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 
boundary in the obstacles offered to 
the spread of the average animal. Each 
species broadens its range as far asitcan. 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 
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 
Barriers to 
diffusion. 
Holarctic realm. 
