CORRELATION OF VEGETATION AND CLIMATE. 99 
It is obvious that the importance of slope exposure lies in the topo- 
graphic control of the physical factors which form the environment of 
the plants concerned. It is possible to know, on purely a priori grounds, 
that two slopes of the same inclination, which lie in opposed positions 
so that one faces north and the other south, will present to plants two 
environments differing in almost every essential physical feature. The 
temperature of the air on two such slopes might be identical as deter- 
mined by the thermometers of a carefully established meteorological 
station, but they are distinctly different as they affect the vegetation, 
for the plants not only receive the direct rays of the sun but receive 
very different amounts of heat through diurnal terrestrial radiation. 
This circumstance is of small importance to full-grown trees and large 
plants, but is of great importance to young plants and seedlings. The 
soil temperatures of opposed slopes are also widely unlike, even in the 
presence of the undisturbed cover of natural vegetation. The two 
opposed slopes would in all likelihood receive the same rainfall, al- 
though this is not necessarily the case. An equal amount of rain might 
effect an equal elevation of the soil moisture on the two slopes, and to 
the same depth, but the soil evaporation of the south slope would 
greatly exceed that of the north slope, and a lower moisture would soon 
prevail in the soil of the former. Greater or less differences may thus 
be shown to obtain between the opposed slopes with respect to the 
most vital features of plant environment. Any attempt to explain the 
importance of slope exposure in determining plant distribution is there- 
fore incomplete unless it takes into account every possible environ- 
mental difference between the slopes. Some of these differences are 
undoubtedly of far greater importance than others, but the question 
of their relative importance is always one that must be asked with 
respect to a particular species of plant. To make a thoroughgoing 
answer as to the importance of slope exposure for a single species is in 
itself a very great undertaking. 
The universal occurrence of a large number of species of plants in 
the vegetation of the Santa Catalinas, their commonness within their 
ranges, and the consistency of their distribution with respect to slope 
exposure, all indicate that there has been ample time in the history of 
the mountain for all of these species to attain as wide a distribution as 
it is possible for them to have under existing climatic conditions. It 
is difficult to conceive of any upward or downward movement being 
possible for any of the common species of plants, inasmuch as thousands 
of years have already given an opportunity for such extensions of 
range. In view of the steep climatic gradient of the mountain it is 
easy to believe that all of the common species have reached upper and 
lower limits beyond which their survival is prevented by definite fea- 
tures of the physical environment. The present vertical limit of a 
species, whether upper or lower, must be looked upon as the average 
