PHYSICAL CONDITIONS IN GENESIS OF SPECIES. 389 



In a tropical direction, and is merely dne to tlie more northward ex- 

 tension there of tropical influences. In passing to the plains and the 

 Great Basin west of the Rocky Mountains, however, an entirely 

 different phase of color variation is met Avith. Here, as a general 

 rule, there is a loss of color, this region being characterized by the 

 presence of subdued or faded tints in the representatives of species 

 having a nearly continental range. The transition, however, is as 

 gradual as is that of the climatic conditions, the paleness beginning 

 near the eastern border of the Great Plains and increasing westward, 

 leaching its extreme phase in the arid wastes of the almost whollj^ 

 rainless districts of the far Southwest— south Nevada, Arizona, and 

 I lie contiguous region westward and southward. In respect to this 

 ))art of the subject it is hardly necessary to say more than that the 

 representatives of continental species found here are uniformly much 

 paler than those inhabiting the adjoining regions; that in many 

 cases the paler forms were originally described as distinct species, 

 and are commonly recognized as varietally distinct, though found 

 (o inseparably intergrade Avith the neighboring darker forms. In 

 addition to the general paleness there is often an increase in the areas 

 of Avhite and in some cases an accession of new ones. 



The wooded, mountainous districts embraced in this region also 

 give rise to peculiar local phases of color variation, to give a detailed 

 account of which would too greatly extend the present paper. The 

 tendency is mainly toAvard the development of more or less Avell- 

 marked rufous or fulvous phases of coloration, with sometimes an 

 accession of red, Avhile not a few species have more than the usual 

 amount of black. A most striking instance of increase of red at 

 western localities is seen in the yelloAv-bellied woodpecker {Sphy- 

 rapicus varius)°- which, in some of its forms, ranges in the breeding 

 season over the more northern and elevated Avooded portions of the 

 continent. In its eastern form the male has merely the chin, throat, 

 and crown red, Avhile in the female the red is restricted to the crown. 

 In rare instances there is a trace also of a narrow red nuchal band. 

 In the Rocky Mountain form, howeA^er, there is always a red nuchal 

 band, the red on the throat is more extended in the male, and a small 

 area of red appears also on the throat of the female. In the form met 

 Avith in the Cascade Rangie the red begins to spread OA^er contiguous 

 portions of the plumage, while in the form occurring along the Pacific 

 coast the red oA'erspreads the Avhole head, neck, and breast, through 

 Avhich, hoAvever, the markings of the eastern birds can generally be 

 readily traced. Here Ave liaA^e, at one end of the series, the red con- 

 fined to a few distinct patches about the head, while at the other it 



o In 1877 Sphyrapicus variiis included S. ndjcr as one of its subspecies, now 

 generally held to be a distinct species. — Aitthor'!i note, 1906. 



