ipis-] OF EASTERN NORTH AMERICA. 167 



it is natural to suppose that this part of Asia (or more probably, 

 to allow for the outlying species in California, and the oaks in 

 Mexico, a region east from southeast Asia) has been the center of 

 distribution, and hence the point of origin of the pasania-chestnut- 

 oak group. And Quercus itself, with its black oaks limited to 

 America, its Cyclobalanopsis limited to southeast Asia, and its nu- 

 merous white oak species in both places, undoubtedly differentiated 

 from the pasanias (or their ancestors) in one or other of these re- 

 gions, or more probably between the two. At any rate, the primi- 

 tive, little-differentiated Quercus must have had a distribution that 

 included both regions, as well as the space between them. We are 

 thus brought again to an hypothetical Pacific continent ; for since 

 neither black oak nor Cyclobalanopsis exists or gives evidence of 

 having existed in western Asia or Europe, any cretaceous or earlier 

 connection of the two regions in that direction is well-nigh incon- 

 ceivable. (It is unnecessary to suppose that this Pacific land ex- 

 tended much farther north than the equator). 



According to our hypothesis, the disappearance of this Pacific 

 land isolated the two extremes of the range of Quercus. The genus 

 had already become differentiated ; the Asiatic part of the range re- 

 ceived the stock of Cyclobalanopsis (found nowhere else) as well 

 as the more typical Quercus stock. Certain species of Quercus, 

 even today, form a part of the oldest Asiatic flora, which holds its 

 own in isolated regions, — in parts of the Himalayas, for instance. 

 Some of these ancient endemic species are the white oaks Q. lanata, 

 semecarpifolia, and dilatata, of which the last is said by Schottky 

 to stand nearest of all oaks to the Cyclobalanopsis group. (Ameri- 

 can black oaks, however, show certain features in common with 

 Cyclobalanopsis — apical ovules, type of style). 



The American end of the range received a group of oaks of 

 which (according to evidence from distribution and palaeontology) 

 Quercus chrysolepis is probably our nearest representative; these 

 may have been the basis of both the black and the white oaks of 

 America. It is suggestive to find that Q. semecarpifolia (represen- 

 tative of the ancient oaks of Asia) bears some resemblance to this 

 early American oak. Some of the European oaks are also of this 

 ancient type ; but since one, Q. Ilex, occurs in both Asia and Europe, 



