GENERAL DISCUSSION. 181 



action of the environic sieves was such that out of the hundreds of species which must 

 have been thrown against the barriers only 60 reached the beaches and germinated. The 

 last of the series of screens would be the conditions of the beaches themselves, which varied 

 from year to year. To complete the figure it would be necessary to imagine the last of 

 the series of sieves in the great selecting machine as being constantly changed in pattern 

 and size of mesh. 



Furthermore, the selective action does not cease with the germination of the seed which 

 has been carried to the beach. Here a new series of sieves was to be encountered, con- 

 sisting chiefly of the phenomena of progressive desiccation, which would soon bring many 

 species to the limits of endurance and to death. Others siirviving under extreme tension 

 would offer suggestive possibilities as to responses of possible importance in their evolution- 

 ary development, such as have been suggested by the behavior of Prosopis, Aster, Sctrpus, 

 and Atriplex. 



It became obvious during the course of the work that the origination of qualities or 

 structures upon which dissemination would depend might, in many instances at least, have 

 no possible connection in a causal way with the agencies themselves. Thus, for example, the 

 desert gourd which was carried about the lake and deposited on various beaches owes this 

 dissemination to structures which could hardly be attributed to any excitation action on the 

 part of water or to any previous selecting action. The same mechanical qualities of flotation 

 and dissemination are displayed by the fragments of pumice which were carried about at 

 the same time. 



The communal life and successions on the beaches showed some interesting diversities. 

 The manner of the occupation of the zones laid bare by the receding waters was chiefly 

 determined by the water, and hence the first communities of plants were of the nature of 

 strand-steppes. The history of such formations showed two distinct phases, both also 

 determined chiefly by edaphic conditions. The strands of the more gently sloping alkaline 

 beaches were at first occupied by a greater number of species than the bared strips on the 

 steeper gravelly shores, and the pioneers gained, a foot-hold earlier. This difference may 

 be attributed in greater part to the fact that the narrower, more steeply sloping beaches 

 were subject to the action of storm waves for a longer period than the broader zones on 

 the gentler slopes, and also to the fact that the latter actually presented a greater area of 

 soil for the reception of seeds. 



The arrangement of the pioneers on any beach would be characterized as open, and 

 on the gentler alkaline slopes the tendency in general was toward a decrease, both in the 

 number of species and individuals, with few or no secondary introductions, thus making 

 a direct change toward the true open or desert formations. The gently sloping beach at 

 Mecca, however, receiving some seepage water, did not exhibit such simple results. Steeply 

 sloping beaches, as represented by the Travertine Terraces and the shores of Obsidian 

 Island, showed two phases of succession, differing chiefly in degree. The open formations 

 on the shores of Obsidian Island showed some tendency to become closer or denser in 

 irregular areas, which soon began to thin in the final change toward the open desert forma- 

 tion. The benches of Travertine Terrace were characterized by the development of dense 

 ranks comprising a half dozen species or less at the upper margins of the annually bared 

 strands, which were soon thinned in accordance with the general tendency toward desert 

 formations, but at the same time the greater part of the surface of the terrace was knit 

 together in a close formation by a mat of Distichlis. This closed formation, however, soon 

 began to show the effects of desiccation, and the progression toward the desert formation 

 with the introduction of xerophytes would be seen within three or four years after the 

 zone had been laid bare by the lake. The transition is so rapid, or so abrupt, that species 

 appearing on strands two years old are also included in ancient beach ranks marking the 

 positions of strands 300 to 400 years old. The revegetation of the area submerged by the 



