194 BIRDS — LATITUDINAL VAEIATION. 



" The Choerocampa tersa, an elegant niiller, was taken in our garden, in the month 

 of May last. Dr. Harris describes it as a native of South Carolina, where it 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 shoul4 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 chiefly 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 

 AUeghenian. 



LATITUDINAL VARIATION. 



In deference to the authority whose nomenclature I adopt, and on wliom 

 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 

 an explanation. It is to be found in the fact that many birds inhabiting or 

 breeding in widely separated regions, and once considered different species, 

 are found to gradually lo^e their distinctive characters, as individuals 

 from intermediate localities are examined. In some instances, an ex- 

 amination of members of the same species from different localities has shown 

 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 :* 



" Id North America, a geographical variation exhibits two marked phases (as has 

 already been stated), a differentiation with difl'erences of latitude and elevation; and, 



* Proceedings of the Boston Society of Natural History, Vol. XV., June, 1872. 



