102 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 
say erroneous impression, .1f applied to plant species. Furthermore, it 
can not be doubted that “the arid-plains forms tend to become the climax 
type’’ in the southwestern United States, but in this region, where such 
tendencies are liable at any moment to be corrected by the process of 
volcanic uplift, the future disposition of plant and animal associations 
and habitats can only be tentatively suggested. 
Several recent papers of Professor Wheeler (1907) should also be 
noticed, although they are quite too voluminous to permit more than 
the barest reference. In the course of his investigation of the fungus- 
growing ants of North America, it was found that in the wash near the 
Desert Laboratory a catclaw (Acacia greggit) in the neighborhood of 
the nests of Atta versicolor had been completely defoliated by colonies 
of these ants, which suggests on their part a relation to vegetation which 
may often be prejudicial to the latter, however efficient they may be as 
agents of seed dissemination. Another paper of this writer (Wheeler, 
1907), though not directly discussing questions of plant distribution, 
is of special interest in its bearing on the origin of habits and adapta- 
tions, fundamental as they are in relation to distributional problems, as 
studied in a group of animals which is described as having become the 
dominant invertebrates of tropical America. 
Experiments conducted by Prof. W. L. Tower involving the mutual 
relations of certain plants and animals are now in progress in the vicin- 
ity of the Desert Laboratory, where colonies of Leptinotarsa have been 
established. No report of this can yet be made, but the previous work 
of this investigator, which has already been referred to, is of special 
interest and value as embodying, with much else, a study of a single 
genus of animals with reference to the local distribution and wider migra- 
tions of its species, serving thus to indicate a rational method of pro- 
cedure in following out the geographical history of desert plants. (See 
Tower, 1906.) 
Reference should also be made to the well-known papers of Adams 
(1902) and especially to his views regarding the probable existence of a 
characteristic and varied fauna and flora in the Southwest during the 
Ice Age. These, in general, are in accordance with the results indicated 
in the special contributions presented with this paper and referred to 
in the closing discussion. 
