100 E E. Clements 



which appear to diiiFer in no important particular. Quercus minor 

 and gam belt are in close proximity in western Texas; the latter 

 occurs in the mountains o£ western Arizona and Nevada, and 

 garryana extends into the southern Sierra Nevada. Q. alba and 

 lobata are likewise nearly related, though the present gap be- 

 tween them reaches from central Texas to the edge of the Great 

 Valley in California. A similar lacuna exists between Q. velutina 

 and \elloggi, which might well be regarded as varieties of the 

 same species. 



An even more striking example is afforded by the virginiana 

 phylad, of which the primary species ranges from Virginia 

 through the South to Mexico and Lower California, where it 

 meets Q. dumosa of central and southern California. 0. emoryi 

 of the same group stretches from the mountains of western 

 Texas through those of southern Arizona, and Q. engelmanni 

 lies in contact with dumosa in southern California and in close 

 proximity to virginiana. The phylad Q. breviloba-undulata- 

 douglasi is morphologically continuous, the respective ranges 

 being from Alabama to central Texas, western Texas to the east- 

 ern border of the desert, and from northern California to the 

 rim of the Mohave. Similarly, in the phyletic line Q. myrti folia- 

 hypoleuca-wislizeni-agrifolia, the first extends from South Caro- 

 lina to Mississippi, the second from western Texas to Arizona 

 and Sonora, and the third from desert slopes to Lower California 

 and northward, often meeting agrifolia in southern California. 



Phylads of similar significance in respect to evolution and 

 migration are exhibited by Juglans, Fraxinus, Platanus, Populus, 

 Pinus, and Juniperus, as well as by the more shrublike Arbutus, 

 Aesculus, and Prunus. Juglans nigra-rupestris-californica is 

 essentially a structural continuum, and from Massachusetts to 

 California is interrupted only by the desert. The continuity in 



