128 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 
Putting all these things together, we are in a position to formulate the 
following theoretical and at the same time highly probable statement 
concerning the distribution of the sahuaro and its causes. 
It is essentially a subtropical species and could not, under existing 
climatic conditions, or any that are supposed to have preceded those 
of the present time, have originated north of its present limits. If 
migrations have been made by it, the general course of its advance has 
been northward, rather than southward; but the fact must again be 
emphasized that on southern exposures in the vicinity of Tucson what 
appear to be essentially optimum conditions for its development pre- 
vail, and it is entirely conceivable that this plant has never, within all 
its history as a distinct species, undergone any extensive migrations 
whatever. 
As it stands to-day, its capacity for the utilization of light rains and 
of getting on with an exceedingly meager annual precipitation adapts 
the sahuaro, as regards the water relation, to a fairly wide range in the 
southwestern United States and northern Mexico, but this is greatly 
restricted by its inability to cope with low temperatures. We have, 
accordingly, in the giant cactus, a plant which, though extraordinary 
in the perfection of its adaptations to the arid regions in which it lives, 
is so limited by its lack of capacity to endure cold that, beyond certain 
lines, its progress to higher latitudes and altitudes is inevitably stopped, 
and locally, far within these limits, the successful occupation of northern 
exposures is impossible. ‘Thus bounds are set by its relation to one of 
_ these two factors, temperature and water-supply, to what, as far as the 
other is concerned, might be a far more extensive distribution than it 
exhibits to-day. There can be no doubt, moreover, that the limits of 
distribution of this species are defined by its physiological relation to 
these two physical factors; others, however important to plant life in 
general, may be omitted from consideration. ‘There is not a particle of 
evidence that light, for example, is to be reckoned as a factor limiting 
the distribution of the sahuaro. It would apparently find light enough 
for its needs hundreds of miles north and east of its present limits, and 
nowhere within its limits, as far as can be seen, does it suffer from too 
intense insolation. Here, however, observations of its deportment in 
the southern part of its range, which are not at hand, are greatly needed. 
If wide distribution is assumed to be an advantage to any given spe- 
cies of plant, the criticism of the giant cactus would be that while—with 
its superficially placed root-system and its exquisite mechanical structure, 
in short, with its essentially perfect construction as regards the water- 
relation—it is adapted to a much greater area of arid country than it 
has yet occupied, such occupation is rendered entirely impossible by its 
physiological relation to temperature. It will be understood that this 
applies to it in its northward extension. Whether the same principle 
