322 THE OAK-VEGETATION OF AMERICA. 
of Sunda 37 species. The northern coast-lands of Africa possess seven 
sorts, the Canary Islands (Madeira) one sort; but none is met with in 
middle and south Africa, or the islands belonging thereto. New Holland 
and Australia have no oaks, nor South America, south of the line. Thus 
Europe counts 20, Asia 97, and Africa 8 species. But since several 
of the South European Oaks occur again in Asia Minor and the ad- 
joining countries, and in northern Africa, the sum total of Oaks in 
Europe, Asia, and Africa must be reckoned at 110 species. 
From America 101 species of Oaks have already been described ; 
which number, however, will probably suffer a not inconsiderable re- 
duction, when the species are critically revised. I venture, neverthe- 
less, to assert, that the American Oaks surpass in number the aggregate 
amount from all other parts of the world, with reference only to the 
forms preserved in the herbaria of Europe—nay, to those only which 
are at present in my hands*. 
It is deserving of notice, that while other parts of the world have 
several Oak-species in common, the case is not so as regards America, 
where not one single species exists from thence. The cause of this is 
to be traced, partly in the circumstance, that neither in America nor 
Asia do the Oaks extend so far north, as to be able to migrate from 
one quarter into another, where the transition is shortest; and partly 
in the extraordinarily brief vitality of the acorn, by which the ocean is 
prevented from transporting them in a living condition, from one part 
of the globe to the other. 
As in Europe, north of the Alps, the Oaks are deciduous, and continue 
leafless during all the winter, while in the Mediterranean they are ever- 
green; so also do we find, that, in North America, to the north of the 
equator, and as far as a considerable difference exists between the tem- 
perature of summer and winter, the Oaks are deciduous; while those in 
its tropical and subtropical zone, are evergreen. 
It has hitherto been a prevailing notion, that the oak-form is pecu- 
liarly characteristic of the temperate zone. But whether we look to 
the number of species, the beauty of the forms, or the size of par- 
ticular organs (leaves, fruits, cups), we shall find their maximum in 
* Among the contributions to the study of American Oaks, which I have received 
from abroad, I must above all advert to the rich calleeticis o. berally confided to me 
by Sir William J. Hooker, containing as it does, besides most of the published Ameri- 
can Oaks, a number of un ibed i ; : "HESS 
he author, in a letter to the translater)” [This maine added by desire 
