1907] Beebe: Geographic Variation in Birds. 29 



the epithelial pigment of the choroid is as truly epidermal, trac- 

 ing its derivation, in the embryo, from the involuted epiblast of 

 the medullary canal. 



With the sixth moult, experiments have for the present 

 ended, but the blackest dove now in the collection exhibits an 

 interesting condition which may, in future work, prove as sig- 

 nificant as it was unexpected. In wild specimens of Scardafella 

 inca there appears to be no trace whatever of any iridescence or 

 metallic tints, the darker tips of the feathers being a dull and life- 

 less brown. In the Venezuelan S. ridgivayi the largest dark-tipped 

 feathers show, in a very favorable light, a trace of dull greenish 

 or bluish sheen. 



In the darkest bird under experiment, this metallic color is 

 strongly developed, and in a most interesting way. The feathers 

 of the lower parts show little iridescence, and this of a bronzy 

 character, but above, the plumage is as iridescent as that of any 

 East Indian fruit pigeon. The entire back and rump are similar 

 in hue, each feather having a broad tip of iridescent bronze, while 

 the remaining exposed part of the feather is of a brilliant green, 

 this tint extending down along the rhachis as a shaft stripe, a 

 little way into the distal bronze portion. The primaries and 

 outer secondaries are dull black, but on the inner secondaries, 

 coverts and scapulars the most brilliant iridescence is found. 

 On some of the feathers blue and green seem arranged in cross 

 bars, but, on the whole, the metallic coloring is irregular, the en- 

 tire feather changing in color as the angle of incidence of the 

 light is changed. This alar distribution of iridescence becomes 

 significant when we consider the occurrence of metallic color in 

 the plumage of many genera of doves — some rather closely re- 

 lated to Scardafella and others separated by considerable struc- 

 tural differences. 



The rather closely related genera Columbula and Chamae- 

 pelia — ground doves of the Neotropical Region — are ornamented 

 with metallic colors on the wing coverts; in the first mentioned 

 forming a band of steel blue on the lesser coverts, and in Chamae- 

 pelia being arranged as a number of spots and blotches, both on 

 wing coverts and inner secondaries. These spots are of various 

 colors, as bronze in one species and purplish-green in another. 

 In a future paper I shall discuss variation in the genus Chamae- 

 pelia and the effect of artificially changed conditions of environ- 

 ment on these metallic patches ; but here I mention them only to 

 show the tendency, in certain groups of doves, to iridescence on 

 the wing coverts. In our northern mourning dove, Zenaidura 

 macroura, the wing coverts exhibit dark spots which are glossy 



