154 ANIMAL LIFE IN DESERTS 



always found it associated with much greater num- 

 bers of a pale buff form of the same species ; inter- 

 mediates between this and the chocolate form do not 

 occur in my experience. It is remarkable that two 

 varieties of one species, living in identical environ- 

 mental conditions at the same season of the year, 

 should be so differently coloured, the common form 

 resembling the surrounding stones and dead herbage 

 in colour, as do nearly all other members of the 

 family, the rarer form being nearly black and very 

 conspicuous. Another member of the same family 

 {Poecilocerus bufonis) in Sinai is dimorphic in a 

 similar manner. 



In most parts of the Great Palaearctic Desert, 

 birds which are mainly or entirely blaick are found. 

 Excellent examples are afforded by the Ravens and 

 by a large number of species of black and white 

 Wheatears and Chats (Saxicola). The latter range 

 right across the Great Palaearctic Desert, from Rio 

 de Oro and Morocco to Turkestan, Sind and Raj- 

 putana. Inside the belt of desert and semi-desert 

 they are abundant both as species or as individuals, 

 and outside it they are rarer, and occur for the most 

 part as stragglers. Blackness is not an ancestral 

 colour for Wheatears and Chats, and is quite excep- 

 tional among them. One can trace a progressive 

 series among the desert members of the genus, 

 showing the gradual replacement of buff by white, 

 and then the replacement of the white by black. 



In some species the blackening has proceeded 

 further in the male than in the female, in other 

 species both sexes are equally black. It appears 



