ei) 
Recent History of the Rocky Mountain Flora. 24 
Populus balsamifera L. Prosopis juliflora DC. 
» __Fremonti Watson. Acacia Greggii Gray. 
»  monilifera Ait. (=P. deltoides | Platanus Wrightii Wates. 
Marsh.). Yucca brevifolia Engelm. 
» tremuloides Michx. Fraxinus anomala Torr. 
trichocarpa Torr. & Gray. > pistaciaefolia Torr. 
Betula occidentalis Hook. » viridis Michx. f. 
>» _ papyrifera Marsh. Chilopsis saligna Don. 
Ulmus americana Linn. Arbutus Menziesii Pursh. var. 
Morus microphylla Buckley (= M. | Sambucus glauca Nutt. (= S. caerulea 
rubra L.). Cereus giganteus Engelm. [Raf.). 
Juglans californica Wats. Negundo aceroides Moench. 
»  rupestris Engelm. Acer grandidentata Nutt. 
Quercus Emoryi Torr. Sapindus marginatus Willd. 
» hypoleuca Engelm. Prunus emarginata Walp. 
» undulata Torr. »  pennsylvanica L. 
» grisea Liebm. »  angustifolia Nutt. 
Condalia obovata Hook. Pyrus sambucifolia Cham. & Schl. 
Olneya tesota Gray. Crataegus rivularis Nutt. 
Parkinsonia Torreyana Wats. Amelanchier alnifolia Nutt. 
Prosopis pubescens Benth. 
It includes practically all of the trees inhabitating a region extending from 
the eastern base of the Rocky Mountains to the eastern base of the Sierra 
Nevada and Cascade ranges and north of the Mexican boundary, well into 
British America. The character of the flora at the two extremes are most 
widely different. There is a denser forest in the north, but it consists of 
fewer species. In the southern part on the other hand, and undue appearance 
of richness is given to a scanty sylva ı. by the exclusion of shrubs and 2. by 
including species which belong to the southern part of Arizona and New Mexico. 
Of the latter sort are Yucca brevifolia, Cereus giganteus, Pinus chihuahuana, 
Pinus arizonica, Sapindus marginatus, Arbutus Mensiesi, Fraxinus anomala, 
Frazxinus pistaciifolia, Platanus Wrightii, Ouercus Emoryi and Quercus hypo- 
leuca. From the whole region oaks are conspicuously absent, as trees, though 
the forms just referred to as Mexican types form small trees in the southern 
portions of Arizona. 
It is important to analyze the various floral elements of the Rocky 
Mountain flora in order to determine their origin. As has been mentioned 
after the retreat of the continental glacier which covered the Rocky Mountain 
region, as far, as central Montana and Idaho, the region south of this line 
‚became drier and drier owing to the causes above enumerated. An extensive 
-territory both in the northern and southern Rocky Mountains was thus opened 
for the migration of trees, shrubs and herbs. Upon the direction of the mi- 
gration and the character of the plant migrants depends the character of the 
floras in the northern and southern Rocky Mountain regions respectively. 
