are indigenous to Eurasia 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 
