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 {81}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. +-------------+-----------------+ | | VERTEBRATA. | | REGIONS. |Families|Peculiar| | | |families| +-------------+--------+--------+ | Palæarctic | 136 | 3 | | Ethiopian | 174 | 22 | | Oriental | 164 | 12 | | Australian | 141 | 30 | | Neotropical | 168 | 44 | | Nearctic | 122 | 12 | +-------------+--------+--------+