inches across, while the little popgun elder of the 
East is supplanted here by a species 12-20 inches 
in diameter. 
Most of the trees mentioned are indigenous to 
California and three-fourths of them are found 
only in that state. Why this great prodigality of 
Nature in behalf of the Pacific Slope, and espe¬ 
cially of little California? 
The solution of this problem involves a brief 
discussion of certain controlling factors. 
At the outset we may observe that an impassable 
climatic barrier is set up at present, by Nature, 
preventing migration north and south. The Torrid 
Zone, in which no resinous trees can grow except 
on high peaks, .separates the world’s forests into 
unequal and very different floras. 
The Southern Hemisphere is the home of the 
Araucaria, the Eucalyptus and the Acacia, while in 
the Northern Hemisphere are found—in addition 
to the hosts of broad-leaved, non-resinous trees, 
such as oak, ash, hickory, etc.—all of the large 
families of pine, larch, cedar, spruce, and fir, with 
the redwood, cypress, and juniper; the distribution 
of these trees across the two continents, however, is 
very unequal. 
DISPARITY OP AREAS AND DISTRIBUTION. 
The northern part of the eastern continent— 
Eurasia—is approximately 9,000 miles across. 
North America is but 3,000. We would naturally 
expect, for instance, three times as many pines in 
Eurasia as in America. Just the reverse is the 
case. Of the 80 species of known pines only 20 
