236 Loomis, The Reality of Species. (.April 



record of past events warrants the conclusion that bird species 

 are now in process of evolution through geographic variation. 

 Theorize as we may, the fact remains that we do not know what 

 part geographic variation or other agencies played, or did not play, 

 in the origin of existing bird species, the modus operandi of the 

 evolution being unknown. But we do know that geographic 

 variation is one of the common variations occurring within the 

 bounds of a bird species of to-day, and that it is not the only 

 variation in which geography is a factor. 



Independent of individualism, age, sex, season, or climatic con- 

 ditions, there exists a type of variation known as dichromatism, 

 which perhaps originated in mutations. It is well exemplified in 

 the Jaegers, Albatrosses and Petrels, Herons, Hawks, and Owls. 

 In some species there is a difference in the geographic range of the 

 phases, but it does not correlate with environment as in geographic 

 variation. Instances to the point are found in the Wedge-tailed 

 Shearwater, Red-tailed Hawk, and Screech Owl. 



More than thirty years ago, when our knowledge of variation 

 was far less than it is now, Dr. Stejneger had the discernment to 

 interpret Colaptcs auratus (Linnaeus), Colaptcs cafer (Gmelin), 

 and Colaptes hybridus Baird to be dichromatic or trichromatic 

 phases of one species, and not two species that hybridize on a 

 gigantic scale. 1 None of the characteristics of dichromatism are 

 wanting in these extremes and intermediates. They are similar 

 in general character to the extremes and intermediates of well- 

 known dichromatic species, of the Wedge-tailed Shearwater, 

 Neglected Petrel, and Rough-legged Hawk for example. They 

 are not individual and are not dependent upon age, sex, season, or 

 environmental conditions. Moreover, intermediates crop out 

 sporadically in the Eastern States, where the auratus phase is 

 dominant. It is well to bear in mind that these variations of the 

 Flicker are not greater than certain other normal variations; as 

 the age variation of the Western Gull, the sexual variation of 

 Williamson's Sapsucker, the seasonal variation of the Marbled 

 Murrelet, and the dichromatic variation of the Parasitic Jaeger. 



The question naturally arises, whether dichromatism has often 



i Riverside Nat. Hist.. Vol. IV, pp. 8, 9. 



