140 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 
affords confirmatory evidence of the fundamental importance of tem- 
perature in the determination of aspect preference. é 
(5) Light has been thrown on the causal relations involved in habitat 
preferences by the study of inherited habits and structures of certain 
species. Some at least, and possibly a large proportion of the plants 
which have become established within the Laboratory domain, have 
entered it with correspondingly great differences of habit and power of 
accommodation to conditions which exist here. These differences are con- 
spicuously manifested in choice of habitat, especially as regards aspect 
and soil preferences, and also in their relation to other members of the 
same association. fEncelia is a genus of subtropical species, chiefly, and 
its representative on the Laboratory domain finds most congenial con- 
ditions on southerly exposures, just as Delphinium and other plants of 
cooler and moister regions find their places here on northern slopes. The 
characteristics of the root-system of Cereus giganteus which have been 
worked out by Dr. W. A. Cannon, taken in connection with its arrange- 
ments for water storage and high temperature requirement, satisfactorily 
explain its limitations in both local and general distribution, pp. 59-66. 
Peculiarities of structure and habits, correlated with water-supply on 
the one hand and temperature relations on the other, are exhibited by a 
large percentage of plants of the Laboratory domain. These structures 
and habits have determined, and are still determining, the continued 
existence of the various species in places where they are now growing. 
(6) It is none the less certain, however, that there are here examples 
of plants in which the capacity for individual adjustment to changes 
of condition is a highly potent factor in the determination of local dis- 
tribution. The creosote-bush presents a case in point. It can not be 
doubted that to its extraordinary power of adjustment this plant owes 
its capacity of retaining its place on the long slopes, where, under the 
extremely adverse conditions there prevailing, it forms a conspicuous 
belt of vegetation, from which other perennial plants are largely absent, 
and at the same time flourishes in the low ground of the wash and grows 
luxuriantly along irrigating ditches, where it has an abundant water- 
supply. The same plant, together with the ocotillo and some others, 
shows its capacity for individual adjustment in its perfect indifference 
to change of aspect, to which so many species of Tumamoc Hill are 
exceedingly sensitive. The ready growth of Suweda moquini in the worst 
salt-spots, and at the same time along irrigating ditches conveying fresh 
water, is still another illustration. 
Fundamentally these cases of adjustment are the same as those of 
adaptation just cited. In the one case, as in the other, inherited peculi- 
arities determine the limits of choice. It is true, however, that a species 
without prominent structural adaptations may, by its inherited capacity 
of ready physiological adjustment, be a far more successful element of 
