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, i. e., isotely, 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 

 gulls and columbid birds, for instance, the characteristic gray colors 

 are produced by conspicuous transverse bars of dark pigment on a 

 transparent background in the barbules (pi. 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 (pi. 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 

 (pi. 29, fig. 69a). 



Structural colors, i. e., colors which are produced by modifica- 

 tions of structures causing interference or diffraction of light, may 



