56 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 
bush on the slopes above the flood-plain can not be thought of as gaining 
and holding its place through competition, in any just sense of the term. 
It has become singularly fitted to the hard conditions with which it is 
here confronted, and which, in times of excessive drought, try even its 
extraordinary endurance, and the almost complete absence of other 
perennials is due to their inability to cope with the severe test which 
only the creosote-bush has been able to meet successfully. In this zone, 
distinctively desert conditions prevail, competition between species is 
reduced to a minimum, and the struggle with environmental factors 
determines the case. None the less, the limited number of individuals 
of Larrea occurring on any given area of this zone, and the small size 
it here attains, are presumptive proof of severe competition between 
individuals of the same species. It may yet be possible by determina- 
tion of the available water-content of the soil in times of drought to find 
an expression for the possible aggregate growth of the creosote-bush on 
a given area, but this has not yet been attempted. 
By the invasion and establishment of such plants as those already 
described a succession results, and consequently at a given period the 
associations in which invasion has occurred present a more or less dif- 
ferent composition from that in a preceding stage, often becoming strik- 
ingly dissimilar to what they were before the invasion took place. 
In many of these cases, especially such as those described for the flood- 
plain, we are dealing with successions that are partial or incomplete 
and more or less ephemeral. In contrast with these are certain cases 
of invasion which result in successions of more permanent character. 
As already indicated, coincidently with the process of base leveling, 
several well-marked associations of plants in the vicinity of Tumamoc 
Hill are slowly advancing beyond the areas which they previously occu- 
pied. As the flood-plain of the Santa Cruz River becomes wider, at any 
point, the mesquite association also widens. As by the process of erosion 
the lower part of the hill becomes replaced by the long slopes or by the 
alluvial fans which form one of the most striking topographic features 
of the region, the creosote-bush association advances and takes posses- 
sion of the ground with its characteristic vegetation; and as the washes 
occupied by mesquite, catclaw, and Cercidium torreyanum shift their 
places, or are extended toward the hills, the plants of this association 
advance step by step with them. In each case an essential change of 
topographical boundaries has been promptly followed by the advance of 
each of the associations named into an extension of its appropriate habi- 
tat; in other words, we have in these cases a definite and well-established 
succession. 
