ORNITHOLOGICAL NOTES FROM THE WEST. 403 
though some of their call notes may be quite different. At other 
times they differ in habits, especially in respect to the situation 
and form of the nest, the same species at some localities breeding 
always on trees and at others almost as uniformly on the ground, 
in cases where the identity of the species is admitted by every 
ornithologist. In like manner some that at some localities build 
domed nests, at others build an open nest; others vary from a 
somewhat elaborate bulky nest, to a much simpler and more slight 
one; whilst in all cases the material varies in accordance with 
the respective abundance of whatever may be most suitable to 
the wants or habits of the species. Thus, on the Plains, many 
species line their nests with the soft hair of the buffalo in place of 
the down from certain plants they are accustomed to choose at 
other localities. 
The question of the occurrence of supposed hybridity among 
certain of the birds of the West is one of constantly increasing 
interest. The facts of the case are simply these :— that between 
several congeneric but widely diverse forms occur individuals over 
the region where the habitats of the two adjoin, which combine in 
varying degrees the characteristics of both forms. These individ- 
uals have been generally supposed to be hybrids between the forms 
they respectively resemble, but whether such or not, in a technical 
Sense, they are evidently fertile with either of the original forms, 
and also among themselves. Furthermore, that on either side of 
the area of « hybridity,” either form exhibits in varying degrees 
an admixture of the characters of the other, the degree lessening 
On either hand over an area of usually several hundred miles in 
breadth, till each form assumes nearly constantly its maximum 
divergence from the other. Such in general is the nature of what 
I have termed longitudinal variation, or the differentiation of con- 
Specific forms at localities differently situated in respect to lon- 
&itude. Similar differentiation occurs at localities differing in lat- 
arm which hybridity has never been assumed to explain, though 
it is difficult to see why it should not be called in as well in the 
_~ ©ase as in the other. In each case we have a similar grad- 
ual differentiation over extensive areas. Hybridity has been gen- 
my regarded as an unfailing test of specific diversity, but here 
one of two things must be assumed :— either that hybridity fails 
Sa test of specifie diversity, or else that these widely differing 
ic forms are only geographical modifications of the same 
