JTo.l.] GRAY AND HOOKER ON THE ROCKY MOUNTAIN FLORA. 71 



To comi)lete tlio view, let it bo noted that tlio divisiou of these forests into coniftjr- 

 ous and non-coniferous is, for tlie — 



Genera. Specica. 



European non-coniferous 20 C8 



European coniferous 7 17 



3:! 85 



Japan-Mauchurian non-coniferous 47 123 



Japan-Manchurian coniferous 19 4'> 



66 168 



In other words, a narrow region in Eastern Asia contains twice as many genera and 

 about twice as many species of indigenous trees as are j)ossessed by all Europe; and 

 as to coniferous trees, the former has more genera than the latter has species, and 

 over twice and a half as many species. 



The only question about the relation of these four forest regions, as to their com- 

 ponent species, which we can here pause to answer, is to what extent they contain 

 trees of identical species. If we took the shrubs, there would be a small number, if 

 the herbs, a very considerable number, of species common to the two New World and 

 to the two Old World areas, respectively, at least to their northern portions, even after 

 excluding arctic-alpiue plants. The same may be said, in its degree, of the North 

 European flora compared with the Atlantic North American, of the Northeast Asiatic 

 compared with the northern part of the Pacific North American, and also in a peculiar 

 way (which I have formerly pointed out, and shall have soon to mention) of the North- 

 eastern Asiatic flora in its relations to the Atlantic North American. But as to the forest 

 trees there is very little community of species. Yet tliis is not absolutely wanting. 

 The Red Cedar {Jun'iperus Virginlana) among coniferous trees, and Pojmlus fremuloides 

 among the deciduous, extend across the American continent specifically unchanged, 

 though hardly developed as forest trees on the Pacific side. There are probably, but 

 not certainly, one or two instances on the northern verge of these two forests. There 

 are as many in which eastern and western species are suggestively similar. The Hem- 

 lock-Sjjruce of the Northern Atlantic States and the Yew of Florida are extremely like 

 corresponding trees of the Pacific forest. Indeed, the Yew-trees of all four regions 

 may come to be regarded as forms of one polymorphous species. The White Birch of 

 Europe and that of Canada and New England are in similar case, and so is the com- 

 mon Chestnut (in America confined to the Atlantic States), which, on the other side 

 of the world, is also rej)resented in Japan. A link in the other direction is seen in 

 one Spruce-tree (called in Oregon Menzies Spruce) which inhabits Northeast Asia, 

 while a peculiar form of it represents the species of the Rocky Mountains. 

 But now other and more theoretical questions come to be asked, such as these : 

 Why should our Pacific forest region, which is rich and in some respects unique in 



coniferous, be so poor in deciduous trees ? 

 Then the two Big-trees, Sequoias, as isolated in character as in location — being 



found only in California, and having no near relatives anywhere — how came California 



to have them? 



Such relatives as the Sequoias have are also local, peculiar, and chiefly of one 



species to each genus. Only one of them is American, and th.at solely eastern, the 



Taxodium of our Atlantic States and the plateau of Mexico. The others are Japanese 



and Chinese. 

 Why should trees of six related genera, which will all thrive in Europe, be restricted 



naturally, one to the eastern sideof the American continent, one genus to the western 



side and very locally, the rest to a small portion of the eastern border of Asia f 



Why should coniferous trees most affect and preserve the greatest number of types 



in these parts of the world ? 



