. Heredity and evolution in plants . ertiary. If a similar subsidence should occur in thetwo limited regions where the species is now found itwould become extinct unless, by some combination ofcircumstances, it could migrate and become establishedin new localities. It is not unlikely that species haveoften been exterminated in this way. 8. Encroachment of Fresh Water over Land Areas.—Previous to about the year 1900, the Salton basin, in [98 III RKDtTV AND EVOLUTION IN PLANTS lower California, was a saline area of a so pronounceddesert type that its flora contained less than 140 speciesof ferns
. Heredity and evolution in plants . ertiary. If a similar subsidence should occur in thetwo limited regions where the species is now found itwould become extinct unless, by some combination ofcircumstances, it could migrate and become establishedin new localities. It is not unlikely that species haveoften been exterminated in this way. 8. Encroachment of Fresh Water over Land Areas.—Previous to about the year 1900, the Salton basin, in [98 III RKDtTV AND EVOLUTION IN PLANTS lower California, was a saline area of a so pronounceddesert type that its flora contained less than 140 speciesof ferns and flowerng plants, five of which were the winter of 1904-1905 the fresh waters of theColorado River began to debouche into this basin, andby early 1907 had formed a brackish lake, over 80 feet deepand of about 450 square miles in area, known as the SaltonSea. At the end of ten years it still had an area of some-what less than 300 square miles. Some three or fourhundred-years previously the entire Salton Basin was. FIG. 85.—Sketch map showing the geographical distribution of the sun-dew, Droscra fdiformh. (After M. L. Fernald.) occupied with a lake of over 2,000 square miles in area,which, in turn, had dried up and given place to the desertconditions above mentioned. It is not improbable thatsuch drastic changes as this may have resulted in theobliteration of one or more species, though the flora wasnot well enough known previous to the last inundationto make a definite statement on this point possible. Forexample, the presence there of endemic species was notknown until the recent botanical survey of the regionlying between the late water level and that of the ancient PALEOBOTANY I99 sea. According to MacDougal,1 if the water had risen in1907 to its ancient level of three or four hundred yearsago, it would have destroyed all these endemic species. 7. Transformation of fresh water lakes into salt lakes,as in the case of the Caspian Sea, and the Great Salt L
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