244 CALIFORNIA ACADEMY OF SCIENCES. 



much more intensely red toward Mexico and Central 

 America. The house finch (Oarpodacus mexicanus fron- 

 talis) is an especially instructive instance with respect to 

 the spreading and intensification of the red color. To- 

 ward the south (G. mexicanus) the red becomes intense, 

 but sharply restricted in extent. In the western United 

 States it is less intense but more diffused, while upon 

 Guadalupe Island (G. aviplus) it is both more intense 

 and more diffused, but the most deeply colored portion 

 is restricted to the same areas as in G. mexicanus. 



The transformations in the genus Sphyrapicus are of 

 so much interest that I have figured the heads of the 

 three forms, arranging them in a regular series of in- 

 creasing red (Plate XV). It is to be observed that 

 the series is a double one — from young to adult female, 

 to adult male, increasing in redness, and from east to 

 west the same. The upper map in Plate IX. indicates 

 roughly the areas occupied by the three forms, A being 

 the distribution of the S. varius group and B of S. ruber. 

 In spite of the fact that these two forms are classed as 

 distinct species, there can be no doubt that they are 

 climatic modifications of the same stock. The young of 

 S. varius and 8. varius nuchalis are mottled brown, and 

 with the colored areas of the adult male white. This is 

 doubtless the plumage of the primitive bird from which 

 the genus arose. The next stage of differentiation is 

 shown in fig. 2 of the adult female in winter. The 

 acquisition of a black plumage, not very different from 

 the present coloration of Dryobates, may have been due 

 to sexual selection, to a surplus of pigment in the sys- 

 tem, to direct climatic influence, or to the need of recog- 

 nition markings. However this may be, the brown 

 color of the young would seem to indicate the presence 

 of red pigment in the system in conjunction with the 

 black. This is deposited, in small quantities at first. 



