PALEOBOTANY 59 I 
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 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. 
9. Diminution of carbon dioxide in the atmosphere. There 
are reasons for thinking that in cert^fcLpast 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. In the course of 
ages even lofty mountains are planed down by erosion, 
and the arctic and sub-arctic species of the high altitudes 
thus undergo extinction. Furthermore, erosion may be 
coupled with general subsidence. In fact, not only do 
geologists now recognize numerous old mountain "roots," 
such for example as the Adirondack region of New York 
State, but there are also abundant evidences of periodic 
emergence and subsidence of areas of continental extent, 
quite throughout geologic time. The climatic and other 
environmental disturbances accompanying such changes 
would inevitably result in the extinction of certain species. 
(See also If 505.) 
