This region is of immense extent, comprising all the temperate portions of the great eastern continents. It thus extends from the Azores and Canary Islands on the west to Japan on the east, a distance not far short of half the circumference of the globe. Yet so great is the zoological unity of this vast tract, that the majority of the genera of animals in countries so far removed as Great Britain and Northern Japan are identical. Throughout its northern half the animal productions of the Palæarctic region are very uniform, except that the vast elevated desert-regions of Central Asia possess some characteristic forms; but in its southern portion, we find a warm district at each extremity with somewhat contrasted features. On the west we have the rich and luxuriant Mediterranean sub-region, possessing many peculiar forms of life, as well as a few which are more especially characteristic of the Ethiopian region. On the east we have the fertile plains of Northern China and the rich and varied islands of Japan, possessing a very distinct set of peculiar forms, with others belonging to the Oriental region, into which this part of the Palæarctic region merges gradually as we approach the Tropic of Cancer. Thus, the countries roughly indicated by the names--Northern Europe, the Mediterranean district, Central and Northern Asia, and China with Japan--have each well-marked minor characteristics which entitle them to the rank of sub-regions. Their boundaries are often indefinable; and those here adopted have been fixed upon to some extent by considerations of convenience, dependent on custom and on the more or less perfect knowledge we possess of some of the intervening countries. [Illustration: PALAEARCTIC REGION] {181}_Zoological Characteristics of the Palæarctic Region._--The Palæarctic region has representatives of thirty-five families of mammalia, fifty-five of birds, twenty-five of reptiles, nine of amphibia, and thirteen of freshwater fishes. Comparing it with the only other wholly temperate region, the Nearctic, we find a much greater variety of types of mammalia and birds. This may be due in part to its greater area, but more, probably, to its southern boundary being conterminous for an enormous distance with two tropical regions, the Ethiopean and Oriental; whereas the Nearctic has a comparatively short southern boundary conterminous with the Neotropical region only. This is so very important a difference, that it is rather a matter of surprise that the two north temperate regions should not be more unequal in the number of their higher vertebrate forms, than they actually are. It is also to the interblending of the Palæarctic with the two adjacent tropical regions, that we must attribute its possession of so few peculiar family groups. These are only three; two of reptiles, _Trogonophidæ_ and _Ophiomoridæ_, and one of fishes, _Comephoridæ_. The number of peculiar genera is, however, considerable, as the following enumeration will show.