Vol.1] Hall. — Bofanical Survey of San Jacinto Mountain. 49 



had no connectiou with the Rockies by intervening ranges since 

 Triassic times, and therefore not since our present flora was de- 

 veloped. 



The presence of the northern forms on the higher mountains 

 of the southwestern part of the United States is probably best 

 accounted for by a theory advanced by Asa Gray in 1878.* ac- 

 cording to which all the species normally occurring in the Arctic 

 regions were compelled to migrate southward during the glacial 

 period, on account of the great reduction in temperature. Geol- 

 ogists tell us that in California extensive glaciers extended at 

 least as far south as the southern High Sierras, and it may well 

 be imagined that the climate in Southern California must then 

 have been much less temperate than at the present time, and 

 therefore better adapted to a boreal flora. At the close of the 

 glacial period the conditions were reversed and the boreal species, 

 now finding the climate gradually growing warmer, were forced 

 to retreat to colder regions and therefore not only migrated to 

 the north but also "took to the mountains," where they found at 

 the higher altitudes a set of conditions somewhat similar to those 

 of their northern home. Here many species have continued to 

 exist up to the present time without undergoing any great 

 change; others, finding the conditions unsuited to their growth, 

 have been forced out, while a few have taken on a more or less 

 modified form, thus adapting themselves to their new environ- 

 ment. This may possibly account for the presence of certain 

 high-mountain species which are found only in Southern Cali- 

 fornia but which are represented in the Sierras by closely re- 

 lated species. Thus it was that certain boreal species of plants 

 were stranded on the highest mountains and that we find many 

 which are common to the higher peaks of Southern California, 

 Arizona, and New Mexico, from which they extend along the 

 crests of the Sierra Nevada and Rocky Mountains to the Arctic 

 regions. 



* "Forest Geography and ArchaBology," a lecture delivered before the Har- 

 vard University Natural History Society, Apr. 18, 1878, by Asa Gray. Printed in 

 Am. .Journ. Sci. & Arts, cxvi. 85-94, 183-19« (1878); also in Bull. U. S. G. S. vi. 



62 (1882). 



