18 
Introduction: Variation. 
papillae destroyed by the beaks of antagonists; the face of the adult Rook from 
which the feathers fall at the base of the bill as a result of dirt and AA'ear for 
generations; the head and face of the adult Moleo, naked owing to ages of 
attrition from the sand in which it burrows; and so on. Examples drawn from 
man and other animals could be given. The principle is of importance, as a 
cause of, or directive stimulus to, variation ; it should therefore not be accepted 
without criticism. For some cases the principle of natural selection atfords an 
explanation (e. g. the remiges), but for others the argument furnished thereby 
can hardly be made to commend itself to impartial judgment (e. g. the rectrices), 
and for others again this principle appears to fail completely (e. g. the rackets 
of Prionitiirus, the comb of the Cock’)). 
Effect of light. In course of time most colours in mounted specimens 
and skins of birds fade with exposure to the light. Among Celebesian birds 
the effect is particularly well seen in the buff of the Nutmeg Pigeons and the 
wash of salmon-colour on the under parts of the Moleo, which soon fade in 
exposed skins, leaving the respective parts white. Nor does light seem to be 
operative solely upon the dead. 
Where the wing rests u])on the body. — In nearly all birds a change of colour 
takes place on the under side of the remiges where they rest upon the body 
Avith the wing closed, so that this part differs from the distal ends and more 
external parts of these feathers. Sometimes merely a slight change of gloss is 
seen, but all stages of difference may be found from this up to the most marked 
contrasts. Among Celebesian birds some of the most striking examples are: the 
Cuckoo-shrike, Grmicalus bicolor, wuth the remiges white below where they rest 
upon the body, black on the other portions; the Parrots, Prioniturus Loriculus, 
wdth the remiges below verditer-blue against the body and partly where they 
cover one another, black elsewhere; the Roller, Coracias temmincfd — remiges 
blue against the body, black changing with the light to bronze on the free 
parts ^); the Flycatcher, Zeocephus, Avith the said parts ferruginous and blackish 
respectively; and so on. A tendency to blackness is generally seen on the distal 
ends and external portions of these feathers. 
Where the tail-feathers are concealed by the upper tail-coverts. — A change 
of colour in the shafts and webs of the rectrices is generally seen on their con- 
cealed bases, very commonly a tendency to paleness or white, suggesting a loss 
of pigment. The most striking examples occurring in Celebes are the Cuckoo- 
shrike, Graucalus bicolor; the two Nutmeg Pigeons, Mpristicivora bicolor and luctuosa; 
the Pratincole, Glareola isabella (as also G. orientalis). In these birds all that 
part of the tail Avhich is concealed by the npper tail-coverts is white, and all 
') As shown by Stolzmann (P. Z. S. 1885, 430) this may bring disaster to the wearer of it, but the 
author attempts to explain this by natural selection. 
2) On the upper surface of the remiges these colours are reversed, being blue above where they are 
bronze below, and bronze above where they are blue below, but the lines of demarcation do not exactly coincide. 
