Geographical Distribution 



165 



tain summits and arctic lands in North America, while some 

 extend around the world. On the mountain summits they 

 form isolated groups, cut off from their congeners of the 

 north by the wide intervening plains and valleys. How have 

 they come there? In glacial days, when the ice sea swept 

 southward to New Jersey, Illinois and Nebraska, and gla- 

 ciers covered the higher slopes of our western mountains, 

 plants and animals were forced to move before it ; for the 

 Ice King is an inexorable landloi-d, and when he undertook 

 to dispossess the tenants of the lands there was no gainsay- 



PiKA, OR EocKY Mountain Hare 



An inhabitant of rock slides both above and below timber line. Photo 

 by E. R. Warren. From Metcalf, "Organic Evolution," 

 By permission of the Maimillan ComiJany. 



ing his wishes in the matter. But with the retreat of the 

 ice the former tenants returned to their old abodes. Some 

 of them however instead of moving north once more after 

 the retreating ice, found a more convenient path up the 

 mountain sides and thus came to settle in a new home, on 

 the bleak mountain tops where they found the climate to 

 their liking. 



So too the animal life of alpine summits contains many 

 species, common alike to inonntain top and ban-en ground of 

 tlie far North, tiiongh the number repoi'ted for the San Fran- 

 cisco Mountain is too few to allow any generalizations con- 



