112 VEGETATION OF A DESERT MOUNTAIN RANGE. 
other to a common geographical and habital limit. The physical 
requirements of plants are so varied and so elastic that the composition 
of a series of communities occupying similar habitats in widely sepa- 
rated places shows the constant overlapping of the ranges of individual 
species which is due to the physiological inequivalence of these species. 
It is particularly true of the plant communities of arid and semi- 
arid regions that the most closely associated individuals are not alike 
in their life requirements, and this is true to a less pronounced extent 
in all plant communities. The members of the many diverse biological 
types, or growth forms, which are found together in Desert and Encinal 
find their soil water at different levels, procure it at different: seasons, 
and lose it through dissimilar foliar organs, at the same time that they 
react differently to the same temperature conditions. In brief, these 
associated plants are not living in the same climate but are living in 
different sections of the same climate, the demarcation of these sections 
being either temporal or spatial. 
The use of the physical characteristics of the habitat as a criterion 
in the definition of a plant community does something to give a greater 
rigidity and a wider applicability to the definition. On the other hand 
it confuses cause with effect, and makes it impossible to investigate 
the relation of physical conditions to a community defined in that 
manner without reopening the whole question as to the nature and 
identity of the community. There is much strong logic to support 
the view that all necessary definitions and classifications of vegetation 
should be made on the basis of the vegetation alone. When units of 
vegetation are thus defined they lend themselves to the further study 
of their life requirements, and it is such study—applied to individual 
species as well as to vegetation—that affords the most promising and 
important field for ecological activity. 
The distribution of vegetation in the Santa Catalina Mountains is 
strongly controlled by a steep climatic gradient; the vegetation itself. 
is diversified in its display of growth forms; and the secular changes 
of vegetation due to physiographic phenomena, and to the reaction 
of the plant upon its habitat, are in almost complete abeyance. These 
circumstances have made it possible to give a delineation of the vege- 
tation upon purely vegetational characteristics, without regard to the 
secular changes which are taking place in very restricted areas, and 
with particular emphasis upon the individualism of behavior among the 
characteristic species. The same circumstances have also made it 
possible to lay side by side the facts respecting the vegetational gradient 
and those respecting the climatic gradient in such manner as to reveal 
the correlations between the two and to indicate some of the physical 
controls which operate in the limitation of the activities and of the 
ranges of species and of vegetations. 
