28 LIFE ZONES AND CROP ZONES. 



last-mentioned valleys the summer rainfall (May to September) is less 

 than 5 inches, while in western Washington (not including the coast 

 or mountains) it varies from 5 to 10 inches. The region as a whole is 

 one of relatively uniform temperature, the wide seasonal differences 

 usual in other parts of the Transition zone being unknown. The 

 temperature of the summer season, the hottest part of the year, is 

 phenomenally low for the latitude, enabling northern or Boreal types to 

 push south as far as latitude 35°. On the other hand, the summer sea- 

 son is so j)rolonged (from the standpoint of temperature) that the total 

 quantity of heat for the entire season is phenomeuallj^ high for the 

 latitude, enabling southern or Austral species to push north as far as 

 Puget Sound, where the total quantity of heat is even greater tlian at 

 Philadelphia, Pittsburg, Cleveland, and Omaha, although Puget 

 Sound is 500 miles north of tlie latitude of these i^laces. Even at 

 Cape Flattery — the extreme northwestern point of the United States — 

 which is exposed throughout the year to the cold coast fogs, the total 

 quantity of heat is 500° F. greater than at Eastport, Me., although 

 the latter is the more southern locality and lias the liigher mean 

 summei' temperature. The low summer temperature along the Pacific 

 Coast i)ei'mits northern species to come far south, while the high sum 

 total of heat ena])les southei-n species to push northward as far as 

 Puget Sound. Sucli an extensive overlapping of Boreal and Austral 

 faunas does not occur elsewhere in North America, and for the 

 evident reason that no area approaching it in oxtrcnt has so equable a 

 temperature. 



In most parts of tlie Ilnitod Stales it is easy to distinguish the 

 boundaiios between the Transition and Upper Austral zones, but in 

 the T*acilic Transition area these distinctions are nearly obliterated, 

 a large proportion of the species ranging in common over both belts. 



To the soutliward, particularly'^ away from the coast, where the 

 Pacific Transition area receives less moisture aiul more summer heat 

 and tlie seasonal changes of tcmpei-ature are greater, the faunal rela- 

 tions become moi'e and more complex. The decrease in humidity 

 allows Aiid Tiansition species to creep in, while the increase in total 

 quantity of heat biings with it n marked increase in the number of 

 Ujiper Sonoi'an species, so that types ordinai'ily chaiacteristic of the 

 humid ]*acitt(' Transition, the Arid Transition, and the Upper Sonoran 

 occur together.' The total (piantity of heat in most parts of the Wil- 

 lamette and Rogue Rjver valleys is about 2,000° F. more than in 

 the Puget Sound I'egion, and is as great as on the adjacent plains of 

 tlie Columbia and slightly greater than at St. Louis, JNIo., and Wash- 

 ington, D. C. 



These warmer and less humid parts of the Pacific Transition area, 



' Hood River, at the north base of the Cascade Range in Oregon, is situated in 

 the belt of overlappinj?. where the humid Pacific Transition of the gorge of the 

 Cohimbia merges into the ai-id Upper Sonoran of tlie adjacent Great Basin. 



