Recent History of the Rocky Mountain Flora. 247 
Crataegus columbiana Howell, Bitter-- _ Salix sitchensis Sanson, Bitterroots. 
> Douglasii Lindl. [roots.  Spiraea arbuscula Greene, Bitterroots. 
Kalmia glauca L. var. microphylla ' Sorbus occidentalis Wats. 
Hook. Taxus brevifolia Nutt. 
Pentstemon fruticosa Pursh. (= P. | Thuja plicata D. Don. 
Menziesii Hook.), Bitterroots. »  gigantea Nutt. 
Populus trichocarpa Hook. Tsuga heterophylla Sarg. 
Rhamnus Purshiana DC. »  Pattoniana Engelm. (= T. Mer- 
Rubus leucodermis Dougl., Bitterroots. tensiana Sarg.). 
Sambucus melanocarpa Gray, Bitter- »  Mertensiana Carr. 
roots. Vaccinium occidentale Gray. 
The following species of the northern Rocky Mountains have a range 
northward: 
Rhododendron (Azaleastrum) albi- Ribes irriguum Dougl. (= R. divarica- 
floruım (= Azalea albiflora Hook.). tum Dougl. var. irriguum Gray). 
Larix Lyallii Parl. Ribes niveum Lindl. (= R. gracile 
Rosa pisocarpa Gray, while eleven Michx.). 
occur only in the Flathead Lake | Rubus parviflorus Nutt. (= R. nut- 
Region of Idaho and the Bitterroot kanus Michx.). 
Mountains, viz. Rubus vitifolius Cham. & Schlecht. 
Alnus sinuata E. Regel. (= R. ursinus Cham. & Schlecht.). 
Chrysothamnus (Bigelowia) graveolens | Salix Nuttallii Sarg. 
Pentstemon mollis Sudw. [Nutt. | Vaccinium scoparium Leiberg. 
The northeastern and eastern element entered in all probability by a path 
which crossed the northern end of the great plains and followed the eastern 
flank of the Rocky Mountains southward to Idaho and Montana, where the 
migration of this contingent seems to stop. 
Abies balsamea Mill. [Ehrh.). | Picea mariana B. 5. P. 
Alnus viridis DC. (= A. alnobetula | Populus balsamifera L. 
Atragene americana Sims (= Clematis | Prunus virginiana L. 
verticillaris DC.). Quercus macrocarpa Michx. 
Betula papyrifera Marsh. Ribes Hudsonianum Richards. 
Bryanthus empetriformis Gray. » rotundifolium Michx. 
Crataegus macracantha Lodd. (= C. |, Rosa blanda Ait. 
coccinea L.). S.P.). | Salix vestita Pursh. 
Picea alba Link (= P. canadensis Be | Ulmus americana L. 
The flora of the northern Rocky mountains is clearly a derivative one, 
because it occupies a territory at one time glaciated and because the affınities 
of the plants clearly point to a western and eastern derivation and not a con- 
tinuation of the flora of the southern Rocky mountains. Its characters are 
clearly negative and the forest flora is distinguished more by what is absent 
than by what is present. For example, it lacks very many of the peculiar 
