1916] Chandler: Structure of Feathers 275 
gories, namely, pigment colors, structural colors, and compounded 
colors, produced by combinations of pigment and structure in differ- 
ent parts of the same barb. 
It is not the purpose of the present chapter to deal with pig- 
ments or methods of actual color production, except in so far as 
the morphology of the feather parts is concerned, but to show what 
different modifications occur in feathers of different groups of birds 
to produce the same results, 1. e., wotely, to use a word coined by 
Gadow (1911) to mean the attainment of a similar end by different 
processes in different organisms. 
Colors which are produced by a single pigment, evenly distributed 
in the rami and barbules, with no objective color effects, seldom in- 
volve any modification in the morphology of the barbs. For example, 
in feathers which have light and dark bars in which the colors are 
of purely pigment origin, there is no appreciable difference in the 
form of the barbs in the light and dark areas. The only colors 
which are produced merely by an even distribution of pigment are 
blacks, browns, including rufous, and lemon yellow. Although red 
occurs very frequently as a pigment, it is almost always accom- 
panied by some structural modification. In the Musophagidae there 
occurs a green pigment, turacoverdin, which is not accompanied by 
any special structural modification. Grays, tinged with bluish, rang- 
ing from pale pearl gray to deep slate gray, are usually produced 
by an uneven distribution of black or dark brown pigment. In 
eulls and columbid birds, for instance, the characteristic gray colors 
are preduced by conspicuous transverse bars of dark pigment on a 
transparent background in the barbules (pl. 29, figs. 70c, d). In 
herons nearly the same effect is obtained by a dilute, even pigmenta- 
tion in the bases of the barbules, supplemented by elongated unpig- 
mented pennula (pl. 20, fig. 20e). The same method is employed to 
produce the hoary color of terns and other birds, except that in this 
case the effect of the unpigmented pennula is accentuated by the long, 
brush-like ventral cilia. A pretty olive-green color is produced in the 
back feathers of Osmotreron vernans by a combination of slate and 
lemon yellow, the former being the effect of dark pigment bars in the 
transparent bases of the barbules, the latter produced by a lemon- 
yellow pigment in the pennula, which have large blunt ventral cilia 
(pl. 29, fig. 69a). 
Structural colors, i. e., colors which are produced by modifica- 
tions of structures causing interference or diffraction of light, may 
