GEOGRAPHICAL VARIATION IN NORTH AMERICAN BIRDS. 539 
riation upon the question of origin of genera and species? Hav- 
ing approached the subject from a geographical standpoint, my 
own impression of the importance of the conditions of environ- 
ment in modifying the characteristics of animals may have unduly 
impressed me; yet that they exercise a greater influence than is 
currently recognized I think must be admitted. How, for instance, 
can natural or sexual selection satisfactorily account for the occur- 
rence of pallid forms in arid, semi-desert regions, and of brighter 
colored forms in contiguous humid districts, or the generally in- 
creased intensity of color southward, and its maximum develop- 
ment only toward and within the tropical regions? In many 
cases, it is true, the change in color may be protective, as it doubt- 
less is in the assimilation of the pale tints of birds and other ani- 
mals inhabiting arid plains to the generally gray color of the 
vegetation and the earth itself in such localities; yet, as the re- 
semblance of the birds of these arid districts when young or in 
fresh plumage to those of the adjoining regions at the same season 
is much greater, as a general rule, than at the end of the breeding 
Season, we have thus palpable evidence of the direct modification 
of color by environing conditions. Again, it is hard to see how 
the intenser and darker shades of the iridescence of the Quiscali 
in the South Atlantic and Gulf States, or their slenderer and more 
decurved bill, or the greater breadth of the transverse black bars 
on the breast of the southern form of Ortya Virginianus can be in 
the one case any more “protective,” or in the other give greater 
facility in obtaining food, than the different colors and the differ- 
ently proportioned beaks of the northern forms of these species ; 
or of what advantage the large claws and long tails can be at 
Southern localities rather than at northern. The variation in color 
is not apparently.any better explained by sexual selection than are 
the other modifications by natural selection, for it is hardly sup- 
posable that sexual selection should act in so uniformly an acceler- 
ated degree toward the southward, or so generally from arid 
regions toward moister ones. On the contrary, it is just this grad- 
ual and general modification over wide areas that apparently 
Points to climatic influence as the differentiating cause. There is, 
farther, frequently a closer assimilation of the sexes at the south- 
y »48 among the Jcteridw, through the greater increased bril- 
lianey of the female as compared with the male, which is rather 
