196 



FOOT-NOTES TO EVOLUTION. 



The various divisions or realms into which the sur- 

 face of the earth may be divided on the basis of the 

 differences in animal life, each has its 

 Barriers to boundary in the obstacles offered to 



diffusion. , , r 1 • 1 -r^ 1 



the spread of the average animal. Each 

 species broadens its range as far as it can. It struggles, 

 knowingly or not, to overcome the barriers of ocean or 

 river, of mountain or plain, of woodland or desert, of 

 moisture or drought, of cold or heat, of lack of food or 

 abundance of enemies — whatever these barriers may be. 

 Were it not for these barriers, every species would be- 

 come what only man now is, practically cosmopolitan. 

 Man is pre-eminently the barrier-crossing animal. The 

 degree of hindrance offered by any barrier to the exten- 

 sion 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 is the chosen home of the ouzel. 



In spite of the great variety among the barriers ex- 

 isting 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 

 Holarctic realm. „, , , 



zones. I he 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 



