132 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 
individuals of which are found on the Tumamoc Hills, and about the 
same number of Semmondsta californica have their nearest representatives 
a mile or two to the west on the slope of the Tucson Mountains. ‘The 
single specimen of Yucca elata growing on the Laboratory domain is 
found in a shady place in the wash, several miles from its kind, which is 
well represented, at a higher altitude, on the slopes toward the Sierritas 
a number of miles away. 
These several perennials have made so little progress in the occupation 
of the ground that they can hardly be considered to have become con- 
stituent members of the associations in which they occur, and there is 
every reason to expect that they will later exhibit wide differences in 
the degree to which they become established in the places they have 
entered. The single individual of Yucca elata may possibly maintain or 
even reproduce itself to some extent in the sheltered nook it has found 
on the Laboratory domain, but it is at least quite as probable that it 
will die out and leave no trace behind. It is of interest to note that 
several species (Spheralcea coultert, Anagallis arvensis, and Erodium 
moschatum) have been found by Professor Thornber growing on the 
Laboratory domain, but only for a year or two, and subsequently have 
disappeared. The Cercidium, on the other hand, may unquestionably be 
regarded as a forerunner of its kind, which, as the processes of erosion 
and deposition finally make the gulch and wash continuous, may be 
expected to move up with the extension of the latter, just as it is doing in 
other places at the present time, and to become there, as it is now in the 
wash, one of the most numerous and characteristic species of its association. 
- Without multiplying examples, it may be gathered from these and 
similar cases that various species of both annuals and perennials have 
very recently entered the Laboratory domain; that some of these have 
rapidly multiplied and competed successfully with earlier occupants of 
the ground; that others are barely holding their places as single indi- 
viduals, making practically no progress, and with a highly problematical 
future before them; and that still others, now well settled in their respec- 
tive associations, are holding their places by accommodation rather than 
competition. Thus, arriving at different times and by various routes, 
members of the associations here represented have come to hold the 
most various relations with their associates. 
The terms just employed—competition and accommodation—are to be 
taken literally. The widely prevalent view according to which desert 
plants are to be considered in relation to their environment, rather than 
to each other, can only be accepted for those areas where distinctively 
desert conditions prevail; but where these conditions are modified, as 
they are on the flood-plain, in the wash, and on the hill, in short over 
a large part of the Laboratory domain, there, as we have seen, the different 
plants stand in close relation to each other, and their relations are of 
