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type of landscape,—so common north and northeast of here,—is of in- 
terest because it brings into juxtaposition the commonest tree of the 
southwestern forests and the commonest plant of the Great Basin. As 
seen here in the Cuyamaca Mountains this combination is doubtless 
near its southernmost limit. Nearer Descanso some hills were seen 
which bore a heavy stand of Adenostoma, apparently unbroken by the 
occurrence of other shrubs. Throughout the return journey we saw 
heavier chaparral on the hills, larger oaks in the valleys, and fewer of 
all the xerophytice shrubs. Yucca was absent, and not a single plant of 
Opuntia littoralis was seen, although we had observed it up to 2000 feet 
on the outward trip. These facts, together with the presence of pines, 
indicate a substantial difference in the physical conditions of the regions 
traversed by the two highways. North of Descanso lies Cuyamaca Peak, 
about 2000 feet higher than the pass east of Campo, a circumstance 
which probably brings a heavier rainfall to the former locality, and 
explains the observed differences of vegetation. 
The opportunity which was afforded us for comparing the coastal 
and desert slopes of the Cuyamaca Mountains emphasized the difficulty 
of interpreting the distributional phenomena of this region in terms of 
“‘life-zones’’ so extensively used by faunistic and floristie workers. ‘Two 
almost wholly dissimilar assemblages of plant and animal life occupy 
the eastern and western slopes of these mountains, and the entire com- 
plex of physical conditions is very unlike on the two slopes. 
In Southern California, as an example, the delimitation of the 
Lower Sonoran zone, proceeding from a basis of temperature conditions, 
requires the embracing of at least three very distinct floristic areas: the 
coastal lowlands, the Mohave desert, and the Colorado desert—with 
the Imperial and Coachella valleys. The fact that these areas possess 
dissimilar floras is not of so much importance in this connection as the 
fact that their plants exhibit a striking dissimilarity of habit, structure 
and anatomy, by reason of their adjustment to the three unlike sets of 
environmental conditions. After we had witnessed the transition from 
the chaparral west of Campo to the junipers, cacti, nolinas and desert 
shrubs east of Jacumba, it was difficult to realize that we were in the 
Upper Sonoran zone on both occasions. When we reached the edge of 
the desert, with its extremely open stand of Covillea and Fouquwieria, it 
was still more difficult to realize that we had re-entered the Lower 
Sonoran zone, which we had traversed for a short distance on leaving 
San Diego. 
Such considerations emphasize the fact that we should not attempt 
to adjust distributional areas to conform to our knowledge of any of the 
controlling factors, for these inconsistencies have arisen from a supposed 
