80 DISTRIBUTION OF ANIMALS. [PART J. 
Encyclopedia Britannica, thinks that as regards that class it 
can hardly claim to be more than a sub-region of the Neotropical. 
These views are mutually destructive, but it will be shown in 
the proper place, that on independent grounds the Nearctic 
region can very properly be maintained. 
Subdivisions of the Nearctic Region—The sub-regions here 
depend on the great physical features of the country, and have 
been in some cases accurately defined by American naturalists. 
First we have the Californian sub-region, consisting of Cali- 
fornia and Oregon—a narrow tract between the Sierra Nevada 
and the Pacific, but characterized by a number of peculiar 
species and by several genera found nowhere else in the region. 
The second, or Rocky Mountain sub-region, consists of this 
great mountain range with its plateaus, and the central plains 
and prairies to about 100° west longitude, but including New 
Mexico and Texas in the South. 
The third and most important sub-region, which may be 
termed the Alleghanian, extends eastward to the Atlantic, in- 
cluding the Mississippi Valley, the Alleghany Mountains, and 
the Eastern United States. This is an old forest district, and 
contains most of the characteristic animal types of the region. 
The fourth, or Canadian sub-region, comprises all the northern 
part of the continent from the great lakes to the Arctic ocean ; 
a land of pine-forests and barren wastes, characterized by Arctic 
types and the absence of many of the genera which distin- 
guish the more southern portions of the region. 
Observations on the series of Sub-regions—The twenty-four sub- 
regions here adopted were arrived at by a carefui consideration 
of the distribution of the more important genera, and of the 
materials, both zoological and geographical, available for their 
determination; and it was not till they were almost finally 
decided on, that they were found to be equal in number through- 
out all the regions—four in each. As this uniformity is of great 
advantage in tabular and diagrammatic presentations of the 
distribution of the several families, I decided not to disturb 
it unless very strong reasons should appear for adopting a greater 
or less number in any particular case. Such however have not 
