PALEOBOTANY I 99 



sea. According to MacDougal,^ if the water had risen in 

 1907 to its ancient level of three or four hundred years 

 ago, 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 Lake 

 of Utah (i 8 per cent, salt^ . This change gradually extermi- 

 nates plant and animal life until the given body of water 

 becomes a true "dead" sea, where practically nothing 

 remains alive, as in the Dead Sea (24 per cent. salt). 

 A more extreme case yet is Lake Van, in Turkey, where 

 saHne matter constitutes over one-third of the contents. 

 In the last stages of such transformations the lake may give 

 place to a salt marsh or plain (salina). South of Lake 

 Titicaca, in the Andes Mts. of BoUvia, ar several sahnas, 

 one of some 4000 square miles in area, with a layer of 

 salt three or four feet thick. 



10. Disturbance of Symbiotic Relationships. — The inter- 

 relationships of organisms are very complex, affording 

 innumerable opportunities for extinction by a disturbance 

 of adjustments. Shade-loving forms in a forest may 

 perish by the destruction of those affording the shade; 

 obligate parasites may perish from the destruction of the 

 necessary host; plants dependent upon certain insects for 

 cross-pollination may perish on account of the extinction 

 of the necessary insects. 



11. Diminution of Carbon Dioxide in the Atmosphere. — 

 There are reasons for thinking that in certain past ages 

 the atmosphere was richer than now in carbon dioxide, 

 and that that condition was favorable to the development 

 of certain vegetatively vigorous species which cannot live 

 in an atmosphere like the present, having a smaller per- 

 centage of carbon. 



• In a letter to the author. 



