THE COLOURS OF DESERT ANIMALS 155 



therefore that these birds have evolved their 

 blackness because in some way it fits them to a desert 

 existence ; in this respect they resemble Eugaster 

 guyoni and the dark form of Calliptarmos code- 

 syriensis. This is the more remarkable because 

 within the limits of the same genus (Saxicola) 

 there are birds which inhabit the same deserts and 

 which are coloured like the soil on which they live ; 

 examples are the Isabelline Wheatear {8. isdbeU 

 Una) and the Desert Wheatear {8. deserti), both of 

 which are widely distributed in the Great Palae- 

 arctic Desert, and occur side by side with various 

 black and white congeners. The habits of the black 

 and white Wheatears closely resemble those of the 

 other members of the genus, that is to say that they 

 are wary birds, given to perching on upstanding 

 rocks and, at any rate to my eyes, easy to see when 

 they are sitting still, and extremely conspicuous 

 when they move. 



Among the reptiles there is one species which 

 becomes increasingly black beneath as its habitat 

 becomes more and more strongly desert. Hartert 

 has recorded that the spiny-tailed Hzard ( Uromastix 

 acanthinurus) of Biskra, El Kantara, and other 

 places in the northern part of the Algerian Sahara 

 is replaced farther south in the Mzab country by a 

 form which is much blacker on its ventral surface, 

 and that this in turn is replaced in Tademait and the 

 central Sahara by a form in which the males are quite 

 black, and the females nearly so, aU over the under 

 surface of the body and the tail. This very remark- 

 able case can scarcely be grouped with those which 



