286 CALIFORNIA MAMMALS. 



latitude around the northern hemisphere, deflected here and there 

 by the effects of warm or cold ocean currents on the shores they 

 wasli. The presence of mountain ranges breaks up such uniform- 

 ity of climate and renders the definition of life zones very diffi- 

 cult, nowhere more so than in California, where, in many moun- 

 tains, island-like areas are detached from the main bodies of their 

 zones or long points project, or narrow bands curve to follow the 

 sinuosities of the mountain sides. The peculiar topography of 

 this state produces a variety of life zones which is probably equal- 

 ed by no other similar area elsewhere. Bordered as California is by 

 the sea; traversed its whole length by a mountain range, in places 

 carrying perpetual snow; possessing considerable areas lying be- 

 low sea level; having a range of annual rainfall varying from 80 

 inches in the northwestern part of the State to 3 or 4 m the south- 

 eastern part, it offers the student of climatology and of the dis- 

 tribution of life facilities unsurpassed in any civilized country, and 

 problems unknown in most other parts of the world. 



Long ago geographers divided the earth's surface into five 

 zones, giving them definite boundaries of certain parallels of lati- 

 tude founded on astronomical considerations. Biologists have 

 also divided the earth's surface into life zones and other divisions. 

 These divisions seldom have very definite boundaries, but blend 

 into one another. 



For my present purpose I shall follow the division of the 

 northern hemisphere into three Life Realms, as follows: The 

 Arctic Life Realm, surrounding the north pole and passing south- 

 ward to the northern limit of trees, or about the annual isotherm 

 of 32 degrees; the North Temperate Life Realm, extending 

 southward from the Arctic Life Realm to about the annual 

 isotherm of 70 degrees ; and a Tropical Life Realm. These Life 

 Realms are subdivided into Life Zones as follows : An Arctic 

 Life Zone, consisting of all the Arctic Life Realm; a Boreal Life 

 Zone, consisting of the upper or northern part of the North Tem- 

 perate Life Realm south to about the summer isotherm of 63 

 degrees; a Transition Life Zone, consisting of that part of the 



