extension of species is only relative. That which constitutes an impassable barrier to some groups is a high-road to others. The river which opposes the passage of the monkey or the cat would be the king's highway to the frog or the turtle. The waterfall which checks the ascent of the fish would be the chosen home of the ouzel.
In spite of the great variety among the barriers existing on the earth, we may divide the globe roughly into five realms or areas of distribution, having their boundaries in the sea or in differences of climate. One or two of these realms are sharply defined; the others are surrounded by a broad fringe of debatable ground, which forms a region of transition to some other zone.
The largest of these realms is the holarctic realm, which comprises nearly all of Asia, Europe, and North America, the arctic and north temperate zones. The north temperate zone has practically a continuous climate, the chief variations being in elevation and rainfall. The close union of Alaska to Siberia forms an almost unbroken land area from the eastern coast of America around to western Europe. To the south the species increase in number and variety; the arctic regions are remarkable for what they lack, yet the general character of the life is almost unbroken over this vast district. Alfred Russel Wallace refers to this unity of northern life in these words:
"When an Englishman travels by the nearest sea route from Great Britain to northern Japan, he passes by countries very unlike his own both in aspect and in natural productions. The sunny isles of the Mediterranean, the sands and date-palms of Egypt, the arid rocks of Aden, the cocoa-groves of Ceylon, the tiger-haunted jungles of Malacca and Singapore, the fertile plains and volcanic peaks of Luzon, the forest-clad mountains of Formosa, and the bare hills of China pass successively in review, until after a circuitous journey of thirteen thousand miles he finds himself at Hakodadi in Japan. He is now separated from his starting-point by an almost endless succession of plains and mountains, arid deserts or icy plateaus; yet, when he visits the interior of the country, he sees so many familiar natural objects that he can hardly help fancying he is close to his home. He finds the woods and fields tenanted by tits, hedge-sparrows, wrens, wagtails, larks, redbreasts, thrushes, buntings, and house-sparrows, some absolutely identical with our own feathered friends, others so closely resembling them that it requires a practiced ornithologist to tell the difference. . .. There are also, of course, many birds and insects which are quite new and peculiar, but these are by no means so numerous or conspicuous as to remove the general impression of a wonderful resemblance between the productions of such remote islands as Britain and Yesso" (Island Life).
