40 VEGETATION OF A DESERT MOUNTAIN RANGE. 
Some of the members of this group are of wide distribution in the 
north, as Populus tremulotdes, Achillea lanulosa, and Disporum trachy- 
carpum. In the northern mountain contingent are also a few species 
which range eastward to the Atlantic coast, a few which are found at 
least as far south as Maryland (Heracleum lanatum, Rudbeckia lacini- 
tata, Apocynum androsemifolium, Vicia americana, and Asplenium tri- 
chomanes), not to mention Achillea lanulosa, which scarcely deserves 
separation from the cosmopolitan Achillea millefolium. 
The relationship with northern California and the northwestern 
states is weakly expressed in the occurrence of Salix lasiolepis and 
Prunus emarginata. Genera characteristic of the sub-arctic regions are 
sparingly represented at higher elevations by species of Primula, Sazi- 
fraga, and Androsace. 
Some of the most conspicuous components of the vegetation belong 
to northern genera, but to species which are characteristic of the Mexi- 
can cordillera, as Pinus arizonica, Pinus strobiformis, Alnus acuminata, 
Salix bonplandiana, Quercus hypoleuca, and Quercus reticulata. Such‘ 
genera of herbaceous plants as Solidago, Eupatorium, Erigeron, Pentste- 
mon, Mimulus, Potentilla, Gilia, and Gentiana—all of which are richly 
developed ia the Rocky Mountains—are chiefly represented in the 
Santa Catalinas by species not found in Colorado nor Wyoming. The 
extent to which these species are characteristic of the Arizona-New 
Mexico region or are components of the flora of the higher Mexican 
mountains is only partially known. 
The relationship of the Forest flora to that of the extended mountain 
regions to the south is still further strengthened by the occurrence of 
members of genera which are not found in the Rocky Mountains of 
Colorado and northern New Mexico, as Arbutus, Calliandra, Micro- 
stylis, Drymaria, Cologania, Stevia, and Tagetes. 
To summarize for the mountain as a whole, it may be said that the 
floristic relationships of the Desert and Encinal regions are almost 
wholly with the Mexican deserts and foothills to the south, while those 
of the Forest region are divided between the Mexican Cordillera and 
the Rocky Mountains. The Mexican group is the more conspicuous 
in the make-up of the vegetation, while the Rocky Mountain contin- 
gent is apparently preponderant in number of species. 
It will beimpossible to summarize the floristic relationships of the Santa 
Catalinas in a thorough manner until very much more is known of their 
own floraand also of the floras of themany adjacent mountain rangesand 
desert valleys, both in the United States andin Mexico. Fortheexplana- 
tion of these relationships a closer acquaintance is needed with the 
actual mechanisms of transport which are effective in the dispersal of the 
seeds of desert and mountain plants. A fuller knowledgeis also required 
of the fluctuations of climate within recent geological time, and of the 
consequent downward and upward movements of the Encinal and Forest 
belts of all the southwestern mountains. Such movements would alter- 
