desert-breeder and is dark iron-grey 

 above, winters in many of the Great Palsearctic 

 deserts ; in these regions one finds also the Saker 

 Falcon (Falco cherrug) and the Lanner Falcon 

 {F. biarmicus), which are pale desert-breeders. One 

 cannot see that the desert species have any advantage 

 over the Peregrine during the months during which 

 they five side by side. 



With the Falcons the theory of protective colora- 

 tion, or rather of effecting coloration, fails to cover 

 the facts, but the effacing colour of other predaceous 

 animals does help them to catch their prey. In the 

 CaHfornian deserts near the Turtle Mountains there 

 exists, according to Camp, a Rattlesnake (Grotalus 

 cerastes) which is coloured so that when it Hes 

 buried in sand flush with the surface it is well-nigh 

 invisible. Invisibihty, or some close approach to it, 

 really is attained, and the snake and the Hzards 

 which form its prey are active by dayhght at a 

 time when the value of colour as an effacer must 



