109) 
at 
2% 
So 
Alpine and Boreal Floras of Western North America. 
Carex festiva Dewey. Trifolium longipes Nutt. 
» - filifolia Nutt. Valeriana silvatica Rich. 
»  tenella Schkuhr. Juncus Parryi Engelm. 
Claytonia Chamissonis Esch. »  subtriflorus E. Mey. 
Deschampsia caespitosa Beauv. Mitella pentandra Hook. 
Draba stenoloba Ledeb. Phleum alpinum L. 
Epilobium anagallidifolium Lam. Phlox Douglasii Hook. 
Erigeron compositus Pursh. Polygonum bistortoides Pursh (=P. 
> salsuginosus Gray. bistorta L.). 
» uniflorus L. Pulsatilla occidentalis Wats. 
Festuca ovina L. var. brevifolia Wats. | Rumex Geyeri Meisn. 
Glyceria pauciflora Presl. Saxifraga nivalis L. 
Heuchera rubescens Torr. Sedum roseum L. (= S. rhodiola DC.'. 
Saxifraga punctata L. Tellima tenella Nutt. 
Spraguea umbellata Torr. Trisetum subspicatum Beauv. 
It appears from this list, that nearly one half are common to the three 
regions, a fact which emphasizes the community of origin of all our boreal 
vegetation. About one-third of the whole number are confined to the Sierra 
Nevada. This is a large percentage for the endemic constituent of 
any region which climatically so closely resembles other portions of the same 
continental area. The broad geographic isolation of the Sierra Nevada, and 
the great antiquity of this isolation may explain the phenomenon in question. 
These lists indicate that the flora of the higher Sierra Nevada of California 
has an affınity quite as close with that of the Rocky Mountains of Colorado, 
as with that of the Cascade mountains of Oregon and Washington. In sug- 
gesting a reason for this remarkable fact, we must consider the theoretic cause 
of their original migration. 
Several glacial periods are recognizable in North America, when the con- 
tinental ice sheet advanced and then retreated, giving rise to one or two inter- 
glacial periods, when the surface exposed by the retiring ice was rapidiy 
occupied by vegetation, which for example in many places in Iowa, Minnesota 
o the depth of 25 feet. It was 
that the first Een of boreal plants migrated along the Cordilleran system 
this stage was not one of 
of mountains southward. For in all probability, 
maximum glaciation, but one in which only the mountain glaciers existed 
giving rise to conditions favorable for the rapid advance of many boreal plants. 
Consequent on the first retreat of the glacial ice and the ushering in of the 
first interglacial period, this vanguard of boreal plants remained stranded on 
the mountain summits of the Sierra Nevada, Cascade and Rocky Mountains 
and during this period of isolation, they were modified into the forms which 
are confined to these mountains and which characterize them floristically spea- 
king. To this category belong the plants listed under divisions a and b above. 
