294 CALIFORNIA ACADEMY OF SCIENCES. 



the working of sexual selection in accordance with the 

 law of the assortment of pigments. Plate XVI shows 

 the generalized plumage of the group. It will be noticed 

 that the Arizona hooded oriole (I. cucullatus nelsoni) in 

 female plumage is a dull-colored greenish olive bird 

 without any special markings, except the partial band 

 of white on the wings. The young male is similarly 

 colored the first year. Upon the second year the young 

 male acquires a brownish black throat patch, as shown 

 in the plate. In these two phases of plumage, the 

 orchard oriole (I. spurius) is colored almost exactly the 

 same, although in both species there is a large amount 

 of individual variation. 



In the adult female of Bullock's oriole (I. bullocki), the 

 colors of the male are indicated on a much duller key. 

 In the figure of the immature plumage of this species it 

 will be noticed that already the superciliary stripe has 

 appeared. The immature plumage of 1. spurius seems 

 to have even more yellow than later stages, and indi- 

 cates the relation of the colors of the adult of this 

 species to the other orioles which are so differently 

 colored. The female Baltimore oriole (I. galbula) is rap- 

 idly acquiring the plumage of the male, although the 

 colors are much duller and broken. 



While the general tone of color in this plate is olive 

 greenish and brownish, the effect of Plate XVII, show- 

 ing the heads of 'the adult males, is black and yellow or 

 orange. The separation of the component pigments of 

 the undeveloped phases of coloration would produce the 

 coldts of the specialized males. And what factor can 

 be suggested to account for this better than sexual selec- 

 tion, which explains at once the similarity of colors and 

 the difference in their distribution? The colors would 

 first be black and yellow, but by intensification of the 

 latter orange and crimson would be developed. This is 



