MUSEUM OF COMPARATIVE ZOOLOGY. 239 



while it seems to be a marked characteristic of many of the birds of 

 Lower California. The tendency in southern forms to an elongation of 

 the tail seems, however, less general than the southward decrease iu size 

 and the increase in color, or the tendency to an elongation of the bill. 



Among other local variations may be mentioned the white instead of 

 a red iris in the South Florida representatives of Pipilo erythroph- 

 thalmus ; the yellow instead of a black bill in the magpies of the coast of 

 California ; the white basal half of the feathers of the neck of the raven 

 of Southwestern Texas and Mexico, by which it is chiefly distinguished 

 from the common species ; the greater continuation anteriorly of the 

 superciliary stripe in the western forms of Zonotriclda leucophrys, 

 by which alone it is distinguishable from the eastern form ; the white 

 frontlet of one of the western forms of the Parus atricapillus group, 

 etc. There appears frequently to be also a locally greater development 

 of the foot in western and southern forms of wide-ranging species, 

 and occasionally an exceptional increase in general size under identical 

 isothermes. 



Causes of Climatic Variation. — The facts respecting climatic varia- 

 tion are at present too imperfectly known to be fully explained. There 

 are, however, certain peculiarities of climatic variation, especially in 

 color, coincident with certain meteorological peculiarities of the regions 

 where they occur, that demand attention. The increase in color to the 

 southward, especially the tendency to darker tints above shown to be so 

 general, coincides with the increase in the intensity of the solar rays to 

 the southward, and in the humidity of the climate. The southward 

 increase in depth of color and in iridescence in birds specifically identi- 

 cal coincides also with the general increase in brilliancy of color in 

 birds, taken as a whole, in the lower latitudes (as well as in insects 

 and animals generally), the maximum being reached in the tropics. 



The longitudinal variation, or the westward increase in color, seems 

 to be also coincident with the increased humidity to the westward, the 

 darker representatives of any species occurring where the annual rain- 

 fall is greatest, and the palest where it is least. This coincidence is 

 clearly illustrated in the birds of the United States, where the darkest 

 representatives of a species, as a general rule, (indeed without exception 

 so far as known to me,) come from regions of maximum annual rain- 

 fall, and the palest from those of minimum annual rain-fall. In the 

 Northeastern States the amount of rain is only one half to two thirds 



