52 IOWA ACADEMY OF SCIENCES. 



on the part of the animal or rather of some members of its 

 ancestral line. 



Finally there is the form of elimination which occurs 

 apparently as the termination of a long course of gradual 

 decadence or senility and in which distinct elements of 

 destruction are difficult to discover. A process which we 

 may designate as exhaustion. It may be compared perhaps 

 to that running out or deterioration which cultivators 

 recognize in a variety or race that has been kept through 

 a long course of generations. Such exhaustion appears to 

 occur in certain protozoans as paramoecium after a period 

 of fission and which seems to be counteracted by the 

 process of conjugation. Upon such a basis as this we may 

 account for the disappearance of certain types of animal 

 life which so far as we can see have not been forced out 

 by the other factors. Or it might be looked upon as a 

 protoplasmic exhaustion which rendered the type suscep- 

 tible to the action of other factors or a combination of 

 factors no one of which could be counted as predominant. 



To summarize these factors then we may recognize: 



First. — That extinction which comes from modification 

 or progressive evolution; a relegation to the past as a result 

 of the transmutation into more advanced forms. 



tiecond. — Extinction from changes of physical environ- 

 ment which outrun the powers of adaptation. 



Tkird. — The extinction which results from competition. 



Fourth.- — The extinction from extreme specialization and 

 limitation to special conditions the loss of which means 

 extinction. 



Fifth. — ^Extinction as a result of exhaustion. 



I realize that these groups do not represent a classifica- 

 tion based on hard and fast lines and such groups are 

 seldom found in nature but it seems to me that they indi- 

 cate in a tentative way what may be recognized as a 

 number of quite different processes by which organic groups 

 may suffer disappearance. 



