EVOLUTION AND DISTRIBUTION OF ANIMALS. 319 



extension of species is only relative. That which constitutes an 

 impassable barrier to some gronps 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 com- 

 prises nearly all of Asia, Europe, and North America, the arctic 

 and north temperate zones. The north temperate zone has prac- 

 tically a continuous climate, the chief variations being in eleva- 

 tion 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 Kussel 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 For- 

 mosa, and the bare hills of China pass successively in review, until 

 after a circuitous journey of thirteen thousand miles he finds him- 

 self at Hakodadi in Japan. He is now separated from his start- 

 ing-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 tenante'd by tits, hedge-sparrows, wrens, wagtails, larks, 

 redbreasts, thrushes, buntings, and house-sparrows, some abso- 

 lutely 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 in- 

 sects 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). 



