EVOLUTION OF THE COLORS OF BIRDS. 151 



interspersed throughout the back. In the third variety, 

 S. psaltria mexicana, the black is the predominant color 

 of .the back, while in still more extreme races no trace 

 of the green can be detected. 



Three methods of turning black have been observed 

 among the examples above cited. (1) The general tone 

 of color may darken uniformly, approaching a black, 

 as in Junco hyemalis and Sayornis nigricans; (2) masses 

 of color may spread until they meet, as in Icterus; or 

 (3) dark spots and irregular mottliiigs may become fused 

 into continuous patches, as in the Spinus psaltria group. 

 This latter is the most universal method, and the other 

 two may be combined with it to a greater or less degree. 



As we are not concerned at present with the causes of 

 colors, but rather with the facts of the distribution and 

 localization of the different primary shades, we may 

 rest content for the present with this survey of black 

 as it appears among North American birds, and take up 

 white. 



One of the first points which is noticeable with regard 

 to white is its close interrelation with black. Next to 

 brown, white is more universally present among North 

 American genera than any other color. In spite of its 

 prevalence, however, there is not a single species which 

 is pure white, and only a very few in which it is pre- 

 dominant. The fact that black and white both occur 

 in the same species so frequently might first be consid- 

 ered as a mere coincidence, due to the fact that both are 

 very prevalent, or it might be regarded as a question of 

 utility, white setting off black, or vice versa, in a con- 

 spicuous manner, and affording a useful recognition 

 mark. This may indeed be a partial explanation, but 

 I think not the fundamental one. We have such species 

 as the dusky seaside sparrow (Ammodramus nigrescens), 

 and the black and white warbler (Mniotilta varia), in 



