1872.) ; 217 | [Allen. 
ties; yet, as the resemblance of the birds of these arid districts 
when young or in fresh plumage to those of the adjoining re- 
gions 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 slen- 
derer and more decurved bill, or the greater breadth of the trans- 
verse black bars on the breast of the southern form of Ortyzx virgin- 
wanus can be in the one case any more “protective,” or in the other 
give creater facility in obtaining food, than the different colors and 
the differently 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 gradual and 
general modification over wide areas that apparently points to cli- 
matic influence as the differentiating cause. There is, further, fre- 
quentty a closer assimilation of the sexes at the southward, as among 
the Icteride, through the greater increased brilliancy of the female 
as compared with the male, which is rather the reverse than otherwise 
of what is commonly supposed to be the result of sexual selection. 
Freely admitting, however, that both natural selection and sexual 
selection are causes of modification in the gradual differentiation of 
animals, I am led to regard them as secondary rather than primary 
elements, and that climate and other environing conditions take a 
larger share in the work than the majority of evolutionists seem will- 
ing to admit. Evidently no single law will explain all the phases of 
modification by descent, and in addition to those above alluded to, 
doubtless what Hyatt and Cope, among American zoologists, have 
termed the laws of acceleration and retardation are among the other 
causes of the modification. In birds, even, phenomena are apparent 
that cannot be strictly admitted into the category of geographical or 
climatic variations, but seem to singularly combine some evident, fea- 
tures of this character with a retention of a few embryonic char- 
acteristics, especially in respect to coloration, of allied intergrad- 
ing forms, as occurs in some of the birds of the middle portion of the 
