130 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 
variety of habitats. Its success as a desert species is a striking illus- 
tration of the great advantage in adverse circumstances of physiological 
endurance over highly developed adaptations. In possession of this 
undefined quality, which, however, is capable of definite measurement, 
the creosote-bush ranges from below sea-level in the Salton Basin to 
nearly or quite 6,000 feet in altitude, and on a great variety of soils 
from central Texas to the foothills of the Coast Range, while its occur- 
rence (or a species so closely related as to be indistinguishable from it 
except by a systematist) in Chile and Argentina indicates its prolonged 
and successful fight with vicissitudes which we may now but faintly 
picture. All the evidence, however, points to the conclusion that while 
subject, like the sahuaro, to general physiological laws, its definite rela- 
tion to these two factors, temperature and water-supply, has determined 
the geographical range and the local distribution of the creosote-bush, 
and its wider capacity in regard to both of these, as compared with the 
sahuaro, has given it a foothold, both locally and at a distance, far beyond 
the power of the latter species to reach. 
There is, nevertheless, one point in which even the creosote-bush is 
at a disadvantage. A comparison of the maps (plates 14 and 18) shows, 
as already pointed out, that beyond the sharp line between the Tumamoc 
slope and the flood-plain of the Santa Cruz the creosote-bush does not 
go. It also shuns the salt-spots, and to the south of Tucson one may 
ride for miles through the bolson area which lies between the slopes of 
the Tucson Mountains and the Sierritas without noting a single speci- 
men. This avoidance of the flood-plain, seen in all the great river valleys 
of southern Arizona, of salt-spots, and of some bolson areas, seems 
to admit of but one explanation. The creosote-bush is preeminently a 
plant of well-drained ground. Whatever else it can endure, it is quite 
unable to cope with conditions that exist where there is defective aeration 
of the soil. Thus this most successful plant of the desert, the one which 
seems adapted to a wider range of conditions than any other, is brought 
to a full stop when it reaches the line between its own special habitat, 
that is, the one in which it is most numerously represented, and that 
of the mesquite and the salt-bushes. 
As we proceed to other plants it becomes plain that each species has 
its own physiological requirements, resulting in the introduction of very 
different elements, even into the same association of plants, and making 
the distribution of each a problem by itself. The case of Encelia jarinosa, 
a close companion of the sahuaro, and with the same limitations as to 
aspect preference, goes to show that two species, closely associated, may 
have essentially identical temperature requirements, and at the same 
time may differ as widely as possible in their relations to water-supply. 
Again, the sharp tension line between the domain of the creosote-bush 
and that of the mesquite appears to be due to the fact on the one hand 
