48 University of California Puhlications. [botany 



and the Sierras, all but 18 have also beeu found in the San Ber- 

 nardino Mountains, and this number is sure to be even farther 

 reduced when that range shall have been more thoroughly ex- 

 plored. But from this it is not to be inferred that there are 

 only 18 species common to San Jacinto and the Sierras which 

 are not represented in the San Bernardino Mountains, for this 

 number is taken from the already small list of those species 

 selected as representing the boreal flora, and not from the entire 

 list of San Jacinto Mountain plants. On the other hand, a much 

 larger number of Sierran species, perhaps about four times as 

 many, have been collected on the San Bernardino Mountains 

 which do not occur on San Jacinto. This would seem to indi- 

 cate that, if the high-montane flora of Southern California has 

 been derived from some northern source, many species were un- 

 able to cross the barrier formed by the San Gorgon io Pass, and 

 therefore found the southern limit of their range in the San Ber- 

 nardino Mountains, or, if they did succeed in crossing over to 

 San Jacinto, that they have become extinct on this mountain. 



We have next to consider the probable conditions under 

 which so large a number of boreal species entered Southern Cali- 

 fornia. At the present time we find a series of mountain ranges 

 extending from the High Sierras to San Jacinto Mountain, — 

 everywhere high enough to support coniferous forests except 

 where broken through by passes. Of these, three are so low 

 that the montane flora is divided by strips of the Sonoran, this 

 occurring at the San Gorgonio and Cajon Passes and at the low 

 country between the Sierra Madre Mountains and the southern 

 extremity of the Sierras; the last, which includes Soledad and 

 Tehachapi Passes, being the most serious barrier to the south- 

 ward migration of Sierran species. 



It is possible that the montane flora has been able to cross 

 these gaps. This, however, seems hardly probable, when we con- 

 sider that alpine conditions are nowhere present between the 

 highest peaks of the San Jacinto and San Bernardino Mountains 

 and the southern High Sierras, a distance of something over one 

 hundred miles. It is also to be noted that this explanation 

 would not account for the presence in Southern California of the 

 large number of Rocky Mountain species, since the Sierras have 



