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 California 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, including 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 distinguish 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 careful 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 throughout 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 arisen; and it is hoped that these divisions will prove as satisfactory and useful to naturalists in general as they have been to the author. Of course, in a detailed study of any region much more minute sub-division may be required; but even in that case it is believed that the sub-regions here adopted, will be found, with slight modifications, permanently available for exhibiting general results.

I give here a table showing the proportionate richness and speciality of each region as determined by its families of vertebrates and genera of mammalia and birds; and also a general table of the regions and sub-regions, arranged in the order that seems best to show their mutual relations.

Comparative Richness of the Six Regions.

Regions.Vertebrata.Mammalia.Birds.
Families.Peculiar
families.		Genera.Peculiar
genera.		Per
centage.		Genera.Peculiar
genera.		Per
centage.
Palæarctic136310035351745733
Ethiopian17422140906429417960
Oriental16412118554634016548
Australian1413072446129818964
Neotropical168441301037968357686
Nearctic122127424321695231

Table of Regions and Sub-regions.

Regions.Sub-regions.Remarks.
I.Palæarctic1. North Europe.
2. Mediterranean (or S. Eu.)Transition to Ethiopian.
3. Siberia.Transition to Nearctic.
4. Manchuria (or Japan)Transition to Oriental.
II.Ethiopian1. East Africa.Transition to Palæarctic.
2. West Africa.
3. South Africa.
4. Madagascar.

III.

Oriental1. Hindostan (or Central Ind.)Transition to Ethiopian.
2. Ceylon.
3. Indo-China (or Himalayas)Transition to Palæarctic.
4. Indo-Malaya.Transition to Australian.
IV.Australian1. Austro-Malaya.Transition to Oriental.
2. Australia.
3. Polynesia.
4. New Zealand.Transition to Neotropical.
V.Neotropical1. Chili (or S. Temp. Am.)Transition to Australian.
2. Brazil.
3. Mexico (or Trop. N. Am.)Transition to Nearctic.
4. Antilles.
VI.Nearctic1. California.
2. Rocky Mountains.Transition to Neotropical.
3. Alleghanies (or East U. S.)
4. Canada.Transition to Palæarctic.