Oe RT Onn et Te ee ee! 
A. Gray—North American Flora. 333 
understand how Europe came to lose these elements of her 
flora, and Atlantic North America to retain them, we must 
recall the poverty of Europe in native forest trees, to which I 
have already alluded. A few years ago, in an article on this 
subject, I drew up a sketch of the relative richness of Kurope, 
Atlantic North America, Pacific North America and the eastern 
elements were somewhat under-rated. I allowed only 38 
genera and 85 species, while to our Atlantic American forest 
4 I 
were assigned 66 genera and 155 species. I find from Nyman’s 
by the hand of man. On Nyman’s authority i may put into 
this category Cercis Siliquastrum, Ceratonia Siliqua, Diospyros 
otus, Styrax officinalis, the Olive, and even the Walnut, the 
Chestnut, and the Cypress. However this may be, it seems 
clear that the native forest flora of Europe is exceptionally 
by Europe. First, Europe, extending but little south of lat. 
40°, is all within the limits of severe glacial action. Second, 
flanked the mountain ranges, or were stationed south of them, 
Stretched the Mediterranean, an impassible barrier. . . Escape 
Y the east, and rehabilitation from that quarter until a very 
ate period, was apparently prevented by the prolongation of 
the Mediterranean to the Caspian, and probably thence to the 
Siberian Ocean. If we accept the supposition of Norden- 
* This Journal, III, xvi, 85. 
