180 THE SALTON SEA. 



beach line marking the level of Blake Sea which filled the basin to this shore line 

 within comparatively recent time. The situation suggests that these species originated 

 in the Sink since it was last filled and the inference is strongly in favor of such a conclusion. 

 According to Mr. Parish Astragalus limatus is so closely related to A. preussii A. Gray of 

 the Mohave desert to the northward that it has been considered a variety of it, but the 

 other endemic species noted have no near relatives in the immediate region. If the pos- 

 sibility of the origination of these forms within the Sink be allowed it is also suggested 

 that other species might have originated in like manner but become disseminated over a 

 wide area in such manner that their nativity is undiscoverable. 



The origination of a species within the limits of the Sink or in any basin such as the 

 Cahuilla would probably be followed by its dissemination over surrounding arid regions. 

 The recent formation of the lake, in which about 7 cubic miles of water were poured into 

 the Sink, covering an area of 450 square miles, constituted the beginning of an experiment 

 in which exact studies could be made as to the movements of plants on such sterilized 

 areas. Submergence, of course, resulted in the extermination of the vegetation; then the 

 slow recession of the waters due to evaporation exposed a peripheral strip from a few feet 

 to a half mile wide every year. The total area thus laid bare every year was 8 to 10 square 

 miles, and this afforded the means by which the action of various chsseminational agencies 

 might be tested and compared; furthermore, the test areas showed a slight but measurable 

 progressive modification year by year. The concentration and composition of the water 

 with which each area was moistened was characteristic, and the constitution of the vege- 

 tation on the strip nearest above newly bared strips differed every year from that nearest 

 the new beaches of the preceding year. 



The disseminational agencies which were to be taken into account included those 

 which might be active in the dry phase of the basin and others which might have been 

 called into action by the formation of the lake itself. Winds carried seeds about the region 

 in a manner probably but little changed. The run-off streams coming down the slopes 

 continued their characteristic action in the incidental transportation of fruits, seeds, and 

 propagative bodies, much as in the dry periods of the Sink, but the flood streams of the 

 Colorado would constitute an unusual feature, while the wave and current actions of the 

 lake would be features directly connected with the existence of the lake. The activities 

 of birds in distributing seeds has been the subject of a voluminous literature, although their 

 actual efficiency in carrying seeds to new areas has not been adequately tested. Their 

 possible intervention in this region, however, was enormously increased in importance, 

 since vast numbers of cormorants, pelicans, and ducks were attracted by the possibilities 

 of food in the form of fish offered by the lake. 



It is obvious, therefore, that the phenomena connected with the making and desicca- 

 tion of the lake did not include simply the extermination of the species on an area and the 

 simple re-entrance of these and other species into the bared area. The things which exter- 

 minated the vegetation brought many new conditions, including new disseminational 

 agencies and a whole series of soil conditions different from those prevailing in the area 

 previous to flooding. 



The progress of a seed from its place of origin on the slopes of the Cahuilla toward 

 the bared beaches might be imagined as across or over a number of barriers, which would 

 be of such character as to stop a large number and thus form the sieves of natural selection. 

 Thus, for example, a free seed being carried down the shallow channel of a run-off stream 

 during the few minutes or hours in which water would run down its shallow channel at 

 intervals of a year or two, would be liable to be buried under debris, crushed by rolling 

 rocks, or deposited where it might be eaten by rodents— or, being wetted, would be killed 

 by the high temperature. It is as if the moving seeds were thrown against a series of sieves 

 the meshes of which offered openings differing not only in area but in form. The screening 



