RUSSET WHEATEAR. 125 



tleman, widely as they appear separated, a well-cliosen 

 series of the numerous African species of the class, 

 "will exhibit a range of transitions so imperceptible, 

 that it will be found very difficult without careful 

 comparison to draw a line between one species and 

 another."— "Ibis," vol. i., p. 432. 



This bird, says Mr. Salvin, at page 307 of the same 

 work, is found in similar situations, and appears equally 

 distributed with ^S'. aiirita, whose favourite resort is 

 among stony ground at the foot of hills or old ruins. 

 "No difference is noticeable between the earffs of these 

 species." 



In general distribution of colour Mr. Tristram says 

 there is much resemblance to the Desert Chat, but 

 the bill and tarsi are one-third less in length; the 

 black of the throat does not extend so far, and in the 

 latter, the head and back are of a more sombre Isabel 

 colour. Mr. Tristram says "that without exception the 

 upper plumage of every bird, whether Lark, Chat 

 Sylvian, or Sand Grouse, and also the fur of all the 

 small mammals, and the skins of all the snakes and 

 lizards, are of one uniform isabelline or sand-colour." — 

 "Ibis," vol. i., p. 430. 



This beautiful adaptation of colour, so important as 

 a protection against their enemies, is, in my opinion, 

 produced in these animals principally by means of the 

 food. When colour is owing to the deposit of pigment, 

 it is clear that this pigment must exist in the organic 

 productions by which the animals are surrounded, for 

 it produces the same colour in them. In the humming 

 bird which feeds on the nectar of flowers, which being 

 hidden, requires no colour-pigment, the hue of the 

 plumage is owing to a peculiar sculpturing of the 

 ultimate rarnule of the colourless feather. 



