194 BIRDS—LATITUDINAL VARIATION. 
‘“The Choerocampa tersa, an elegant miller, was taken in our garden, in the month 
of May last. Dr. Harris describes it as a native of South Carolina, where 1t feeds on a 
species of plant which does not grow at the North. The food it finds as a substitute: 
has not been discovered.” 
From this description of the peculiarities of the climate of the southern 
shore of Lake Erie, we may not only understand, why many birds, 
usually considered as more southern in their breeding range, should 
choose that region for their summer residence, but also why some north- 
ern birds should find suitable winter quarters there, and for that reason do 
not visit, in numbers, the central and southern portions of the State. 
In other States, localities have been recently discovered which are 
peculiar in their bird fauna, probably for similar reasons. 
Differences in the fauna of the eastern and western portions of the 
State are not marked, and consist ch?efly in a difference in the relative 
abundance of some species. There are a few species, however, which are 
not found in the eastern portions of the State. 
By far the largest part of the State is strictly Carolinian in its faunal 
characters; a small portion, the north-eastern corner, is as emphatically 
Alleghenian. 
LATITUDINAL VARIATION. 
In deference to the authority whose nomenclature I adopt, and on whom 
I have drawn for descriptions, several Ohio birds are described as varieties 
of species which also are given as Ohioan. 
To those who have never made the science of ornithology a study, or 
who are unacquainted with its recent progress, this may require 
anexplanation. Itis tobe found in the fact that many birds inhabiting or 
breeding in widely separated regions, and once considered different species, 
are found to gradually lose their distinctive characters, as individuals 
from intermediate localities are examined. In some instances, an ex- 
amination of membersofthe same species from different localities hasshown 
a constant and uniform difference in characters, which difference is ex- 
pressed in calling one group a variety of the other. | 
As the variation between birds of the same species must follow some 
law or laws to be of any scientific interest or value, the determination of 
these laws has engaged the attention of the most eminent American orni- 
thologists in late years. Among these investigators none have been 
more prominent than Prof. J. A. Allen, of the Museum of Comparative 
Anatomy, from whose writings I quote the following on the subject of 
latitudinal variation :* 
‘‘In North America, a geographical variation exhibits two marked phases (as has 
already been stated), a differentiation with differences of latitude and elevation; and, 
* Proceedings of the Boston Society of Natural History, Vol. XV., June, 1872. 
