EVOLUTION OF THE COLORS OF BIRDS. 243 



pany with a flock of S. j)saUria, which had the back 

 almost, if not entirely, black. It is probable that these 

 exceptional instances are not stragglers from the south, 

 for the bird is generally a resident wherever found, but 

 rather an unusual manifestation of the melanism which 

 normally only asserts itself in a warmer climate. 



Mr. Ridgway also mentions the following instances of 

 melanism toward the south: " Myiarchus Laivrencii, 

 which, starting with a grayish brown crown in the 

 northern examples (var. Laivrencii) gradually assumes 

 a blacker and blacker crown, as we trace it southward, 

 first through var. nigricapillus (Costa Rica and Panama), 

 and finally ending in var. nigricejys (Ecuador), which 

 has the crown deep black. Sayornis nigricans from 

 California and Northern Mexico, has the crissum pure 

 white; Mirador specimens have it clouded with dusky, 

 while in Costa Rica specimens (var. aquaticus), it is 

 entirely blackish, only the middle of the abdomen being 

 white. * ^ -^ The same law as regards the Pacific 

 province of North America is made evident by the well- 

 known cases of Picus viUosus var. Harrisii, P. puhescens 

 var. Gairdneri, Sphyrajyiciis varius vav. ruber, the North- 

 west coast forms of Falco peregrinus, F. Columbarius , 

 Bubo Virginianus, Scojjs asio, and numerous other simi- 

 larly affected species." 



Mr. Ridgway mentions Xanthoura luxuosa and var. 

 guattmalensis , and the different races of Geothlypis as 

 examples of intensification in yellow toward the tropics. 

 In Geothlypis the yellow not only becomes intensified 

 but extended upon the white of the belly both in the 

 warm moist climate of the Mississippi Valley and Gulf 

 States and in Mexico. Sylvania p>usilla pileolata and 

 Helniinthophila celata lutescens are illustrations of the 

 intensification of yellow in the Pacific provinces. 



Gardinalis virginia/iius and Piranga oistiva become 



