1915.] OF EASTERN NORTH AMERICA. 171 



nearest relatives in the modern Asiatic oaks, which were mentioned 

 as having probably reached Asia in Tertiary times from the east- 

 ward. The gambeUi group in the Rockies and the Atlantic group 

 are apparently the separated branches of the latest developed white 

 oaks (and the Californian oaks are perhaps a third corresponding 

 group), which before glaciation may have succeeded in covering the 

 greater part of the continent. Glaciation left survivors of this 

 forest, it would seem, in two parts of the land — mountainous re- 

 gions which projected above the ice — the southern Rockies, and tde 

 southern Alleghanies. From the one Q. gambelii has spread north- 

 ward, keeping rather closely to the mountains and differentiating 

 numerous but similar species ; while from the other the early species 

 (possibly lyratiformis and minor) have recovered an enormous 

 stretch of territory, and have produced a correspondingly large 

 number of varied species. 



IV. White Oaks of Eastern North America. 



The white oaks found east of the Rocky Mountains comprise 

 the following species (see key) : 



I. hreviloha 2. lyrata 



durandii hicolor 



macrocarpa 



3. chapmani 4. michauxii 

 minor prinus 



margaretta muhlenhergii group. 



alha 



These species are all of the deciduous, thin-leaved type of Leu- 

 cohalanus, except that durandii and hreviloha, in ranging from Ala- 

 bama west and south into northern Mexico, show a series of transi- 

 tions towards the smaller, more entire, evergreen type of leaf. It 

 might be that a careful study of these forms would show them to 

 be transitional in other features also. Their range seems to indi- 

 cate an ancient center of distribution in the southwest ; this again is 

 in sharp contrast to all the other species, which may be referred to a 

 more recent center in the southeast. In short, there seem to be 



