30 BOTANY OF THE DEATH VALLEY EXPEDITION. 



ley! 



recession, and must have originated in places so situated that they 

 could ungrate equally well to the southern portion of both mountain 

 systems. The time of such origin must have been before these moun- 

 tains had attained their present wide floral separation, for otherwise the 

 same species would not be found in both; and it must have been later 

 than the first part of the last glacial recession, for these species have not 

 migrated to the northern limits of the two mountain systems. And 

 finally, the species in group (a) presumably have originated since the 

 floral isolation of the Sierra Nevada. 



To explain the present distribution of our third group, we must 

 assume a means of communication between the floras of the Sierra Ne- 

 vada and those of the southern Rocky Mountains shortly before their 

 final separation. It is believed that this communication may easily have 

 taken place across Nevada and Utah by way of the numerous mountain 

 ranges which traverse those regions. Under their present conditions 

 of isolation, the migration of these species would be impossible; but 

 when the present boreal flora was depressed nearly or quite to the 

 base level of the mountains, an excellent route for migration was 

 presented. This explanation is rendered still more probable by the 

 occurrence of nearly all the species of group (c) upon one or more of the 

 higher desert ranges, such as the White, East Humboldt, and Trinity 

 mountains of Nevada, and the Uiuta and Wahsatch mountains of 

 Utah. Besides these ranges there are many others which rise to the 

 height of 2,000 meters or more, while the base level of the intervening 

 plains is above 1,500 meters. 



The tabulation of species here presented may serve as an illustration 

 of two other principles concerning the boreal vegetation of the United 

 States : 



First, a circumpolar plant of the Boreal zone, if it occurs in the 

 United States at all, is usually found over broad areas or at widely sep- 

 arated points. This principle is supported by the fact that the group 

 of species common to all three mountain ranges contains twelve that 

 are circumpolar, while none of the other columns contain a single cir- 

 cumpolar species. Several of these twelve species are found also in 

 the high mountains of the Apalachian system. in the eastern United 

 States. This indicates that the present boreal flora was at one time 

 diffused over the base level of a large transverse zone south of the Can- 

 adian line; in other words, that the flora of the United States in glacial 

 times was far more homogeneous in an east and west direction than our 

 present flora, and conversely that if the flora of our present austral 

 region should be driven northward and to higher altitudes — that is, to 

 the position of our present boreal flora— by a general continental in- 

 crease of temperature, the resulting high mountain floras of the Sierra 

 Nevada and of the Apalachian system would be totally different. 



Secondly, our two western great mountain systems do not show marked 

 isolation as a group, but each has a well-defined individual isolation. 



