50 IOWA ACADEMY OF SCIENCES. 



animals inimical to man are driven out and operations of 

 similar nature doubtless occurred among lower forms before 

 man appeared upon the earth. 



Such com petition may have arisen between forms indigen- 

 ous to a particular region but is evidently most striking 

 when species of different faunae are brought into contact 

 as when from migration a species is introduced to a new 

 locality. European butterflies, sparrows, and other forms 

 including man brought to America by design or accident, 

 are prone to supplant the native species. Such migrations 

 and consequent competitions we can safely assume to have 

 occurred in prehistoric as well as in historic times and that 

 species have been exterminated thereby we can scarcely 

 doubt. 



The opening of some barrier permitting the projection 

 of one fauna upon another would intensify such action 

 producing for a number of forms the conditions that 

 ordinarily occur accidentally. Thus the establishment of 

 land connection between Europe and America permitting 

 migrations of whole faunas and their intermingling has 

 resulted in intense competition. Asiatic and African 

 faunas have probabl}?^ been projected upon European and 

 rapid evolution of European types is thus explained. Lack 

 of such connection and competition may account in part 

 for the conservatism of the Australian fauna. 



Quite different from these it appears to me is the extinc- 

 tion which follows some extreme specialization which has 

 fitted the animal to some very limited sphere of existence. 

 For example, parasitic animals have acquired such a 

 dependence upon a host form that extinction without this 

 host is inevitable. Extermination of the Great Auk 

 doubtless carried with it extermination of the parasites 

 peculiar to that species. Further, the parasites dependent 

 on two or more hosts must be exterminated by the 

 destruction of one of such hosts. 



The liver fluke is doomed to extinction whenever one of 

 its necessary hosts is wanting or even whenever the nec- 

 essary association on common ground of its essential hosts 

 becomes impossible. What occurs locally would be gen- 



