EVOLUTION OF THE COLORS OF BIRDS. 245 



upon the white throat and upon the head, being in- 

 creased by sexual selection to the stage now shown by 

 the adult female of 8. varius in breeding plumage (fig. 

 3). The element of climate is next brought in con- 

 picuously to supplement the work of sexual selection. 

 The birds in the Rocky Mountain district have the red 

 considerably extended. Thus fig. 4 shows the female 

 of S. varius nuchalis, in which the red has spread very 

 considerably. On comparing the breeding plumage of 

 the adult males of the two races (figs. 5 and 6), it will 

 be found that a corresponding difference in the amount 

 of red exists in them. also. The climax is reached in 

 the Pacific Coast form, in which the red has become 

 almost complete upon the head and breast. 



As an instance of an increase in blue from north 

 to south, Mr. Ridgway cites the blue jays of the genus 

 Cyanocitta. The modifications of this genus in North 

 America are so interesting as regards the relation of 

 color to geographical distribution, that they must be 

 considered in some detail. The lower map on Plate X. 

 is a conventional representation of the areas occupied 

 by the different forms of the group. There are but two 

 species, C. cristata (A) and C. slelleri (B), occupying re- 

 spectively the eastern and western half of the North 

 American continent. Coming from the same stock, the 

 eastern bird early developed conspicuous white recogni- 

 tion marks on the wing and tail, which are lacking in 

 C. stelleri in all its forms. It remained very homoge- 

 neous throughout its range, only varying slightly in 

 Florida, where the white on the wings and tail is re- 

 duced, probably by direct climatic influence. In the 

 western provinces C. stelleri is as variable as C. cristata 

 in the east is constant. In the first place, there are two 

 general forms, one occupying the region upon the Paci- 

 fic Coast, and the other the Rocky Mountains south into 



