PALEOBOTANY 59 1 



by changes in the level of the land, resulting in the killing 

 of fresh-water vegetation. 



8. Disturbance of symbiotic relationships. The inter- 

 relationships of organisms are verj- 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-polhnation may perish on account of the extinction 

 of the necessary insects. 



9. Diminution of carbon dioxide in the atmosphere. There 

 are reasons for thinking that in certain past ages the at- 

 mosphere 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 percentage 

 of carbon. 



10. Denudation of the land surface. This has been 

 sufl&ciently discussed in paragraph 505. 



