Without going into the zoogeographical regions in detail, we may note that there is, as already stated, considerable resemblance between the mammals of Europe, Asia, Africa and North America, that is, of the land hemisphere, while South America, which was for long isolated from North America, has a peculiar and relatively primitive fauna, and Australia, whose isolation has lasted longer, has an even more peculiar and a much more primitive fauna. When we look at the fauna of the great land mass formed by the continents of Europe, Asia, Africa and North America, sometimes called by zoogeographers the _Arctogæic_ realm, we find that North America differs from the eastern land mass as regards its land mammals in several respects. Though long separated from South America it has been connected long enough for some of the southern forms to find their way northwards, so that we find skunks, raccoons, and other mammals strikingly different from analogous forms found in the Old World. Again, it is relatively so long since there was any free communication between the eastern and western hemispheres that the two faunas have had time to diverge without destroying the fundamental resemblance. Beginning with the fauna of the Old World, we find that no effective barrier of any sort separates the animals of Europe, even of western Europe, from the animals of temperate Asia, even of eastern Asia. Right across from the British Isles to Japan, through about a hundred and fifty degrees of longitude, there is great general similarity in the land animals. To the south, on the other hand, the Atlas mountains and the African desert cut off the greater part of the continent of Africa, and eastwards the transverse mountain chains, no less than the difference of climate and the cold, barren nature of the uplands of central Asia, cut off the rich fauna of the peninsula of India with Further India, etc., from the habitable regions of temperate Asia, with their scantier fauna. We are thus left with the conception of a very large and tolerably uniform zoological region, stretching right across Europe and temperate and northern Asia. This is the Palæarctic region of zoogeographers. The European section of it is somewhat impoverished as compared with the Asiatic section, partly perhaps because of the effects of the ice, and certainly also because for long ages Europe has been densely populated, and the larger wild animals have thus been exterminated. Asia, with its northern forests and its more southerly steppes, has always been a great reservoir of life, which has periodically overflowed into Europe. Some of these overflowing animals, like the black and the brown rats, succeeded in establishing themselves very firmly; others, like the saiga antelope, died out rapidly except in the extreme east of the European area.