RUSSET WHEATEAR. 
125 
tleman, widely as they appear separated, a well-chosen 
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 30T of the same 
work, is found in similar situations, and appears equally 
distributed with S. aurita, whose favourite resort is 
among stony ground at the foot of hills or old ruins. 
“Xo difference is noticeable between the eggs 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 ramule of the colourless feather. 
