Geographical Distribution 175 



American origin, the short-tailed or Canada porcupine."^ 

 Of birds there are a large number of characteristic species, a 

 mere enumeration of which would hardly carry conviction 

 to the general reader. 



Leaving behind us the forests of Douglas sprace and bal- 

 sam fir, we enter an open "forest of statelj^ pines . . . which 

 average at least ... 100 feet in height. There is no under- 

 growth to obstruct the view, and after the rainy season the 

 grass is knee-deep in places. ..." This forest covers the 

 mountain side between 7,000 and 8,200 feet, some of its trees 

 extending even to 8,800 feet among the spruce and fir. It 



The Beaveb 



From a group in the American Museum of Natural History. 



Courtesy of the Museum. 



marks a debatable area, where boreal forms come down co- 

 mingling with southern types, and hence has been aptly 

 termed the transition zone. It has but few distinctive spe- 

 cies either on San Francisco jMountain or elsewhere, being 

 characterized rather by a mixture of types. In general it 

 occupies the northern half of the United States, bending far 

 southward along the mountain ranges, and running north 

 along the river valleys, which serve as paths of northern in- 

 vasion for southern forms. Southern animals which cross 



'Scott, "History of Land Mammals of the Western Hemisphere," p. 

 151. By permission of the Macmillan Company. 



