Introduction: Variation. 
79 
the terminal exposed part is black, the division of the colours being sharply 
conterminable with the tips of the longest upper tail-coverts. 
The concealed bases of the contour-feathers. — Here again a difference of 
pigmentation or of gloss, or of both, is seen, the bases being usually of paler or 
duller hues than the tips. Thus the bases of the contour- feathers in Corvus 
enca are white, the terminal portions glossy black; in the Parrot, Aprosmictus 
sulaenm, the bases are grey or greenish, the exposed terminal portions bright 
blue on the mantle; and so on. 
Apart from the phylogenetic value of the different parts of a feather, there 
is convincing evidence that light must be cited as an important agent affecting 
the distribution of the pigments of a feather, either through physiological stimu- 
lation, or direct action, or both. No better test case could be found than the 
male of the Celebesian Cuckoo-shrike, Graucalus hicolor. Seen from above with 
its wings closed it is a black bird, for, though the rump and upper tail-coverts 
are white, these parts are then probably concealed by the wings. The under- 
surface is white. The wings and tail are black; yet that part of the tail which 
lies hidden beneath the upper tail-coverts is white, and so is the wing below 
where it rests upon the sides of the body; also the black contour-feathers of 
the upper parts are white on their concealed bases. It may be said that where- 
ever the feathers are exposed to the sun they are black; where they are in 
shadow or concealment they are white. It is preferable to attempt no explanation 
of these facts here, but it seems permissible to suggest that the case is similar 
to that of Priomturus, the difference being that the inherited effects of attrition 
are assigned as the cause of the formation of the racket-feathers of the Parrot, 
Avhereas the action of light is regarded as having in the course of generations 
in some way brought about the distribution of the pigments in Graucalus, 
Direct evidence of the action of sunlight upon plumage is afforded by the 
following statement. As Dr. Russ writes: “The Goldfinch when kept caged in 
a dark place often becomes black, and even in a light room the bright colours 
after moulting often appear fainter and more impure, but this can be prevented 
if the Goldfinch is placed as much as possible in the open air and sun” (Einheim. 
Stubenvog. 1873, II, 265). 
Tbe soft, glossless plumage of nocturnal birds, viz. Goatsuckers and Owls, 
also calls for consideration in this connection, as well as examples already sug- 
gested, and innumerable other ones in which the action of light, or the want 
of it, appears less obvious. 
