THE STRUGGLE TO LIVE 



407 



never or rarely come in sight of land. But most of the 

 familiar animals about us cannot live in water at all. They 

 either burrow in the ground like moles and gophers, or 

 live in trees like squirrels, or fly in the air like birds and 

 butterflies. 



Barriers. Of land animals some can live only in tropical 

 and sub-tropical regions, as the monkeys and most of the 

 parrots, some live only in the snowy regions near the poles, 

 as the polar bear and great walrus, while many prefer neither 



FIG. 204. The Parnassian butterfly, Parnassius smintheus, which Jives 

 in the Rocky Mountains and Sierra Nevada at an altitude of 5000 feet 

 and above. (Natural size). 



of these extremes but live in the temperate zones. Although 

 the word "prefer" has been used, it is usually true that 

 animals which live in arctic regions are not able to live 

 elsewhere; they seem to be adapted solely for an arctic 

 climate, so that the line around the earth south of which 

 there is frost and freezing weather during a part of the year 

 only is a sort of barrier beyond which they cannot safely 

 venture. And, turning to the animals of the tropics, we 

 find that most of them cannot endure any frost or freezing 



