IOWA ACADEMY OF SCIENCES. 49 



It is perhaps unsafe to assert that other factors may not 

 have come in as the immediate agents in extermination 

 when the animals had reached a state of decadence due to 

 unfavorable environment, but so far as we can see the con- 

 tinuance of cretaceous conditions would have permitted 

 the survival of some at least of its characteristic fauna. 



While the change in this instance was one of elevation 

 of land surface and obliteration of ocean we can suppose 

 similar destruction of land fauna by the submergence of 

 land areas. In fact we have pretty strong evidence for 

 particular cases of extinction during quaternary times as a 

 result of extensive submergences. Even in marine life 

 depression if taking place more rapidly than adaptation 

 can follow must result in extermination. Corals limited to 

 certain depths are killed by submergences to lower depths 

 and for species limited to certain areas extinction of the 

 species would result. 



Encroachments of seas upon the land or land upon the 

 seas may each result in destruction of life, possibly the 

 extinction of species though usually such changes are too 

 slow to result in complete extermination. They result 

 rather in migration or variation. 



Advance of polar ice cap and its subsequent retreat has 

 very probably resulted in some extinctions particularly 

 among animals of fixed habit. 



Changes of climate from humid to desert or hot to cold 

 in any area if occuring rapidly would certainly influence 

 the fauna and possibly result in extinction. 



Competition among related forms or among forms requir- 

 ing similar conditions has perhaps been most commonly 

 recognized as a factor in extinction. The "survival of the 

 fittest" is here most strikingly illustrated and observation 

 on existing forms leads to more ready appreciation of its 

 force. Closely related species struggle for mastery in a 

 limited area and one of them is crowded out, or, species in 

 widely separated groups may be thrown into competition 

 in a particular region and one or the other must give way. 

 The Indian gives way to his white competitor, the wild 



