242 CALIFORNIA ACADEMY OF SCIENCES. 



by insensible gradations into the one next south of it 

 (S, psaltria arizonce), found in New Nexico, Arizona and 

 northern Mexico in which the black on the head is 

 somewhat extended, the shoulders have become black 

 and mottlings of black have appeared in the back. Con- 

 tinuing south through Mexico the amount of black on 

 the back steadily increases, the rump patch being the 

 last to relinquish the olive color; " the bird now is var. 

 Mexicana, and continues with nearly the same char- 

 acteristics south to Costa Rica and Panama, from which 

 latter countries we find specimens in which the black is 

 often appreciably more intense and lustrous than in 

 those from Mexico. These three forms all have white 

 on the tail; but in specimens from New Granada, and 

 occasionally in those from Panama, there is usually a 

 total absence of white marks on the tail, or else they are 

 greatly reduced in size." Perhaps the most remarkable 

 thing about this variation towards black, which bears 

 such a direct and obvious relation to the changes in lat- 

 itude, and hence of climate, is the fact that the female 

 remains almost or quite the same through all the changes 

 of the male. 



The following is a possible line of explanation of this 

 state of affairs: the black pigment was first present in 

 the system of the bird in conjunction with the yellow, 

 giving rise to an olive green color. It was set apart in 

 a limited area upon the head by the action of sexual 

 selection or for sexual recognition. Then, the tendency 

 to the production of pure black pigment having once 

 been originated, would be favored by katabolism, which 

 would increase under the more abundant food and higher 

 temperature of the tropics. A specimen of S. psaltria 

 arizona was taken by Mr.W. Otto Emerson at Haywards, 

 Alameda County, some years since, while last summer I 

 saw in Berkeley, Alameda County, a goldfinch in com- 



