EVOLUTION OF THE COLORS OP BIRDS. 255 



northern forms are barred and the southern striped, yet 

 in both these instances geographical distribution must 

 have had a part in the differentiation. 



The lower map on Plate XII shows the distribution of 

 the different forms of the nighthawk (Ohordeiles). The 

 eastern variety extends northwest to the Pacific Coast, as 

 is the case with a number of eastern species. The west- 

 ern race is, as would be expected, paler, and the Florida 

 and Cuba bird darker — both due to direct environmental 

 influence. The differences between G. virginianus and the 

 southern species (B) C, texensis, may be due to climatic 

 influence entirely, the overlapping of the ranges being 

 subsequent; but it seems more probable that the char- 

 acteristic wing markings of this latter species are dis- 

 criminative marks developed to distinguish two diverging 

 forms occupying the same district. 



The difference between the hairy and downy wood- 

 pecker groups (Dryobates villosus and D . jyubescens) cannot, 

 it would seem, have been causedby geographical isolation. 

 To determine positively which of the other forms of isol- 

 ation as tabulated by Gulick has been operative in any 

 particular instance is, in the present state of our knowl- 

 edge, impossible. The red-winged black birds (Agelaius) 

 afford another instance of closely related species which 

 do not owe their differentiation to climatic or geographical 

 influences. A. phoe7iiceus, A. gubernator and A. tricolor, all 

 occur in California, the two latter species being confined 

 to the Pacific Coast. Plate XIV shows the principal mark 

 of distinction between these three species in the adult 

 male. These diiBferences are hardly striking enough to 

 be of great use as discriminative marks, particularly 

 between A. phoeniceus and A. gubernator, and would 

 seem to be the result of some form of isolation, rather 

 than of selection. In the case of the king birds, Tyran- 

 nus verticalis and T. vociferans, the differences appear to 



