are indigenous to Kurasia while 60 are flourishing 

 in America. 



Again, the Pacific Slope region, from the Rocky 

 Mountains to the Pacific shore, is about i ,000 miles, 

 one-third of the distance across, yet it has 40 out 

 of the 60 American species, 15 being in Mexico and 

 25 in the western United States, with 20 of these 

 in California, a narrow strip of coast only 800 miles 

 long by 150 wide, yet containing as many pines as 

 all Eurasia! 



Now if the distribution was equal, Eurasia hav- 

 ing 60 pines and North America 20, the Pacific 

 Slope, being one-third of America, would be en- 

 titled to but 6% species, and little California, 

 which embraces about one-tenth of the Pacific 

 Slope, would have little more than half a chance to 

 get one species! 



This excessive prodigality of Nature in favor of 

 the Pacific Slope and especially the California part 

 of it, is due to a combination of factors, chief of 

 which are the contours of continents, the trend of 

 principal mountain ranges, the behavior of certain 

 oceanic and atmospheric currents, the alternate 

 elevation and depression of continental areas, to- 

 gether with the ability of all these factors to 

 modify the effects of certain crucial climatic 

 periods, called 



THE ICE AGE AND THE THERMAL AGE. 



The phenomenon of hot and cold periods in 

 the earth's history compelling the migration, the 

 change of location of the entire organic world 

 the kingdoms of the animals and plants is a 



(5) 



