EVOLUTION OF THE COLORS OF BIRDS. 241 



• 



then produce a divergence in the character of the bill 

 and feet of the two species without altering their colors. 

 Incomplete and unsatisfactory as this explanation cer- 

 tainly is, it is merely introduced to show that it is not 

 absolutely necessary to assume so improbable an hypo- 

 thesis as a definite complicated coloration being painted 

 on, so to speak, by the environment. 



Mr. Ridgway, in 1873, called attention to the relation 

 between color and geographical distribution in North 

 American birds.* He showed that certain species hav- 

 ing a wide geographical range varied in one of two 

 ways, either in a tendency towards melanism " which 

 may be either an increase in the intensity of color or in 

 the extent, of the black parts of the plumage," or in a 

 tendency towards greater brightness or increase in the 

 extent (hyperchromism), of one of the three primary 

 colors, red, blue, or yellow. This tendency towards 

 melanism and hyperchromism increases towards the 

 equator and towards the Pacific Coast, but with red the 

 color increases in intensity towards the tropics and in 

 amount towards the Pacific. Mr. Ridgway calls especial 

 attention to the different races of the Arizona goldfinch 

 (Spinus psaltria), as illustrating the principle of a tend- 

 ency toward melanism as the tropics are approached. 

 As this is an instance of unusual interest from the com- 

 pleteness and uniformity in the links of progression, I 

 have figured three of the races (Plate VII). The typical 

 race of the species (S. psaltria), is the one occupying the 

 northern-most limit of the range, north to about latitude 

 40° The back is in this form plain olive green with a 

 cap of black confined to the top of the head, and with 

 the wings and tail a dark brownish. This race shades 



' Am. Journ. Science and Arts, iii, Ser., Vol. iv, pp. 454-460, Vol. v, 

 pp. 39-44. 

 16 



