EVOLUTION OP THE COLORS OF BIRDS. 235 



south on the Sierra Nevada Mountains irregularly into 

 northern Lower California, upon the Rocky Mountains 

 irregularly into Arizona and New Mexico and in more 

 isolated spots of the Alleghany Mountains south to 

 Tennessee and North Carolina; the Upper Sonoran area 

 lying in the center of the continent including the Great 

 Basin region, the upper prairie and Mississippi Valley 

 district, and touching both the Atlantic and Pacific in 

 restricted areas; the Lower Sonoran area, comprising 

 the south Atlantic and Gulf States the greater part of 

 Mexico and Lower California; the Lower Calif ornian area, 

 occupying only the Cape region; and the Tropical area 

 extending from Central America north along both coasts 

 of Mexico, and the southern extremity of Florida, and 

 including the West Indies. Between the Boreal and 

 Sonoran areas extends a transition region in which the 

 life is typical neither of the northern nor southern area, 

 but rather a more or less complete intermingling of the 

 two. 



The birds of the Boreal area are similar to the birds 

 of the northern Pala^arctic region, becoming practically 

 identical in the circumpolar zone, and progressively dis- 

 tinct southward. There seems to be little doubt that 

 the birds of this northern region are, for the most part, 

 the descendents of the original land birds of this dis- 

 trict. Especially is this the case with such forms as are 

 typical of the Boreal area. The birds of the Sonoran 

 area, while originally descended from the same stock, 

 present forms which have probably come from the Neo- 

 tropical region since the decline of the ice age. It is 

 possible, indeed, that the upper and lower Sonoran areas 

 represent two separate invasions from the south. The 

 tropical fauna of North America is, in all probability, 

 comparatively speaking, a very recent feature. 



Wallace furnishes the following list of Boreal genera 



