100 VEGETATION OF A DESERT MOUNTAIN RANGE. 
point at which some particular feature of its physiological activities 
is met by some particular environmental condition that is preventive 
or uaduly inhibitory to it. The minor fluctuations of climate, which 
have their minimal and maximal values within periods that are as 
brief as the normal life of a perennial plant, are registered in the infre- 
quency of every species as it approaches its distributional limit and 
in the scattered individuals which lie farthest out from the main area 
of occurrence. The secular changes of climate which have their maxi- 
mal and minimal points many centuries apart are registered in slight 
movements of the limits of species, the marginal region of scattered 
occurrence being, of course, the first affected by such movements. 
The writer has seen no evidence indicating that competition between 
plants is at any place in the Santa Catalinas responsible for the limi- 
tation of any species. There is, of course, competition such as that 
between seédling pines in heavy stands of 10 to 40 years in age, and 
such competition as occurs between individuals of the same or different 
species of herbaceous plants in small areas of moist flood-plain. While 
competition may thus determine the surviving individuals of a stand 
of young trees, or may determine the composition of a small community 
of ephemeral or root perennial plants, it is not responsible for the find- 
ing of a plant in one habitat rather than in another, and is not respon- 
sible for the exclusion of a species from an area in which it might find 
favorable conditions. 
It is only consistent with our knowledge of the diversified physical 
requirements of plants that there should be such great diversity in 
the location of the belts of altitude occupied by different species, and 
it accords with our knowledge of the distribution of plants in general 
that these belts should be wider in some cases than in others. It is 
possible, however, to pick out groups of plants the limits of which 
correspond roughly with the limits of the Desert, Encinal, and Forest 
types of vegetation respectively. Even the plants of equatorial regions, 
in which there is a notable constancy of climate, both daily and annual, 
are able to endure small ranges of climate, or occasionally to endure 
changes in individual factors which are many times as great as the 
normal fluctuations of their native climates. In addition to the 
fluctuations of climate from month to month or from year to year which 
must be endured by any plant, there are often even greater differences 
which must be simultaneously endured by the most remotely separated 
individuals of the same specific stock. For example, Asclepias tuberosa 
is found from Maine to Minnesota in the north and from Florida to 
Texas in the south, and thence sporadically in the mountains westward 
to Arizona. In view of the prodigious range of this somewhat poly- 
morphous plant it is surprising not to find it reaching a greater elevation 
than 6,500 feet in the Santa Catalinas. With regard to possible differ- 
ences in the physiological behavior of the most remotely separated 
individuals of a plant stock of such wide range, we know little. 
