LOCAL DISTRIBUTION OF SPECIES. ol 
taining what plants had become established on ground laid bare by the 
recession of the water. At this time the shore-line had receded about 
half a mile from where it was at the maximum high-water level of Feb- 
ruary 10 and 11, 1907. They found that in all cases new introductions 
to any strand appeared to have come by water, although taken from the 
parent plant by wind. A striking case was observed on Obsidian Island, 
on the side facing the inflowing currents from the Alamo and New Rivers 
and subject to the action of wind-driven waves, where, out of 24 species 
collected, no less than 19 appeared to have come from seeds deposited 
by water. At a point on the southwest shore of the sea, where a long 
gravel delta comes in from the westward, 8 species of plants were found 
growing on the emersed zone, 2 of which, judging by obtainable evi- 
dence, floated there on the water of the lake, while the occurrence of 
the remaining 6 was such as to indicate the transportation of their seeds 
by currents of small streams from the slope above. From these and 
similar observations made during this expedition, it appears that water, 
whether of the sea or of streams flowing into it, has thus far been the 
most important agent in transporting the seeds of plants that have become 
established about the shores of the Salton Sea since the recent recession 
of its waters began. 
INVASION, COMPETITION, AND SUCCESSION. 
The succession of certain societies of plants on Tumamoc Hill and in 
its vicinity is a matter of simple observation involving no special diffi- 
culties. Thus, we have the frequently cited case of the creosote-bush, 
which is everywhere seen as the dominant growth on alluvial fans and 
the long slopes, and which moves upward with the changes due to erosion 
and deposition. Equally plain is the advance of the palo verde-catclaw 
association, as the washes extend upward through the mesa-like slopes. 
In this case, although the succession is plainly marked, the displacement 
of the preceding association is partial, not complete, since Larrea, its most 
characteristic constituent, remains in the new association and evidently 
profits by the change of conditions. 
But however simple such general movements may appear, a very few 
years of continued observation are sufficient to establish the fact that 
minor changes are constantly taking place, and that by this means the 
invasion of new forms is gradually changing the composition of the vari- 
ous existing associations. For example, the extension of the gulch just 
to the southwest of the Laboratory has brought in its train, in place of 
the association characteristic of the gentle north slope of the hill, the 
establishment of two remarkably different ecological groups on its oppo- 
site sides, which have already been described. 
Of much interest, though perhaps of less permanent influence on the 
character of the vegetation, is the invasion of certain species of annuals 
