30 Zoologia: N. Y. Zoological Society. [Vol. I 



but hardly metallic. As the opposite extreme, in an otherwise 

 rather dull-hued bird, I may mention Ocyphaps lophotes, the 

 Australian crested pigeon, in which the greater wing coverts are 

 a shining, bronzy green, while the inner secondaries glow with 

 the richest iridescence — changing from purple to blue. 



Until my observations have been supplemented by micro- 

 scopic work I shall hazard no speculations as to whether, in 

 Scardafella, the continued effect of humidity brings about an 

 actual change of structure in the barbs of the feathers, when 

 the dull-brown or white color of the wing coverts gives place 

 later, in the same individual, to metallic tints. If so, according 

 to Gadow's theory, this would be iridescence by refraction. On 

 the other hand, iridescence may result merely from the abnormal 

 abundance and extreme concentration of spherical melanin gran- 

 ules producing the effect of Newton's rings, or thin-plate in- 

 terference colors. The latter would seem the more probable 

 theory, apparently supported by the careful work of Strong on 

 the domestic pigeon. I have not yet had the opportunity of 

 verifying this in the case of Scardafella. 



C — Summary of the Effects of Humidity. 



1. When typical specimens of Scardafella inca are con- 

 fined in a superhumid atmosphere, a radical change in the pig- 

 mentation of the plumage takes place with each succeeding an- 

 nual moult. 



2. A change, apparently similar in extent and direction, 

 results from an artificially induced monthly renewal of the 

 feathers. 



3. The change at the first annual moult brings the bird 

 close to the Scardafella inca dialeucos type. 



4. At the second annual moult, the plumage approximates 

 either the Brazilian type or the typical S. ridgwayi, the succeed- 

 ing changes being unrepresented by wild species of the genus. 



5. Intrinsically the change is at first a segregation and in- 

 tensification of the melanin, resulting in a clearing up and ex- 

 tension of the white or whitish areas. 



6. A period of equilibrium later ensues, until the increase 

 of melanin is such that it begins to encroach upon the white 

 areas, this continuing until all trace of white has disappeared. 



7. Coincident with this intense blackening of the plumage 

 part of the epidermis, occurs an increase of pigment of the 

 choroid coat of the eye. 



